Structures and solution conformational dynamics of stylissamides G and H from the Bahamian sponge Stylissa caribica.

Two new peptides, stylissamides G and H, were isolated from extracts of a sample of Stylissa caribica collected in deep waters of the Caribbean Sea. A single sample of S. caribica among a collection of 10 samples that were examined by LC-MS appeared to be a different chemotype from the others in that it lacked the familiar pyrrole-2-aminoimidazole alkaloids, stevensine and oroidin, and contained peptides of the stylissamide class. The structures of the title compounds were solved by integrated analysis of the MS and NMR spectra and chemical degradation. The solution conformation of stylissamide G was briefly examined by electronic circular dichroism and temperature-dependent (1)H NMR chemical shifts of amide NH signals, which supported a conformationally rigid macrocycle.

The marine sponge Stylissa caribica Lehnert & van Soest, 1998 is found at depths in excess of −25 m in the Caribbean Sea. The cyclic heptapeptides stylissamides A–F (1a–d,[1]e, f;[2,3] see Supporting Information for structures) have recently been described from S. caribica from the Bahamas. Many related peptides exhibit biological activity; for example, stylissamide ‘X’, from an unidentified Stylissa species from Indonesia, inhibited EGF-induced migration of cultured HeLa cells.[4] The cyclic peptides are minor components of these sponges with the major secondary metabolites being, largely, brominated pyrrole 2-aminoimidazole alkaloids[5] (PAIs) such as oroidin (2a),[6,7] hymenidin (2b),[8] and stevensine (3)[9] and their higher-order oligomeric congeners.

Recently, we demonstrated de novo biosynthesis of 15N-labeled PAIs (nagelamide H and benzosceptrin C) from 15N-2a,[10] providing the first experimental evidence that oroidin is a biosynthetic precursor of higher-order PAIs and participates in single-electron C–C bond forming reactions catalyzed by a metallo-enzyme (or enzymes) with molecular oxygen as the terminal oxidant.[11] The provenance and biosynthetic origins of the stylissamides, however, are less certain.

Results and Discussion

The secondary metabolite profile of S. caribica has been investigated previously by HPLC.[12] In our screening for antifungal and cancer cell-inhibitory marine secondary metabolites, we examined several samples (N = 10) of S. caribica collected from the Bahamas in 2007 and 2008. Most showed in vitro antifungal activity against fluconazole-resistant Candida albicans, Can. krusei, Cryptococcus neoformans var. grubbii, and Cryp. neoformans var. gatti. A single sample of S. caribica (08-045Q) collected from Sweetings Cay, with a reversed-phase LCMS profile different from other samples, lacked antifungal activity, but showed significant cytotoxic activity toward cultured human colon tumor cells (HCT-116). While mainly 3 and smaller amounts of 2a were evident in most samples of S. caribica,[11] these two compounds were not detected in 08-045Q (LCMS analysis). Instead, the latter sample contained hymenidin (2b) as the major PAI along with late-eluting components that lacked Br and with m/z values in the range expected for stylissamides. Sequential solvent partitioning of a MeOH extract of the sample separated the constituents by polarity, to give an n-BuOH-soluble fraction that was further purified by preparative HPLC to provide a cytotoxic fraction (IC50 = 5.5 μg·mL–1). Rechromatography of the latter gave pure samples of two new heptapeptides, stylissamides G (1g) and H (1h).

The molecular formula of 1g, C45H61N7O7, was established from HRESIMS data. The high N content of 1g and preliminary analysis of 1H NMR data (Table 1) suggested a heptapeptide; this was further supported by the presence of cross-peaks in the HSQC spectrum consistent with seven α-CH signals (δ 51.6–61.6 ppm) and seven amide carbonyl signals (δ 170.6–171.7 ppm). Analysis of COSY and HMBC spectra of 1g provided assignments of spin systems due to one each of Leu and Ile, two Phe, and three Pro residues (Table 1). All three Pro residues could be assigned cis conformations based on empirical rules[13] that correlate with 13C NMR chemical shifts (δ Cγ < 23.3 ppm) and differences between the β and γ carbons [Δδ(Cβ–Cγ) > 8.0 ppm].

