Literature DB >> 25484721

Conformation and crystal structures of 1-amino-cyclo-hexa-neacetic acid (β(3,3)Ac6c) in N-protected derivatives.

Naiem Ahmad Wani1, Vivek K Gupta2, Rajni Kant2, Subrayashastry Aravinda1, Rajkishor Rai1.   

Abstract

N-Protected derivatives of 1-amino-cyclo-hexa-neacetic acid (β(3,3)-Ac6c), namely Valeroyl-β(3,3)-Ac6c-OH [2-(1-pentanamidocyclohexyl)acetic acid, C13H23NO3], (I), Fmoc-β(3,3)-Ac6c-OH [2-(1-{[(9H-fluoren-9-yloxy)carbonyl]amino}cyclohexyl)acetic acid, C23H25NO4], (II), and Pyr-β(3,3)-Ac6c-OH {2-[1-(pyrazine-2-amido)cyclohexyl]acetic acid, C13H17N3O3}, (III), were synthesized and their conformational properties were determined by X-ray diffraction analysis. The backbone torsion angles (ϕ, θ) for β(3,3)-Ac6c-OH are restricted to gauche conformations in all the derivatives, with a chair conformation of the cyclo-hexane ring. In the crystal structure of (I), the packing of mol-ecules shows both carb-oxy-lic acid R 2 (2)(8) O-H⋯O and centrosymmetric R (2) 2(14) N-H⋯O hydrogen-bonding inter-actions, giving rise to chains along the c-axis direction. In (II), centrosymmetric carb-oxy-lic acid R 2 (2)(8) O-H⋯O dimers are extended through N-H⋯O hydrogen bonds and together with inter-ring π-π inter-actions between Fmoc groups [ring centroid distance = 3.786 (2) Å], generate a layered structure lying parallel to (010). In the case of compound (III), carb-oxy-lic acid O-H⋯Npyrazine hydrogen bonds give rise to zigzag ribbon structures extending along the c-axis direction.

Entities:  

Keywords:  conformation; crystal structure; disubstituted-β-amino acids; hydrogen bonds; π–π inter­action

Year:  2014        PMID: 25484721      PMCID: PMC4257263          DOI: 10.1107/S1600536814020777

Source DB:  PubMed          Journal:  Acta Crystallogr Sect E Struct Rep Online        ISSN: 1600-5368


Chemical context

β-Amino acids are homologues of α-amino acids, which are constituents of several bioactive natural and synthetic products. β-Amino acids have been used as building blocks in peptidomimetic drug design (Cheng et al. 2001 ▶). The introduction of β-amino acids into pharmacologically active peptide sequences has shown improved biological activity and metabolic stability (Yamazaki et al., 1991 ▶; Huang et al., 1993 ▶). The backbone conformation of a β-amino acid is defined by the torsional angles ϕ, θ and ψ (Banerjee & Balaram, 1997 ▶), as shown in Fig. 1 ▶. The monosubstitution at the α- and β-carbon atoms plays an important role in the folding of oligomers of β-amino acids (Seebach et al., 2009 ▶).
Figure 1

Definition of backbone torsion angles for β-amino acids.

In order to investigate the effect of protecting groups and disubstitution on the conformation of β-amino acids, N-protected derivatives of 1-amino­cyclo­hexa­neacetic acid (β3,3Ac6c), i.e. Valeroyl-β3,3-Ac6c-OH (I), Fmoc-β3,3-Ac6c-OH (II) and Pyr-β3,3-Ac6c-OH (III) were synthesized. The crystal structures of the three compounds were determined and are reported herein, together with their comparative conformational features.

