Nuclear hyperfine and quadrupole tensor characterization of the nitrogen hydrogen bond donors to the semiquinone of the QB site in bacterial reaction centers: a combined X- and S-band (14,15)N ESEEM and DFT study.

The secondary quinone anion radical QB(-) (SQB) in reaction centers of Rhodobacter sphaeroides interacts with Nδ of His-L190 and Np (peptide nitrogen) of Gly-L225 involved in hydrogen bonds to the QB carbonyls. In this work, S-band (∼3.6 GHz) ESEEM was used with the aim of obtaining a complete characterization of the nuclear quadrupole interaction (nqi) tensors for both nitrogens by approaching the cancelation condition between the isotropic hyperfine coupling and (14)N Zeeman frequency at lower microwave frequencies than traditional X-band (9.5 GHz). By performing measurements at S-band, we found a dominating contribution of Nδ in the form of a zero-field nqi triplet at 0.55, 0.92, and 1.47 MHz, defining the quadrupole coupling constant K = e(2)qQ/4h = 0.4 MHz and associated asymmetry parameter η = 0.69. Estimates of the hyperfine interaction (hfi) tensors for Nδ and Np were obtained from simulations of 1D and 2D (14,15)N X-band and three-pulse (14)N S-band spectra with all nuclear tensors defined in the SQB g-tensor coordinate system. From simulations, we conclude that the contribution of Np to the S-band spectrum is suppressed by its strong nqi and weak isotropic hfi comparable to the level of hyperfine anisotropy, despite the near-cancelation condition for Np at S-band. The excellent agreement between our EPR simulations and DFT calculations of the nitrogen hfi and nqi tensors to SQB is promising for the future application of powder ESEEM to full tensor characterizations.

Introduction

Quinones are ubiquitous and versatile redox mediators in biological energy conversion systems. Their versatility stems from the ability to tune their redox properties over a very wide range, achieved by interactions with the protein environment of the quinone binding site. However, in no case is the nature of these interactions fully characterized or understood. The bacterial photosynthetic reaction center (RC), with two quinones, QA and QB, has proven to be an exceptional model for studying the underlying structure–function relationships.

In RCs from Rhodobacter (Rb.) sphaeroides, the quinones acting as QA and QB are both identical ubiquinone-10 molecules but exhibit very different functionalities.[1−3] The primary acceptor QA is a tightly bound prosthetic group, while the secondary quinone QB serves as a mobile carrier of two reducing equivalents. The properties of the two quinones are largely determined by the protein environments of the two quinone sites.[3−5] Existing RC crystal structures show significant uncertainty in the conformations of the two quinones, and especially a wide variability of the QB binding.[5] There are several different proposed orientations of QB, including binding in a location distal from a central FeII–(His)4 complex.[6,7] However, the proximal binding of QB seen in earlier structures is also that seen in illuminated crystal structures, which likely traps the semiquinone state (SQB) in the active position.[6,7] The potential hydrogen bond donors to QB include the four residues His-L190, Ser-L223, Ile-L224, and Gly-L225, inferred from X-ray structures.

An alternative way to characterize the SQ state in proteins is through EPR, exploiting its paramagnetism.[8,9] EPR measures the interaction between the electron spin of the SQ and the nearby magnetic nuclei (1H, 13C, 14N or 15N, 17O) of the quinone and protein. Using high-resolution pulsed EPR techniques (ENDOR, ESEEM), detailed information on the O···H···N hydrogen bonds can be obtained. This includes the 1H and 14(15)N hfi tensors (which depend on the geometry of the H-bonds and the spin density distribution over the SQ and H-bonds) and the 14N nqi tensors (which reflect the chemical type of the coupled nitrogen, and the population and configuration of its electronic orbitals). X-band (∼9.7 GHz) 14N and 15N 2D ESEEM spectra clearly show the interaction of two nitrogens with SQB, each carrying transferred unpaired spin density.[10] Quadrupole coupling constants estimated from the 14N spectra indicate them to be a protonated nitrogen of an imidazole residue and an amide nitrogen of a peptide group. The imidazole nitrogen (isotropic coupling a(14N) = 1.5 MHz) can only be assigned to Nδ of His-L190, consistent with existing X-ray structures. The second nitrogen (a(14N) = 0.5 MHz) could not be specified between two candidates (Ile-L224 and Gly-L225), and selective 15N isotope labeling is needed for unambiguous assignment of this nitrogen. However, computational work assigned the a(14N) = 0.5 MHz coupling to the peptide nitrogen (Np) of Gly-L225.[11] The hfi coupling of other protein nitrogens with SQB was not resolved (<0.1 MHz), indicating that H-bonds with other nitrogens were much weaker or absent.