Table 1

1H and 13C NMRa Data for 1g (DMSO-d6, 600 MHz)

	position	13C, type	1H, mult (J in Hz)
Ile	C=O	171.7, C
	α	53.1, CH	4.26, t (9.3)
	β	37.1, CH	1.59, m
	βCH3	14.5, CH3	0.75, m
	γ	23.8, CH2	1.98, m
			1.45, m
	δCH3	10.6, CH3	0.77, t (7.3)
	NH		7.88, d (9.3)
Pro1	C=O	169.6, C
	α	61.0, CH	4.34, brd (9.1)
	β	31.0, CH2	2.00, m
			2.18, m
	γ	21.3, CH2	1.32, m
			1.72, m
	δ	45.6, CH2	3.31, m
Phe1	C=O	167.7, C
	α	51.8, CH	4.37, m
	β	35.2, CH2	3.36, m
			2.98, dd (13.6, 0.6)
	Ph	129.6 (o)	6.91, d (6.4)
		127.7 (m), CH	7.22, m
		126.4 (p), CH	7.21, m
		135.5, C
	NH		6.55 brs
Pro2	C=O	171.3, C
	α	57.1, CH	4.61, d (8.0)
	β	30.4, CH2	2.16, m
			1.92, m
	γ	21.1, CH2	1.89, m
			1.81, m
	δ	46.7, CH2	3.52, m
			3.31, m
Phe2	C=O	168.8, C
	α	52.9, CH	4.41, ddd (9.2, 6.4, 1.8)
	β	36.0, CH2	3.09, dd (13.5, 6.4)
			2.95, dd (13.5, 9.2)
	Ph	128.9 (o), CH	7.28, m
		128.5 (m), CH	7.37, t (7.6)
		126.7 (p), CH	7.28, m
		135.7, C
	NH		8.94, s
Pro3	C=O	169.9, C
	α	60.1, CH	3.27, m
	β	30.0, CH2	1.89, m
			1.04, m
	γ	21.4, CH2	1.62, m
			1.27, m
	δ	45.8, CH2	3.27, m
			3.18, t (9.8)
Leu	C=O	170.6, C
	α	51.1, CH	4.06, m
	β	38.0, CH2	1.52, m
			1.10, m
	γ	24.5, CH	1.51, m
	δ	20.3, CH3	0.73, d (6.3)
	δ′	23.1, CH3	0.81, d (6.4)
	NH		8.83, d (7.1)

See structures of 1g and 1h for the amino acid residue key. 13C NMR shifts from indirect 2D NMR (HSQC, HMBC, and bsctHMBC).

Sequence assignments of 1g were obtained by analysis of HMBC and NOESY data. A band-selective constant-time HMBC[14] experiment (ΔδC 161–181 ppm, optimized for J = 8 Hz) showed correlations from δ 4.37 (Phe1Hα) to δ 169.9 (Pro1C=O), δ 4.34 (Pro1Hα) to δ 171.7 (IleC=O), δ 7.88 (IleNH) and δ 4.26 (IleHα) to δ 170.6 (LeuC=O), δ 8.83 (LeuHα) to δ 169.9 (Pro3C=O), δ 3.27 (Pro3Hα) to δ 168.8 (Phe2C=O), and δ 8.94 (Phe2NH) to δ 171.4 (Pro2C=O). The foregoing data were sufficient to define the sequence Pro1-Phe1-Pro2-Phe2-Pro3-Leu-Ile. Support for this sequence was also obtained from NOESY data. Cross-peaks in the NOESY spectrum of 1g were observed between the following pairs of amino acid residue NH and α-CH NMR signals: Phe2NH/Pro2Hα, Pro2Hα/Phe1Hα, Pro1Hκ/IleHα, IleNH/LeuHα, LeuNH/Pro3Hα, and Pro3Hα/Phe2Hα (Figure 1).

Figure 1

Selected NOESY and HMBC correlations for stylissamide G (1g).

Stylissamide G (1g) is a variant on the heptapeptide motif detected in cyclic peptides, from not only Stylissa (family Dictyonellidae) but other sponges in the family Axinellidae. Cytotoxic phakellistatins constitute a family of over a dozen peptides characterized from different samples of Phakellia spp. from the Indian Ocean,[15] Western Pacific,[16] and the South China Sea[17] mostly with the typical heptapeptide motif of stylissamides. In terms of sequence and amino acid composition, peptide 1g most closely resembles phakellastatin-2 (4)[15] and stylisins-1 (5a) and -2 (5b) from S. caribica collected in Jamaica.[18]