Structural commentary

The mol­ecular conformations of Valeroyl-β3,3-Ac6c-OH (I), Fmoc-β3,3-Ac6c-OH (II) and Pyr-β3,3-Ac6c-OH (III) are shown in Fig. 2 ▶. The backbone torsion angles (ϕ, θ) (C0′—N1—C1B —C1A and N1—C1B—C1A—C1′) adopt a gauche conformation in all three compounds [ϕ = 61.9 (3)°, θ = 57.2 (3)° for (I); ϕ = 56.7 (3)°, θ = 66.1 (3)° for (II) and ϕ = 65.5 (2)°, θ = 55.0 (2)° for (III). The torsional angle ψ restricts the extended (trans) conformation for (I) [166.9 (2)°] and (III) [157.9 (2)°]. In the case of (II), it is restricted to a gauche conformation [i.e. ψ = −63.6 (3)°]. In a 3,3-disubstituted β-amino acid residue, β3,3-Ac6c-OH, the cyclo­hexane ring imposes a restriction on the torsion angles ϕ and θ. The protecting groups at the N-terminus of (I) adopts a trans geometry [ω0 (C4—C0′—N1—C1B) = 177.4 (2) for (I), ω0 (O—C0′—N1—C1B) = −175.64 (19) for (II) and ω0 (C6—C0—N1— C1A) = −170.04 (17)° for (III)]. In the case of the N-protected tert-butyl­oxycarbonyl (Boc) group, the protecting group adopts a cis geometry with ω0 = 14.50° (Vasudev et al., 2008 ▶). The cyclo­hexane ring adopts a chair conformation with axial amino and equatorial CH2CO groups in all the derivatives. In Pyr-β3,3-Ac6c-OH (III), an intra­molecular N—H⋯N inter­action is observed between NH of the β3,3-Ac6c-OH residue and N3 of the pyrazine ring as shown in Fig. 3 ▶ c. There are no intra­molecular hydrogen bonding inter­actions observed in the crystal structures of derivatives (I) and (II).
Figure 2

ORTEP view of the mol­ecular conformation with the atom-labelling scheme. for Valeroyl-β3,3-Ac6c-OH (I), (b) Fmoc-β3,3-Ac6c-OH (II) and (c) Pyr-β3,3-Ac6c-OH (III). The displacement ellipsoids are drawn at the 40% probability level. H atoms are shown as small spheres of arbitrary radii.

Figure 3

(a) Packing of Valeroyl-β3,3-Ac6c-OH (I) down the b-axis showing the alternative hydro­philic and hydro­phobic layers (b) space-filling model.

Supra­molecular features

In the crystals of compounds (I) and (II), inter­molecular hydrogen-bonding inter­actions generate primary centrosymmetric dimeric but different substructures (Figs. 4 ▶ and 5 ▶). In (I), N1—H⋯O1ii bond pairs (Table 1 ▶) give a cyclic (14) motif which is extended into a ribbon structure along the c-axis direction through a second but non-centrosymmetric cyclic carb­oxy­lic acid (8) O2—H⋯Oi hydrogen-bond motif (Fig. 4 ▶ a). In (II), the inter­molecular dimeric association is through the centrosymmetric (8) carb­oxy­lic acid hydrogen-bonding motif. Structure extension is through N1—H⋯O1′ (carbox­yl) hydrogen bonds (Table 2 ▶), generating a two-dimensional layered structure lying parallel to (010) (Fig. 4 ▶ c). Also present in the structure are π–π inter­actions between the Fmoc groups with an inter­centroid distance of 3.786 (2) Å. Fig. 4 ▶ c shows the aromatic rings of Fmoc groups stacked in a face-to-face and edge-to-face manner, together with inter-plane distances that are within the range for stabilizing π–π inter­actions (Burley & Petsko, 1985 ▶; Sengupta et al., 2005 ▶) and have been reported to induce self-assembly in peptides (Wang & Chau, 2011 ▶). In the case of (I) and (II), the mol­ecular packing in the crystals leads to the formation of alternating hydro­phobic and hydro­philic layers. In the crystals of (III), in which no dimer substructure formation is present, the mol­ecules are linked by an inter­molecular carb­oxy­lic acid O2—H⋯N2i hydrogen bond (Table 3 ▶) with a pyrazine N-atom acceptor, leading to the formation of a zigzag ribbon structure extending along the c-axis direction.
Figure 4

(a) Packing of Fmoc-β3,3-Ac6c-OH (II) down the a-axis. (b) Space-filling model showing the alternative hydro­philic and hydro­phobic layers (packing down the c-axis). (c) The environment of the Fmoc group showing the aromatic inter­action. The centroid–centroid distances are shown.