In our previous study,[10] the principal values of the hfi tensors and their directions relative to the g-tensor axes for the two H-bonded nitrogens were undetermined because simulations of 15N 2D ESEEM spectra were performed separately for each nitrogen without a common coordinate system defining the principal axes for all nuclear tensors. Additionally, the principal values and directions of the nqi tensors of the two 14N (with hfi couplings |a(14N)| ≤ 1.5 MHz) were not determined experimentally from the X-band spectra due to substantial deviations from the cancelation condition |νN – |a|/2| = 0. This prevented the observation of the nuclear transitions with pure zero-field nqi frequencies that would provide the principal values of the nqi tensor. Performing the experiments at lower microwave frequencies and consequently a lower Zeeman frequency (ν14N) should improve the fulfillment of the cancelation condition and allow for the observation of the zero-field nqi frequencies.

In this article, therefore, we present the results of an S-band (∼3.6 GHz) 14N ESEEM study of SQB and its analysis in conjunction with previously reported X-band 14,15N 1D and 2D ESEEM spectra. Simulations of X- and S-band spectra provide the hfi and nqi tensors of both nitrogens to SQB and a reliable estimation of the Nδ quadrupole coupling constants. This topic is important, as the experimental background for theoretical investigations relies upon an understanding of the relationship between tensor characteristics and the strength (energy) and geometry of H-bonds. Unexpectedly, we find that the nqi and hfi values for the peptide Np of Gly-L225 lead to suppression of its contribution to the spectrum, despite the near-cancelation condition of Np at S-band. An explanation for this observation, with full support from DFT calculations, is presented.

Materials and Methods

Sample Preparation

Reaction centers used in this study were isolated from a strain of Rb. sphaeroides expressing RCs with a histidine tag on the M subunit.[12] Cells were grown under the natural abundance of 14N or in a 15N background using 15N-labeled ammonium sulfate (Cambridge Isotopes). In order to isolate SQ EPR signals, the native high spin Fe2+ must be replaced by diamagnetic Zn2+. Procedures for biochemical metal exchange, along with the methods of bacterial cell growth and RC isolation, were as previously described.[10] The SQB radical was generated by exposing the RCs to a single 532 nm Nd:YAG laser pulse in the presence of ferrocytochrome c (to quickly rereduce the bacteriochlorophyll dimer after charge separation). Upon radical formation, samples were frozen promptly in liquid nitrogen.

S-Band ESEEM

The S-band EPR experiments were performed on an S-band upgraded pulsed EPR ELEXSYS E-580 Bruker spectrometer equipped with an Oxford CF 935 cryostat and ER 4118S-MS5 resonator at 80 K. The frequency of the microwave source was 3.6 GHz, which corresponds to a resonance field of 129 mT for paramagnetic species with a g-factor of ∼2. Spectral processing of ESEEM time-domain patterns, including subtraction of the relaxation decay (fitting by polynomials of 3–6 degree), apodization (Hamming window), zero filling, and fast Fourier transformation (FT), was performed using Bruker software WIN-EPR V2.22 Rev. 10. Processed data were then imported into Matlab R2013a to be simulated by EasySpin.[13]

X-Band ESEEM

In this work, we have also used X-band spectra of SQB obtained from previously described experiments[10] using 14N and 15N three-pulse ESEEM and the 2D ESEEM technique, HYSCORE (hyperfine sublevel correlation spectroscopy).

Factors Influencing 14N Powder ESEEM Spectra

The 14N nucleus interacting with an unpaired electron spin S = 1/2 can produce up to six lines in an ESEEM spectrum. These lines are from transitions between three nuclear energy sublevels in each of the two electron spin manifolds with mS = +1/2 or −1/2. Because of differences in orientation dependence, not all transitions contribute equally to the spectra in ESEEM measurements of powder-type samples (such as the frozen RC suspensions used in these experiments). The ESEEM spectrum expected from 14N with a predominantly isotropic hyperfine coupling is governed by the ratio between the effective nuclear frequency in each manifold, νef±, given by νef± = |ν14N ± |A(14N)|/2| (where A(14N) is the effective hyperfine coupling), and the quadrupole coupling constant (qcc), 2Qq/4h.[14,15]

If νef–/K ∼ 0, i.e., νef– ∼ 0 (the situation known as the cancelation condition, because ν14N ≈ A(14N)/2), the three nuclear frequencies of the corresponding manifold will be close to the three pure (zero-field) nuclear quadrupole resonance frequencies with 14N transitionswith the energy levels defined by the principal values of the nqi tensorBoth eqs 1 and 2 are completely described by K and the asymmetry parameter η, which ranges in value from 0 to 1. In this case, three narrow peaks at the frequencies given in eq 1 will be present in the powder ESEEM spectrum. These frequencies possess the property ν+ = ν– + ν0 and can appear in the spectrum up to a ratio of νef/K ∼ 0.75–1. However, the intensities of these peaks are also strongly dependent on the time τ chosen between the first two pulses of the ESEEM experiment, so it is important to perform the experiment at multiple values of τ so as to not miss any transitions.