The molecular formula of 1h, C44H58N8O8, was established from HRESIMS data. The 1H NMR spectrum of cyclopeptide 1h exhibited conformational heterogeneity in several solvents (CDCl3, DMSO-d6, and CD3OD) that significantly complicated interpretation and assignment. Similar solvent-dependent complexity has been observed in the NMR spectra of 4.[19] A relatively well-resolved 1H NMR spectrum of 1h was obtained in CD2Cl2, and analysis of the spin systems by proton, COSY, HMQC, and HMBC NMR (Table 2) showed the presence of Trp, Ser, Ile, Val, Phe, and two Pro residues. Pro1 was assigned the cis conformation, while Pro2 was assigned as trans according to empirical rules. Attempts to sequence 1h by HMBC and NOESY failed due to relatively weak carbonyl signals dispersed between multiple conformers. Alternative MS4 analysis also failed to deliver interpretable fragment ions. Gratifyingly, partial hydrolysis of 1h (2 M HCl in CH3CN–H2O, 60 °C, 90 min) gave a linear peptide from single amide bond hydrolysis that was identified as Pro1-Trp-Pro2-Ile-Ser-Phe-Val by LC/MS/MS (Figure 2). The composition and amino acid sequence of 1h are identical to those of euryjanicin A (6),[20] which was isolated by Rodriguez and co-workers from the Caribbean sponge Prosuberites laughlini (Díaz, Alvarez & Van Soest, 1987). However, peptides 1h and 6 are configurational isomers: Pro2 is in the trans configuration in 1h, while both proline residues in 6 adopt the cis configuration. No interconversion of 1h into 6 was observed upon heating the former in DMSO (70 °C, 30 min), suggesting that 1h is the thermodynamically favored isomer.

Table 2

1H and 13C NMRa Data for 1h (CD2Cl2, 600 MHz)

	position	13C, δ, type	1H δ, mult (J in Hz)
Ile	C=O	168.9, C
	α	59.7, CH	3.88, t (5.1)
	β	35.8, CH	1.39, m
	βCH3	15.5, CH3	0.77, d (6.9)
	γ	25.3, CH3	1.16, m
	δ	11.3, CH3	0.63, t (7.4)
	NH		5.63, d (4.8)
Ser	C=O	171.9, C
	α	54.2, CH	4.75, d (9.9)
	β	63.22, CH2	4.64, d (11.8)
			3.42, d (11.8)
	NH		7.21, m
Phe	C=O	171.7, C
	α	63.2, CH	3.79, dd (15.5, 7.9)
	β	36.1, CH2	3.67, m
			3.53, dd (14.0, 7.9)
	NH		8.36, s
Val	C=O	N.D.
	α	56.0, CH	4.30, m
	β	33.0, CH	1.86, m
	γ	17.1, CH3	0.87, d (6.9)
	γ′	20.1, CH3	0.84, d (6.9)
	NH		8.54, d (7.7)
Pro1	C=O	N.D.
	α	61.2, CH	4.29, m
	β	30.8, CH2	2.02, m
			1.95, m
	γ	20.9, CH2	1.57, m
			0.99, m
	δ	46.5, CH2	3.35, m
			3.34, m
Trp	C=O	N.D.
	α	50.0, CH	5.23, m
	β	27.8, CH2	3.29, m
			3.11, dd (15.7, 7.5)
	NH		6.80, d (9.0)
Pro2	C=O	172.9, C
	α	63.1, CH	4.26, t (8.3)
	β	29.7, CH2	2.42, m
			1.86, m
	γ	25.4, CH2	2.07, m
			1.84, m
	δ	47.9, CH2	3.93, m
			3.67, m

See structures of 1g and 1h for the amino acid residue key. 13C NMR chemical shift assignments from indirect 2D NMR (HSQC, HMBC, and bsctHMBC.

Figure 2

LC/MS/MS analysis of the partial hydrolysis product of stylissamide H (1h).

The amino acid residues in all Stylissa peptides reported to date are of the l-configuration. Pure samples of 1g and 1h were separately hydrolyzed (6 M HCl, 95 °C, 15 h), and, after removal of volatiles, the residues were subjected to Marfey’s analysis.[21] The configurations of all amino acid residues in both peptides (except Trp in 1h, which decomposed during hydrolysis) were also found to be l.[22]

Natural product cyclic heptapeptides and octapeptides are presumed to adopt rigid, conformationally constrained structures ordered by minimized torsional strain, especially those imposed by cis and trans Pro residues. Aside from computational modeling studies that determined the minimized energy structures,[18] no experimental data have been reported on the conformational dynamics of the stylissamides. We briefly examined the solution conformational dynamics of 1g and 1h by temperature-dependent CD spectra (Figure 4) and amide signal (NH) 1H NMR chemical shifts (Table 3).