Figure 5

(a) Packing of Pyr-β3,3-Ac6c-OH (III) down the a-axis showing the ribbon structure. (b) Zigzag arrangement of the ribbons along the c-axis.

Table 1

Hydrogen-bond geometry (, ) for (I)

DHA DHHA D A DHA
O2H2OO0i 0.87(4)1.74(4)2.599(3)166(4)
N1H1NO1ii 0.82(3)2.16(3)2.981(3)172(2)

Symmetry codes: (i) ; (ii) .

Table 2

Hydrogen-bond geometry (, ) for (II)

DHA DHHA D A DHA
N1H1NO1i 0.86(2)2.35(2)3.182(3)161(2)
O2H2OO1ii 0.84(3)1.83(3)2.673(3)177(1)

Symmetry codes: (i) ; (ii) .

Table 3

Hydrogen-bond geometry (, ) for (III)

DHA DHHA D A DHA
O2H21N2i 0.93(4)1.86(4)2.791(3)177(4)
N1H1NN30.79(2)2.34(2)2.729(2)111.3(19)

Symmetry code: (i) .

Synthesis and crystallization

Preparation of Valeroyl-β3,3Ac6c-OH (I): β3,n class="Chemical">3Ac6c-OH (5 mmol, 785 mg) was dissolved in 5 ml of a 2M NaOH solution and a solution of 5 mmol of valeric anhydride (931 mg) dissolved in 1,4-dioxane was added, after which the mixture was stirred for 4 h at room temperature. On completion of the reaction, the 1,4-dioxane was evaporated and the product was extracted with diethyl ether (3 × 5 ml). The aqueous layer was acidified with 2M HCl and extracted with ethyl acetate (3 × 10ml) and the combined organic layer was washed with brine solution. The organic layer was passed over anhydrous Na2SO4 and evaporated to give Valeroyl-β3Ac6c-OH (yield: 1.1 g, 85.2%). Single crystals were grown by slow evaporation from a solution in methanol/water. Preparation of Fmoc-β3,3Ac6c-OH (II): β3,n class="Chemical">3Ac6c-OH (10 mmol, 1.57 g) was dissolved in 1M Na2CO3 solution and Fmoc-OSu (10 mmol, 3.37 g) dissolved in CH3CN was added. The reaction mixture was stirred at room temperature for 6 h. After completion of the reaction, the CH3CN was evaporated and the residue was extracted with diethyl ether (3 × 10 ml). The aqueous layer was acidified with 2M HCl and extracted with ethyl acetate (3 × 15 ml). The combined organic layer was washed with brine solution. The ethyl acetate layer was passed over anhydrous Na2SO4 and evaporated. The residue was purified by crystallization in ethyl acetate/n-hexane, affording Fmoc-β3,3Ac6c-OH (yield: 3.0 g, 79%). Single crystals were obtained by slow evaporation from an ethyl acetate/n-hexane solution. Preparation of Pyr-β3,3Ac6c-OH (III): n class="Chemical">Pyrazine carb­oxy­lic acid (3 mmol, 372 mg) was dissolved in dry CH2Cl2 and then 200 µl of N-methyl­morpholine was added, followed by β3,3Ac6c-OMe. HCl (3 mmol, 622.5 mg) and EDCI. HCl (3 mmol,576 mg) at 273 K. The reaction mixture was stirred at room temperature for 12 h. After completion of the reaction, water was added and the reaction mixture was extracted with CH2Cl2 (3 × 5ml). The combined organic layer was washed with 2M HCl (2 × 5ml), Na2CO3 (2 × 5ml) and brine solution (2 × 5ml). The organic layer was passed over anhydrous Na2SO4 and evaporated to give Pyr-β3,3Ac6c-OMe (Yield: 600 mg, 72.2%). Pyr-β3,3Ac6c-OMe (2 mmol, 554 mg) was dissolved in 2 ml of methanol and 1 ml of 2M NaOH, and the reaction mixture was stirred at room temperature for 4 h. Methanol was evaporated and the residue was extracted with diethyl ether (2 × 5ml). The aqueous layer was acidified with 2M HCl and extracted with ethyl acetate (3 × 5ml). The combined organic layer was washed with brine solution (1 × 5ml). The ethyl acetate layer was passed over anhydrous Na2SO4 and evaporated to give Pyr-β3,3Ac6c-OH (yield: 370 mg, 70.3%). Single crystals were grown from an ethanol/water solution.