If νef/K > 1, only a single line is expected, without pronounced orientation dependence from each corresponding manifold. This line is produced by a transition at the maximum frequency, which is actually a double-quantum transition between the two nuclear outer states with m14N = −1 and +1. The frequency of this transition is well described by[14]Two other single-quantum transitions, involving the central states with m14N = 0, usually do not show any resolved peaks because of a significant orientation dependence from the quadrupole interaction.

A more thorough analysis, taking into account the influence of the anisotropic hyperfine interaction, has shown that these simple rules can also be applied to interpret ESEEM spectra when fulfillment of the cancelation condition is considered individually for each principal value of the hfi tensor.[16]

DFT Calculations

The DFT calculations were performed using the B3LYP functional. The EPR-II basis set was used for all atoms except Zn where 6-31g(d) was employed. The model for the calculations was the QM portion of the QM/MM optimized geometry, as described previously.[11] All calculations were performed using the ORCA electronic structure program.[17]

Results and Discussion

14N and 15N ESEEM Spectra of SQB

The interactions of SQB with the protein environment in RCs with a natural abundance of nitrogen (14N isotope - 99.63%) and with uniform 15N labeling have been previously studied in detail by X-band pulsed EPR.[10,11]14,15N HYSCORE spectra clearly resolved the contribution from two nitrogens N1 and N2 carrying unpaired spin density detected through the isotropic hfi coupling. These nitrogens possess different characteristics, i.e., qcc K = 0.35–0.40 MHz and isotropic hfi coupling constant a(14N) ∼ 1.5 MHz for N1 and K = 0.65–0.75 MHz and a(14N) ∼ 0.5 MHz for N2.[10] The values of K characterize the chemical type and electronic configuration of the 14N atoms interacting with SQB. For instance, the value of K for N1 most closely corresponds to the protonated nitrogen from an imidazole residue, and that of N2 is typical for a peptide amide nitrogen. On the basis of the X-ray structure and DFT calculations, N1 was identified as Nδ of His-L190 and N2 was assigned as the peptide Np of Gly-L225 (Figure 1).[10,11]

Figure 1

Interaction of SQB with its two resolved nitrogen donors His-L190 Nδ and Gly-L225 Np, which are H-bonded to carbonyl oxygens O4 and O1, respectively. The principal axes of the g-tensor are labeled as , , and . The g axis lies along the line connecting the two oxygen atoms, which carry most of the spin density; the g axis is perpendicular to the molecular plane, and g is perpendicular to both other principal axes. The principal values of the SQB g-tensor are g = 2.00626, g = 2.00527, and g = 2.00213.[19]

For both nitrogens, the estimated values of hfi couplings and qcc give a ratio of νef–/K in the X-band that deviates by 0.75–1.0 from the cancelation condition (νef–/K ∼ 0). This is in contrast to SQA where two H-bonded nitrogens, from Nδ of His-M219 and the backbone Np of Ala-M260, possess larger couplings of 2.5 and 1.9 MHz, respectively, with smaller deviations of νef–/K from the cancelation condition. In agreement with this conclusion, X-band ESEEM spectra of SQA contain well-resolved nqi lines (eq 1) from the His and Ala nitrogens, both identified by parameters K and η as a consequence of the principal values of their nqi tensors.[11] This analysis suggests that the cancelation condition for N1 and N2 in the QB site, required for the full determination of the nqi tensor, would be achieved at lower microwave frequencies, yielding a smaller ν14N. In this work, we used S-band with a resonant magnetic field of 128.3 mT for SQB, corresponding to ν14N = 0.395 MHz. This decrease in magnetic field strength lowers νef–/K (especially for Np of Gly-L225, to ∼0.2), and should promote the appearance of the nqi triplets from both nitrogens in the S-band spectrum.

Figure 2 shows the S-band three-pulse ESEEM spectrum of SQB in a stacked representation. The peak at 5.5 MHz corresponds to the matrix 1H feature at the S-band resonant magnetic field. Three “cancelation-like” peaks appearing at 0.55, 0.92, and 1.47 MHz (all ±0.06 MHz) can be attributed to 14N. Formal analysis of these peaks using eq 1 for the nqi triplet gives K = 0.40 MHz and η = 0.69, which are strongly characteristic of the protonated nitrogen of an imidazole residue. According to previous studies, the nqi values of Nδ from histidine hydrogen bonded with the SQs of several quinone sites vary within a narrow interval of K ∼ 0.35–0.43 MHz and η ∼ 0.6–0.8.[9,18] Under this interpretation, there is seemingly no trace of Gly-L225 Np in the spectrum, despite the fact that S-band should have brought it even closer to exact cancelation than for His-L190 Nδ. This result is very much unexpected but is accounted for by our simulations of the EPR spectra and DFT calculations as detailed below.

Figure 2

Stacked representation of the two-dimensional set of the S-band three-pulse ESEEM spectra for SQB in bacterial RCs. The spectra show the modulus of the Fourier transform along the time T axis at different times, τ. The initial time τ is 192 ns in the blue trace and was increased in steps of 128 ns in successive traces. The nuclear quadrupole resonance frequencies assigned to His-L190 Nδ are marked with stars. A full 3D view is available in the Supporting Information (Figure S6). Experimental parameters: magnetic field 128.3 mT, microwave frequency 3.601 GHz, π/2 pulse length = 36 ns, temperature 80 K.

15N X-Band HYSCORE and Simulations

The 15N X-band HYSCORE spectrum of SQB (Figure 3) consists of a narrow diagonal peak at (ν15N, ν15N) from weakly coupled nitrogens, and two pairs of cross-peaks 1 and 2 from N1 and N2, respectively. They are located symmetrically around the diagonal peak along the antidiagonal, with maxima at (2.53, 0.49) MHz (1) and (1.83, 1.16) MHz (2) with a1(15N) ∼ 2.1 MHz and a2(15N) ∼ 0.7 MHz, respectively.[10] Previous analysis based on axial hfi tensor simulations of the individual powder 15N HYSCORE spectra of N1 and N2 showed a significant disagreement in the relative intensity of the cross-peaks 1 and 2. Partial improvement was achieved by introducing rhombicity into the hfi.[10] We suggested that further improvement would necessitate simulating N1 and N2 together, with both hfi tensors defined in the same coordinate system.

Figure 3

Comparison of experimental and simulated X-band 15N HYSCORE spectra of SQB in bacterial RCs in stacked (left) and contour (right) presentation. Peaks labeled as 1 and 2 are assigned to His-L190 Nδ and Gly-L225 Np, respectively. Simulations are shown in red. Experimental parameters: magnetic field 345.4 mT, time between first and second pulses τ = 136 ns, microwave frequency 9.688 GHz, temperature 80 K. The experimental spectrum is taken from ref (10).

Simulations of the ESEEM spectra were performed in the g-tensor coordinate system of SQB. The principal axes of the g-tensor (, , ) with respect to the molecular structure of the quinone have been defined by single-crystal EPR experiments for SQA,[19] and are shown in Figure 1. The simulated peak maxima were in strong agreement with the experimental peak positions (Table S1, Supporting Information). The orientations of the hfi and nqi tensor principal axes for Nδ of His-L190 and Np of Gly-L225 were defined relative to the g-tensor axes with Euler angles (α, β, and γ) in accordance with the EasySpin program (http://www.easyspin.org)[13] and as described in the Supporting Information.

Comparison of the experimental and simulated 15N HYSCORE spectra for SQB is shown in Figure 3. All parameters were recalculated for 14N and are listed in Table 1. Simulations were found to be relatively insensitive to Euler angles, especially for α and γ. Therefore, Euler angles were determined with better confidence from the 14N spectra. The weak dependence between the Euler angles and the resulting 15N spectrum is likely due to the low level of hfi anisotropy.

Table 1

14N Hyperfine Simulation Parameters (15N Data Recalculated for 14N)a with DFT Calculated Values in Parentheses

nitrogen	a (MHz)	T (MHz)	δ	Euler anglesb
Nδ His-L190	|1.3|−|1.4| (1.3)	0.3–0.4 (0.3)	0.5–0.6 (0.0)	[50° 130° −10°] ([0° 110° −10°])
Np Gly-L225	|0.4| (0.6)	0.2 (0.2)	0 (0.0)	[0° 130° −100°] ([0° 130° −130°])

Principal values of the rhombic hfi tensor: a + 2T, a – T(1 – δ), a – T(1 + δ); δ ranges from 0 to 1 corresponding to axial and rhombic tensors, respectively.

Hfi tensors with δ = 0 (axial) only require the β and γ Euler angles for their full description, so α was set to zero in these cases.

The choice of hfi tensor principal values, on the other hand, had a large impact on the resulting 15N HYSCORE simulation. Initial parameters for the tensors were taken from the previous analysis of contour line shapes and separate simulations for N1 and N2.[10] The correct intensity ratio of the two features could be obtained when Np of Gly-L225 was assigned an axial hfi tensor and Nδ of His-L190 with a more rhombic tensor. Simulation parameters for the spectrum giving the optimum relative intensity for cross-peaks N1 and N2 and a reasonable line shape in comparison with the experimental spectrum are listed in Table 1. Euler angles included in Table 1 were determined from the 14N spectra as discussed below.