Refinement details

Crystal data, data collection and structure refinement details are summarized in Table 4 ▶. For derivative (I), H atoms for N1 and O2 were located in a difference Fourier map and both their coordinates and U iso values were refined. The remaining H atoms were positioned geometrically and were treated as riding on their parent C atoms, with C—H distances of 0.96–0.98 Å and with U iso(H) = 1.2U eq(C) or 1.5U eq(methyl C). For derivatives (II) and (III), all hydrogen atoms were located from a difference Fourier map and both their coordinates and U iso values were refined. In (II), the carboxyl O—H distance was constrained to 0.84 Å. Although not of consequence with the achiral mol­ecule of (III), which crystallized in the non-centrosymmetric space group Pca21, the structure was inverted in the final cycles of refinement as the Flack parameter was 0.8 (14). The inverted structure gave a value of 0.2 (14) for 1585 Friedel pairs.
Table 4

Experimental details

 (I)(II)(III)
Crystal data
Chemical formulaC13H23NO3 C23H25NO4 C13H17N3O3
M r 241.32379.44263.30
Crystal system, space groupMonoclinic, P21/c Triclinic, P Orthorhombic, P c a21
Temperature (K)291291291
a, b, c ()9.5894(5), 12.5007(7), 12.3709(8)6.0834(4), 12.7642(9), 12.8399(9)8.7135(1), 10.5321(1), 14.3907(2)
, , ()90, 109.984(7), 9094.018(6), 92.295(6), 100.489(6)90, 90, 90
V (3)1393.66(14)976.53(12)1320.66(3)
Z 424
Radiation typeMo K Mo K Mo K
(mm1)0.080.090.10
Crystal size (mm)0.30 0.08 0.080.30 0.05 0.030.25 0.25 0.25
 
Data collection
DiffractometerOxford Diffraction Xcalibur, Sapphire3 CCDOxford Diffraction Xcalibur, Sapphire3 CCDOxford Difraction Xcalibur, Sapphire3 CCD
Absorption correctionMulti-scan (CrysAlis PRO; Oxford Diffraction, 2010)Multi-scan (CrysAlis PRO; Oxford Diffraction, 2010)Multi-scan (CrysAlis PRO; Oxford Diffraction, 2010)
T min, T max 0.797, 1.0000.947, 1.0000.931, 1.000
No. of measured, independent and observed [I > 2(I)] reflections14087, 2737, 17177781, 4166, 203768869, 2878, 2670
R int 0.0470.0470.034
(sin /)max (1)0.6170.6390.639
 
Refinement
R[F 2 > 2(F 2)], wR(F 2), S 0.068, 0.213, 1.030.054, 0.086, 0.970.042, 0.106, 1.04
No. of reflections273741662878
No. of parameters162353240
No. of restraints011
H-atom treatmentH atoms treated by a mixture of independent and constrained refinementAll H-atom parameters refinedAll H-atom parameters refined
max, min (e 3)0.36, 0.300.15, 0.200.27, 0.26
Absolute structure  (Flack, 1983): 1585 Friedel pairs
Absolute structure parameter  0.2(14)

Computer programs: CrysAlis PRO (Oxford Diffraction, 2010 ▶), SHELXS97 and SHELXL97 (Sheldrick, 2008 ▶), ORTEP-3 for Windows (Farrugia, 2012 ▶) and PLATON (Spek, 2009 ▶).