14N X- and S-Band ESEEM and Simulations

The X-band 14N HYSCORE spectrum of SQB (Figure 4) exhibits intense and extended cross-ridges 1 possessing a maximum at (3.96, 1.51) MHz and a second pair of cross-peaks 2 of circular shape with smaller intensity and a maximum at (3.86, 2.98) MHz.[10] Simulations produced reasonable agreement with these experimental maxima (Table S1, Supporting Information). Previous analysis has shown that cross-peaks 1 and 2 correlate double-quantum (dq) transitions from opposite manifolds, i.e., νdq+ and νdq–, for N1 and N2, respectively. The spectrum does not resolve any other cross-peaks. A significant orientation dependence of the single-quantum transitions very likely explains the lack of additional features. The corresponding X-band three-pulse ESEEM spectrum (Figure 5) also shows dq features as an intense line at 1.5 MHz, a weak peak at 2.9 MHz, and a broad feature around 3.8 MHz. At lower frequency, there is a peak at ∼0.3 MHz and a feature around 0.7 MHz that appears to contain overlapping lines. The shapes of these peaks are affected by the procedures preceding the FT analysis, particularly by the degree of polynomial used for extraction of the relaxation decay. Accordingly, our original three-pulse ESEEM spectrum[10] has been reanalyzed in this work to minimize suppression of the low frequency peaks.

Figure 4

Comparison of experimental and simulated X-band 14N HYSCORE spectra of SQB in bacterial RCs in stacked (left) and contour (right) presentation. Peaks labeled as 1 and 2 are assigned to His-L190 Nδ and Gly-L225 Np, respectively. Simulations are shown in red. Experimental parameters: magnetic field 346.1 mT, time between first and second pulses τ = 136 ns, microwave frequency 9.705 GHz, temperature 80 K. The experimental spectrum is taken from ref (10).

Figure 5

Comparison of experimental (blue) and simulated (red) X- (top) and S- (bottom) band 14N three-pulse ESEEM spectra of SQB in bacterial RCs. Experimental parameters: X-band, magnetic field 346.1 mT, microwave frequency 9.705 GHz, time τ = 100 ns; S-band, magnetic field 128.3 mT, microwave frequency 3.601 GHz, time τ = 320 ns.

14N 1D and 2D ESEEM were simulated with full hfi and nqi tensors for both nuclei. It is well established that the principal directions of the nqi tensor for the Nδ imine nitrogen are associated with the molecular axes of the imidazole residue (Figures 1 and S1, Supporting Information).[20−23] These axes are retained if this nitrogen is coordinated to a metal or H-bonded, and thus can be used for the characterization of the imidazole orientation. This approach has been verified by several experiments showing that the principal directions of the nqi tensor determined by magnetic resonance techniques provide the correct description of the ligand geometry by comparison with X-ray crystal structures.[20−22,24]

The principal values of the nqi tensor are |Qmax| = 2K, |Qmid| = K(1 + η), and |Qmin| = K(1 – η). The orientation of the principal axes of the nqi tensor with respect to the molecular frame has been theoretically analyzed for protonated nitrogens of imidazole and peptide bonds in several different systems. For a hydrogen bond between SQB and Nδ of His-L190, an appropriate calculation of the nqi tensor is that of N-methylimidazole H-bonded to a semiquinone.[23] According to this study, the Qmin direction points along the N–H bond (x axis), the Qmax direction is perpendicular to the imidazole plane (z axis), and Qmid is in the imidazole plane, perpendicular to the N–H bond (y axis) (Figure S1, Supporting Information). However, especially short H-bonds (as indicated by a high η value) resulted in an interchange of the Qmax and Qmid principal directions. Under this condition, Qmax lies in the imidazole plane. This orientation of Qmax has been observed experimentally for the NδH (K = 0.35 MHz, η = 0.66) in Cu(II)-doped single crystals of l-histidine hydrochloride[22] and in l-histidine monochloride monohydrate (K = 0.32 MHz, η = 0.946).[20] In contrast, Qmax for NεH (K = 0.366 MHz, η = 0.268) in l-histidine monochloride monohydrate was found to be normal to the imidazole plane.[18] The nqi diversity observed in l-histidine monochloride was explained by differences in intermolecular hydrogen bonding at each nitrogen. The H-bond length H···O 1.58 Å for the Nδ is significantly shorter than 1.94 Å for the Nε.[20] This behavior is in agreement with the calculations of Fritscher.[23] In reaction centers, the length of the O4–HNδ His-L190 H-bond in the optimized structure of SQB is 1.58 Å, suggesting a strong hydrogen bond, as supported by the hfi tensor of the H-bonding proton.[11] Additionally, the characteristics of the nqi tensor of NδH in Cu(II)-doped single crystals of l-histidine hydrochloride[22] are very close to the values observed for the histidine nitrogen H-bonded with SQB. If one accepts that this system mimics well the His-L190 coordinated with Zn(II) and H-bonded with O4 of SQB, then Qmax for Nδ is expected to have a stable value of the order ∼0.70–0.86 MHz and a preferred direction along the y molecular axis in the imidazole plane. Our DFT calculations of SQB were in excellent agreement with this assignment of the nqi tensor orientation, with the calculated principal components having Qmin approximately along the x molecular axis, Qmid perpendicular to the imidazole ring (z axis), and Qmax in-plane with the imidazole (y axis) (Table 2 and Figures S2 and S3, Supporting Information). However, we note that when simulations were performed with Qmax perpendicular to the imidazole plane (Euler angles [170° 30° −10°]) accompanied by a small adjustment of the His-L190 hfi tensor orientation (Euler angles [60° 110° −10°]) but all other simulation parameters unchanged, an equally good fit to the experimental data was achieved. Therefore, the possibility that Qmax takes on a perpendicular orientation relative to the imidazole plane cannot be completely eliminated.