Crystal structure: contains datablock(s) I, II, III, New_Global_Publ_Block. DOI: 10.1107/S1600536814020777/zs2313sup1.cif Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536814020777/zs2313Isup2.hkl Structure factors: contains datablock(s) II. DOI: 10.1107/S1600536814020777/zs2313IIsup3.hkl Structure factors: contains datablock(s) III. DOI: 10.1107/S1600536814020777/zs2313IIIsup4.hkl Click here for additional data file. Supporting information file. DOI: 10.1107/S1600536814020777/zs2313Isup5.cml Click here for additional data file. Supporting information file. DOI: 10.1107/S1600536814020777/zs2313IIsup6.cml Click here for additional data file. Supporting information file. DOI: 10.1107/S1600536814020777/zs2313IIIsup7.cml CCDC references: 1024488, 1024489, 1024490 Additional supporting information: crystallographic information; 3D view; checkCIF report
C13H17N3O3F(000) = 560
Mr = 263.30Dx = 1.324 Mg m3
Orthorhombic, Pca21Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2c -2acCell parameters from 28796 reflections
a = 8.7135 (1) Åθ = 3.7–27.0°
b = 10.5321 (1) ŵ = 0.10 mm1
c = 14.3907 (2) ÅT = 291 K
V = 1320.66 (3) Å3Cube, colorless
Z = 40.25 × 0.25 × 0.25 mm
Oxford Difraction Xcalibur, Sapphire3 CCD diffractometer2878 independent reflections
Radiation source: fine-focus sealed tube2670 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.034
Detector resolution: 16.1049 pixels mm-1θmax = 27.0°, θmin = 3.9°
ω scansh = −11→11
Absorption correction: multi-scan (CrysAlis PRO; Oxford Diffraction, 2010)k = −13→13
Tmin = 0.931, Tmax = 1.000l = −18→18
68869 measured reflections
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.042All H-atom parameters refined
wR(F2) = 0.106w = 1/[σ2(Fo2) + (0.048P)2 + 0.4913P] where P = (Fo2 + 2Fc2)/3
S = 1.04(Δ/σ)max = 0.032
2878 reflectionsΔρmax = 0.27 e Å3
240 parametersΔρmin = −0.26 e Å3
1 restraintAbsolute structure: (Flack, 1983): 1585 Friedel pairs
Primary atom site location: structure-invariant direct methodsAbsolute structure parameter: 0.2 (14)
Experimental. CrysAlis PRO, Oxford Diffraction Ltd., Version 1.171.34.40 (release 27–08-2010 CrysAlis171. NET) (compiled Aug 27 2010,11:50:40) Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm.
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2sigma(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.
xyzUiso*/Ueq
H10.902 (2)0.412 (2)1.0592 (15)0.030 (5)*
H1B21.400 (3)0.435 (2)1.2329 (16)0.041 (6)*
H1B51.272 (3)0.384 (2)1.4161 (18)0.049 (7)*
H1B61.183 (3)0.442 (2)1.3342 (17)0.048 (6)*
H1B11.499 (2)0.380 (2)1.3191 (16)0.033 (5)*
H1D21.651 (3)0.100 (3)1.200 (2)0.061 (7)*
H1N1.161 (2)0.172 (2)1.2259 (16)0.036 (6)*
H1G11.481 (3)0.256 (2)1.1391 (19)0.045 (6)*
H20.709 (3)0.107 (2)0.971 (2)0.051 (7)*