Table 2

14N X- and S-Band Quadrupole Simulation Parameters with DFT Calculated Values in Parentheses

nitrogen	K (MHz)	η	Euler angles
Nδ His-L190	|0.38|−|0.39| (0.40)	0.69 (0.94)	[−120° 90° −100°] ([−120° 80° −100°])
Np Gly-L225	|0.74| (−0.98)	0.45 (0.43)	[−120° 60° −160°] ([−160° 60° −160°])

The amide nitrogen in free peptides, such as in metal complexes of diglycine, which H-bonds to the inorganic sulfur atoms of iron–sulfur clusters,[25−31] has a narrow range of qcc (K = 0.75–0.85 MHz) determined by the electronic structure and the geometry of the planar peptide group. This coupling constant is only slightly perturbed by hydrogen bonding, as has been confirmed by calculations of the nqi tensor.[28,32,33] From these calculations, it was found that the Qmax principal direction is normal to the local peptide plane, the Qmid direction almost coincides with the C(O)–N(H) bond, and Qmin points about 30° off of the N–H bond. Our DFT calculations were again in good agreement with this assigned orientation for the peptide Np Gly-L225 nqi tensor axes (Table 2 and Figures S2 and S3, Supporting Information). Reported qcc’s of peptide nitrogens hydrogen bonded with semiquinones of different quinone sites are also within the interval indicated above.[34−36]

In the crystal structure 1DV3, the main population of QB is in the proximal position. Therefore, coordinates from this crystal structure were combined with the theoretically predicted orientations of the principal directions of the nqi tensor, and the corresponding Euler angles relative to the g-tensor axes for His-L190 Nδ and Gly-L225 Np were calculated and fixed in the EPR simulations. The initial nqi parameters for His-L190 Nδ were obtained from the S-band spectrum for Nδ as K = 0.40 MHz and η = 0.69. For Gly-L225 Np, our 14N ESEEM data for Np of Ala-M260 H-bonded with SQA were used as initial parameters, i.e., K = 0.77 MHz and η = 0.63 MHz, following from the nqi triplet: 1.0, 1.8, 2.8 MHz.[11] No assumptions were made about the Euler angles defining the orientations of the axes for hfi tensors.

To provide justification for the assignment of the lines in the S-band spectra, simulations were done simultaneously along with 14N X-band three-pulse and 14,15N HYSCORE spectra. Therefore, optimization involved judging the parameters’ ability to reproduce all four spectra, as opposed to just one. Once an optimal set was found, a tolerance window of 0.1 MHz was given to the His-L190 Nδ hfi and nqi coupling constant parameters (Tables 1 and 2). This small degree of flexibility reflects the error underlying the various assumptions built into the simulations (e.g., ideal pulse shape and full excitation of the SQ line width). Nevertheless, only minor adjustments to the initial hfi and nqi tensors within the allotted tolerance window were needed to obtain very satisfactory simulations, and the optimized hfi Euler angles were in excellent agreement with the DFT calculated values (Tables 1 and 2, Figures S4 and S5, Supporting Information). All of the 14N simulations are shown in red in Figures 3, 4, and 5, overlaying the experimental spectra. In addition, we have compared the three-pulse S-band simulations performed separately on Nδ from His-L190 and on Np of Gly-L225 to show Np makes only a very small contribution to the total S-band spectrum (Figure 6).

Figure 6

Comparison between separate 14N S-band ESEEM simulations done for His-L190 Nδ (red) and Gly-L225 NP (black). With the given parameters (Tables 1 and 2), Gly-L225 Np makes a minimal contribution to the overall spectral intensity.

An Explanation for the Suppression of the Np Contribution to the S-Band Spectrum

The peculiarities of the S-band spectrum cannot be explained qualitatively using the simple approach based on the approximation of a purely isotropic hfi. Analysis of the simulation results from Tables 1 and 2 allows us to conclude that this approximation is not fully satisfied, especially in the case of Gly-L225 Np. Quantitatively, variation of the hfi coupling in changing from a + 2T to a – T (under an axial approximation) can be characterized by its range 3T, which in our case is comparable with the 14N Zeeman frequency in X-band or even exceeding ν14N in S-band for both nitrogens (Table 3). In this case, the peculiarities of the spectral features can be understood by considering the fulfillment of the condition νef/K < 0.7–1.0 for different principal values of hfi tensors (shown in bold in Table 3).[16]

Table 3

Comparative Characteristics of Magnetic Interactions for Nδ His-L190 and Np Gly-L225 in X- and S-Band ESEEM Experiments

nitrogen	Nδ His-L190						Np Gly-L225
K (MHz)	0.38–0.39						0.74
band	X-band			S-band			X-band			S-band
ν14N (MHz)	1.06			0.395			1.06			0.395
ν14N/K	2.72–2.79			1.01–1.04			1.43			0.53
A1, A2, A3a (MHz)	0.9	1.2	2.0	0.9	1.2	2.0	0.2	0.2	0.9	0.2	0.2	0.9
νef–	0.61	0.46	0.06	0.06	0.21	0.61	0.96	0.96	0.61	0.30	0.30	0.06
νef–/K	1.58	1.19	0.16	0.16	0.55	1.58	1.30	1.30	0.82	0.41	0.41	0.08
νef+	1.51	1.66	2.06	0.85	1.00	1.40	1.16	1.16	1.51	0.50	0.50	0.85
νef+/K	3.92	4.31	5.35	2.21	2.60	3.64	1.57	1.57	2.04	0.68	0.68	1.15

Principal values of the rhombic hfi tensor: a + 2T, a – T(1 – δ), a – T(1 + δ).

This analysis is summarized in Table 3. One can see that for Nδ His-L190 at S-band the two smallest principal values A1 and A2 of the hfi tensor give νef–/K values of 0.16 and 0.55, respectively. These values of νef–/K, which comprise the perpendicular component of the hfi tensor under the axial approximation, justify the appearance of the nqi triplet in the S-band spectra despite the fact that a ratio of νef–/K = 0.8 for an isotropic coupling a(14N) = 1.4 MHz of this nitrogen might suggest a distortion of this triplet. The set of conformations in the (A1, A2) plane produces the triplet nqi spectrum. The spread of νef–/K leads to the observed line broadening surrounding the peak maxima. Nevertheless, the peak maxima satisfy the nqi condition well (eq 1). Simulation of the X- and S-band spectra led to only a very minor correction of the K value estimated from the experimental spectrum (Table 2).

Similarly to His-L190 Nδ, the transition from the X- to S-band shifts νef–/K for all principal values of Gly-L225 Np toward a range appropriate for the observation of the nqi triplet. However, the S-band spectra do not exhibit lines typical for nqi frequencies of Np. An explanation for this comes from previous computational and theoretical analyses showing that the amplitudes of the nqi peaks also vary with ν14N/K. At a low ν14N/K ratio, the energy levels in both manifolds approach each other and ESEEM vanishes at about ν14N/K < 0.3.[14] Finally, the condition of exact cancelation is not sufficient to produce intense ESEEM peaks, with an additional requirement being that A(14N) ≥ K.[15] If A(14N) < K, then the ESEEM intensity is expected to be weak even at exact cancelation.

Transition from X- to S-band decreases the ν14N/K ratio by almost 3-fold (2.68), giving ν14N/K ≈ 0.5 for Np. The simulated spectra (Figure 6) show substantial suppression of spectral intensity from Np in agreement with the theoretical considerations provided above. This suggests that the intensity of the spectrum would increase with an increase of the isotropic coupling. Figure 7 shows that increasing the isotropic coupling 2–3-fold, without changing any other parameters, significantly increases the spectral intensity as expected.

Figure 7

Simulated three-pulse S-band ESEEM spectra of Gly-L225 Np as a function of increasing a(14N) in steps of 0.2 MHz. All other parameters are fixed to the values listed in Tables 1 and 2.