H30.894 (3)−0.021 (2)1.0429 (17)0.047 (6)*
H1B41.269 (3)0.107 (2)1.3669 (17)0.045 (6)*
H1G41.401 (3)0.050 (3)1.232 (2)0.061 (7)*
H1G21.616 (3)0.323 (3)1.173 (2)0.066 (8)*
H1B31.415 (3)0.192 (2)1.3989 (19)0.046 (6)*
H1G31.502 (3)0.005 (3)1.3184 (19)0.065 (8)*
H1D11.667 (4)0.176 (3)1.298 (2)0.071 (9)*
H210.885 (5)0.352 (4)1.458 (3)0.106 (13)*
C10.9028 (2)0.3259 (2)1.05370 (16)0.0401 (4)
N20.7893 (2)0.27249 (19)1.00502 (14)0.0442 (4)
C30.7875 (3)0.1459 (2)1.00191 (15)0.0433 (5)
C40.8954 (3)0.0736 (2)1.04730 (16)0.0450 (5)
N31.0074 (2)0.12584 (16)1.09722 (13)0.0398 (4)
C61.0105 (2)0.25193 (18)1.10026 (14)0.0343 (4)
C01.1318 (2)0.31669 (18)1.15876 (15)0.0382 (4)
O0'1.1604 (2)0.42890 (15)1.14626 (15)0.0658 (6)
N11.19610 (18)0.24153 (15)1.22264 (12)0.0348 (4)
C1A1.2995 (2)0.28157 (17)1.29864 (13)0.0311 (4)
C1B1.2100 (2)0.36660 (19)1.36671 (16)0.0381 (4)
C1'1.0648 (2)0.31013 (19)1.40619 (15)0.0423 (5)
O11.0357 (3)0.2011 (2)1.4129 (3)0.1197 (13)
O20.9700 (2)0.39599 (18)1.43574 (16)0.0677 (6)
C1B11.4369 (2)0.3562 (2)1.26078 (16)0.0394 (4)
C1G11.5373 (3)0.2777 (2)1.19617 (18)0.0504 (5)
C1D1.5948 (3)0.1582 (3)1.2448 (2)0.0575 (6)
C1B21.3586 (2)0.15966 (19)1.34564 (14)0.0366 (4)
C1G21.4634 (3)0.0817 (2)1.28311 (18)0.0483 (5)
U11U22U33U12U13U23
C10.0386 (10)0.0391 (10)0.0428 (10)0.0036 (9)−0.0049 (9)0.0025 (10)
N20.0374 (8)0.0538 (11)0.0414 (9)0.0036 (8)−0.0067 (7)0.0039 (8)
C30.0402 (11)0.0513 (13)0.0386 (11)−0.0063 (9)−0.0045 (9)−0.0018 (10)
C40.0510 (12)0.0399 (11)0.0440 (10)−0.0049 (10)−0.0066 (9)−0.0016 (9)
N30.0445 (9)0.0361 (8)0.0389 (9)−0.0002 (7)−0.0048 (8)0.0035 (7)
C60.0340 (9)0.0368 (10)0.0321 (9)0.0012 (7)0.0002 (7)0.0023 (8)
C00.0388 (10)0.0335 (9)0.0422 (11)0.0024 (8)−0.0048 (8)0.0032 (8)
O0'0.0769 (12)0.0360 (8)0.0843 (13)−0.0095 (8)−0.0359 (11)0.0160 (8)
N10.0365 (8)0.0291 (8)0.0388 (9)−0.0040 (6)−0.0074 (7)0.0016 (6)
C1A0.0299 (8)0.0305 (9)0.0329 (9)−0.0015 (7)0.0010 (7)−0.0030 (7)
C1B0.0352 (10)0.0327 (10)0.0464 (11)−0.0002 (8)0.0058 (9)−0.0069 (8)
C1'0.0418 (11)0.0358 (10)0.0494 (12)−0.0009 (9)0.0134 (9)−0.0037 (9)
O10.0886 (16)0.0440 (10)0.227 (3)−0.0088 (10)0.099 (2)−0.0111 (15)
O20.0533 (10)0.0462 (9)0.1035 (15)0.0071 (8)0.0373 (10)0.0115 (9)
C1B10.0322 (9)0.0395 (11)0.0464 (11)−0.0066 (8)0.0044 (9)−0.0020 (9)
C1G10.0453 (12)0.0557 (13)0.0500 (13)−0.0058 (11)0.0180 (11)−0.0036 (11)
C1D0.0454 (12)0.0660 (16)0.0611 (16)0.0172 (11)0.0121 (12)−0.0050 (12)
C1B20.0371 (10)0.0398 (9)0.0329 (10)0.0042 (8)−0.0012 (8)0.0026 (8)
C1G20.0551 (13)0.0404 (11)0.0493 (12)0.0171 (10)0.0039 (11)−0.0012 (9)
C1—N21.337 (3)C1B—H1B60.95 (3)
C1—C61.391 (3)C1'—O11.180 (3)
C1—H10.91 (2)C1'—O21.297 (3)
N2—C31.334 (3)O2—H210.93 (5)
C3—C41.375 (3)C1B1—C1G11.521 (3)
C3—H20.91 (3)C1B1—H1B20.97 (2)
C4—N31.331 (3)C1B1—H1B11.03 (2)
C4—H31.00 (3)C1G1—C1D1.525 (4)
N3—C61.329 (3)C1G1—H1G10.98 (3)
C6—C01.514 (3)C1G1—H1G20.90 (3)
C0—O0'1.221 (2)C1D—C1G21.505 (4)
C0—N11.336 (3)C1D—H1D21.01 (3)
N1—C1A1.478 (2)C1D—H1D11.00 (3)
N1—H1N0.79 (2)C1B2—C1G21.523 (3)
C1A—C1B11.532 (3)C1B2—H1B41.00 (2)
C1A—C1B1.539 (2)C1B2—H1B30.97 (3)
C1A—C1B21.540 (3)C1G2—H1G40.97 (3)
C1B—C1'1.509 (3)C1G2—H1G31.01 (3)
C1B—H1B50.91 (3)
N2—C1—C6121.03 (19)O1—C1'—C1B126.5 (2)
N2—C1—H1117.5 (14)O2—C1'—C1B112.51 (18)
C6—C1—H1121.3 (14)C1'—O2—H21106 (3)
C3—N2—C1116.57 (18)C1G1—C1B1—C1A112.85 (17)
N2—C3—C4122.0 (2)C1G1—C1B1—H1B2113.6 (14)
N2—C3—H2118.2 (16)C1A—C1B1—H1B2109.0 (14)
C4—C3—H2119.8 (16)C1G1—C1B1—H1B1108.9 (12)
N3—C4—C3121.9 (2)C1A—C1B1—H1B1104.3 (12)
N3—C4—H3117.3 (15)H1B2—C1B1—H1B1107.7 (18)
C3—C4—H3120.8 (15)C1B1—C1G1—C1D110.9 (2)
C6—N3—C4116.44 (19)C1B1—C1G1—H1G1110.5 (15)
N3—C6—C1122.0 (2)C1D—C1G1—H1G1111.0 (15)
N3—C6—C0118.85 (18)C1B1—C1G1—H1G2112.3 (19)
C1—C6—C0119.08 (17)C1D—C1G1—H1G2111.1 (19)
O0'—C0—N1126.08 (19)H1G1—C1G1—H1G2101 (2)
O0'—C0—C6119.79 (18)C1G2—C1D—C1G1111.1 (2)
N1—C0—C6114.12 (17)C1G2—C1D—H1D2106.1 (16)
C0—N1—C1A126.55 (16)C1G1—C1D—H1D2111.2 (16)
C0—N1—H1N115.3 (17)C1G2—C1D—H1D1107.4 (17)
C1A—N1—H1N116.8 (17)C1G1—C1D—H1D1113.3 (18)
N1—C1A—C1B1111.06 (16)H1D2—C1D—H1D1107 (2)
N1—C1A—C1B109.15 (15)C1G2—C1B2—C1A113.00 (17)
C1B1—C1A—C1B108.90 (15)C1G2—C1B2—H1B4110.6 (13)
N1—C1A—C1B2106.91 (15)C1A—C1B2—H1B4109.3 (13)
C1B1—C1A—C1B2108.81 (16)C1G2—C1B2—H1B3110.4 (15)
C1B—C1A—C1B2112.02 (16)C1A—C1B2—H1B3102.9 (15)
C1'—C1B—C1A115.79 (16)H1B4—C1B2—H1B3110 (2)
C1'—C1B—H1B5106.5 (16)C1D—C1G2—C1B2112.6 (2)
C1A—C1B—H1B5108.4 (17)C1D—C1G2—H1G4109.7 (17)
C1'—C1B—H1B6108.0 (15)C1B2—C1G2—H1G4107.0 (16)
C1A—C1B—H1B6107.2 (15)C1D—C1G2—H1G3111.0 (17)
H1B5—C1B—H1B6111 (2)C1B2—C1G2—H1G3109.4 (16)
O1—C1'—O2121.0 (2)H1G4—C1G2—H1G3107 (2)
C6—C1—N2—C3−1.5 (3)C0—N1—C1A—C1B2−173.2 (2)
C1—N2—C3—C40.8 (3)N1—C1A—C1B—C1'55.0 (2)
N2—C3—C4—N30.3 (4)C1B1—C1A—C1B—C1'176.42 (19)
C3—C4—N3—C6−0.6 (3)C1B2—C1A—C1B—C1'−63.2 (2)
C4—N3—C6—C1−0.2 (3)C1A—C1B—C1'—O123.6 (4)
C4—N3—C6—C0177.82 (19)C1A—C1B—C1'—O2−157.9 (2)
N2—C1—C6—N31.3 (3)N1—C1A—C1B1—C1G1−62.6 (2)
N2—C1—C6—C0−176.68 (19)C1B—C1A—C1B1—C1G1177.20 (19)
N3—C6—C0—O0'162.9 (2)C1B2—C1A—C1B1—C1G154.8 (2)
C1—C6—C0—O0'−19.1 (3)C1A—C1B1—C1G1—C1D−57.0 (3)
N3—C6—C0—N1−18.5 (3)C1B1—C1G1—C1D—C1G255.1 (3)
C1—C6—C0—N1159.53 (19)N1—C1A—C1B2—C1G267.3 (2)
O0'—C0—N1—C1A8.4 (4)C1B1—C1A—C1B2—C1G2−52.7 (2)
C6—C0—N1—C1A−170.04 (17)C1B—C1A—C1B2—C1G2−173.13 (18)
C0—N1—C1A—C1B1−54.6 (2)C1G1—C1D—C1G2—C1B2−53.8 (3)
C0—N1—C1A—C1B65.5 (2)C1A—C1B2—C1G2—C1D53.7 (3)
D—H···AD—HH···AD···AD—H···A
O2—H21···N2i0.93 (4)1.86 (4)2.791 (3)177 (4)
N1—H1N···N30.79 (2)2.34 (2)2.729 (2)111.3 (19)
  9 in total