The S-Band Defined nqi Tensor of His-L190 Nδ

Previous analyses of ESEEM[37a] and nuclear quadrupole resonance[37b] measurements determined the values of e2qQ/h and η for imidazole 14Nδ in different compounds to be sensitive to hydrogen-bond formation through a change in electron occupancy of the nitrogen orbitals.[37] It was concluded that a stronger H-bond would lower the pπ orbital spin population and increase the N–H orbital population, subsequently reducing the e2qQ/h value and increasing η. Empirically, this influence can be described with good accuracy by the linear dependence between h/e2qQ and η. Figure 8 shows h/e2qQ and η values for histidine 14Nδ interacting with SQA[34,35,38,39] in bacterial reaction centers and in photosystem II. The points in Figure 8 are clustered in the region with η ≥ 0.7. Also shown is the point (blue) corresponding to the previously reported SQB14Nδ nqi tensor values (e2qQ/h = 1.65 MHz and η = 0.61);[40] it is substantially shifted to a lower η, away from the “QA area”, suggesting the formation of a significantly weaker hydrogen bond. However, this estimation of the 14Nδ nqi tensor was based on the analysis of an X-band three-pulse ESEEM spectrum not satisfying the cancelation condition, and therefore prone to distortion of the peak maxima (see discussion in ref (10)). The characteristics of the nqi tensor reported in this work move the QB point (red) to within the range of the QA points; this is more consistent with the similar value of the anisotropic hyperfine tensor component T ∼ 5.2 MHz for the proton involved in the H-bond between Nδ and SQB, which is similar to the complementary T ∼ 5.4 MHz for the QA site in RCs from Rb. sphaeroides.

Figure 8

Values of h/e2qQ and η for histidine 14Nδ interacting with SQA in bacterial and PSII RCs (black) and SQB in bacterial RCs reported by Lendzian et al. (ref (40)) (blue) and this work (red). The linear fit (dashed line) was performed for only the points corresponding to SQA. (1) QB– in Rhodobacter sphaeroides (ref (40)), (2) QB– in Rhodobacter sphaeroides (this work), (3) QA– in high-pH-treated PSII pH 9.2 (ref (35)), (4) QA– in CN-treated PSII pH 5.5 (ref (35)), (5) QA– in LiClO4-treated PSII pH 6.0 (ref (39)), (6) QA– in high-pH-treated PSII pH 5.0 (ref (35)), (7) QA– in Rhodobacter sphaeroides (ref (34)), (8) QA– in Rhodopseudomonas viridis (ref (38), the nqi tensor was reported incorrectly in the table of this reference: we have reanalyzed their spectra on the basis of their assigned peak positions).

Conclusion

S-band ESEEM was performed on SQB in bacterial RCs with the aim of determining the principal values of the nqi tensor for the hydrogen bond donors Nδ of His-L190 and Np of Gly-L225. The resulting three-pulse spectrum showed a visible contribution only from Nδ of His-L190, despite the shift of the 14N Zeeman frequency toward a region expected to nearly satisfy the cancelation condition for both nitrogens, as predicted from the data obtained in previous X-band experiments. Simultaneous simulation of 1D and 2D X-band and S-band ESEEM spectra confirmed this experimental result. Analysis of the simulation parameters allowed us to conclude that the Np contribution to the S-band spectrum is suppressed as a result of the typically large qcc of Np in combination with its relatively small isotropic hyperfine coupling and comparable hfi tensor anisotropy. ESEEM intensity vanishes under these conditions, despite even better fulfillment of the cancelation condition for Np than for Nδ of His-L190. Nevertheless, simulations successfully reproduced the experimental X-band data for Np, and were well supported by DFT calculations.

The observed triplet of lines for His-L190 corresponding to the zero-field nqi frequencies (eq 1) was simulated as qcc e2qQ/h = 1.54 ± 0.02 MHz with an asymmetry parameter η = 0.69 and defines the principal values (eq 2) of the nqi tensor for Nδ of His-L190. These data significantly improve upon a previous analysis not performed under the cancelation condition.[40] Our results show the strength and applicability of S-band for obtaining accurate nqi parameters for weakly coupled nuclei.

While estimation of the hfi tensor alignment to the molecular frame is best done by EPR on single crystals, it is often very difficult to obtain single crystal radical samples for a protein of interest. In this study, we show that simultaneous simulation of the 14N S- and X-band three-pulse spectra and 14,15N HYSCORE spectra (with a tolerance window of 0.1 MHz) produces remarkable agreement with the DFT calculated tensor orientations. The success of DFT to reproduce the experimentally determined hfi and nqi tensors highlights the reliability of modern DFT for predicting EPR parameters. Additionally, our findings, taken as a whole, show promise for future ESEEM determinations of Euler angles for nuclei not suitable for high-frequency ENDOR measurements[41,42] (such as weakly coupled nitrogens) or in a spin system not readily crystallizable in the radical state.