Review 1.  beta-Peptides: from structure to function.

Authors:  R P Cheng; S H Gellman; W F DeGrado
Journal:  Chem Rev       Date:  2001-10       Impact factor: 60.622

2.  A short history of SHELX.

Authors:  George M Sheldrick
Journal:  Acta Crystallogr A       Date:  2007-12-21       Impact factor: 2.290

3.  Efficient and facile formation of two-component nanoparticles via aromatic moiety directed self-assembly.

Authors:  Weiping Wang; Ying Chau
Journal:  Chem Commun (Camb)       Date:  2011-04-26       Impact factor: 6.222

Review 4.  Aromatic-aromatic interaction: a mechanism of protein structure stabilization.

Authors:  S K Burley; G A Petsko
Journal:  Science       Date:  1985-07-05       Impact factor: 47.728

5.  Aromatic interactions in tryptophan-containing peptides: crystal structures of model tryptophan peptides and phenylalanine analogs.

Authors:  A Sengupta; R Mahalakshmi; N Shamala; P Balaram
Journal:  J Pept Res       Date:  2005-01

6.  Conformations of beta-amino acid residues in peptides: X-ray diffraction studies of peptides containing the achiral residue 1-aminocyclohexaneacetic acid, beta3,3Ac6c.

Authors:  Prema G Vasudev; Rajkishor Rai; Narayanaswamy Shamala; Padmanabhan Balaram
Journal:  Biopolymers       Date:  2008       Impact factor: 2.505

7.  Biological and conformational studies of [Val4]morphiceptin and [D-Val4]morphiceptin analogs incorporating cis-2-aminocyclopentane carboxylic acid as a peptidomimetic for proline.

Authors:  T Yamazaki; A Pröbsti; P W Schiller; M Goodman
Journal:  Int J Pept Protein Res       Date:  1991-05

8.  Cyclic hexapeptide analogs of somatostatin containing bridge modifications. Syntheses and conformational analyses.

Authors:  Z Huang; A Pröbstl; J R Spencer; T Yamazaki; M Goodman
Journal:  Int J Pept Protein Res       Date:  1993-10

9.  Structure validation in chemical crystallography.

Authors:  Anthony L Spek
Journal:  Acta Crystallogr D Biol Crystallogr       Date:  2009-01-20
  9 in total

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