Figure 4

Temperature-dependent CD spectra (CH3CN): (a) stylissamide G (1g) and (b) stylissamide H (1h).

Table 3

Temperature Coefficients (Δδ/ΔT) of Amide NH 1H NMR Signals (δ, parts per billion) for 1g

	T (K)
	298	323	343
	δ (ppm, N–H)			Δδ/ΔT (ppb/K)
Ile	7.88	7.73	7.61	–6.0
Leu	8.83	8.80	8.76	–1.5
Phe1	6.55	6.58	6.61	1.3
Phe2	8.94	8.82	8.72	–4.9

At room temperature (23 °C), the CD spectrum of 1g (CH3CN, Figure 3) was characterized by two Cotton effects (CEs) (λ 196 (Δε +23.2), 223 (Δε −7.2) nm), while that of 1h showed more complex band structure due to the presence of Trp. At elevated temperatures (50 and 70 °C, Figure 4), little difference was observed in the CD spectra of 1g and 1h, although at lower temperature (0 °C) moderate increases in the magnitudes of both CEs of 1g were observed.

Figure 3

CD and UV spectra (CH3CN, 23 °C): (a) stylissamide G (1g) and (b) stylissamide H (1h).

In their study of temperature-dependent 1H NMR amide NH signals and exchange rates in proteins, Baxter and Williamson concluded that temperature coefficients (Δδ/ΔT) greater than −4.5 ppb/K were correlated with hydrogen-bonded amide protons, while NH protons with values less than −4.5 ppb/K were not hydrogen-bonded.[23] Measurement of temperature-dependent 1H NMR amide NH chemical shifts of 1g (T = 25, 50, 70 °C) gives Δδ/ΔT coefficients (Table 3) that are greater than −4.5 ppb/K for Phe1 and Leu, but lower than −4.5 ppb/K for Phe2 and Ile, showing that NH of Phe1 and Leu are strongly hydrogen bonded. These temperature coefficients support a generally tightly constrained amide backbone for 1g with weaker hydrogen bonding at the Phe2 and Ile residues. Taking both CD and Δδ/ΔT measurements into consideration, the data support conformations of 1g and 1h with relatively rigid amide backbones, constrained by intramolecular hydrogen bonds, but higher mobility of the Ile and Phe residues.

Pettit and others have reported potent cytotoxicity of cyclic heptapeptides related to 1g and 1h, for example, phakellistatin-2,[15] related phakellistatins,[16] and hymenistatins G, H, and J.[17] Kobayashi and co-workers showed that stylissamide ‘X’ inhibited EGF-induced migration of HeLa cells in the concentration range 0.1 to 10 μM.[4] In the present work, the fraction of S. caribica extract containing stylissamides 1g and 1h was cytotoxic to HCT-116 cells. Peptide 1h exhibited modest in vitro cytotoxicity against HCT-116 (EC50 5.7 μM) while 1g was essentially inactive (EC50 >200 μM).

In conclusion, two new heptapeptides, stylissamides G (1g) and H (1h), were isolated from a cytotoxic fraction of the sponge S. caribica from the Bahamas and characterized by integrated spectroscopic methods. Studies of the solution dynamics of 1g and 1h by temperature-dependent CD and amide NH 1H NMR temperature coefficients suggested peptide conformations ordered by a relatively rigid, hydrogen-bonded backbone structure with limited mobility.

Experimental Section

General Experimental Procedures

UV–vis spectra were recorded on a Jasco UV V630 dual beam instrument in 1 cm quartz cells. CD spectra were recorded on a Jasco 810 spectropolarimeter in quartz cells (1 or 2 mm path length). 1H NMR and 2D NMR spectra of submilligram samples were measured on a Bruker 600 NMR spectrometer under control of an Avance III console and equipped with a 1.7 mm {13C/15N}1H microcryoprobe, operating at 599.5556 MHz. Additional 1H and 13C NMR spectra were measured on a Jeol ECA 500 spectrometer operating at 500.1599 and 125.6940 MHz, respectively, with samples dissolved in CD3OD or DMSO-d6 (99.8 atom % D, Cambridge Isotopes). Low-resolution MS spectra were recorded on a ThermoFisher MSQ Plus Surveyor single quadrupole mass spectrometer, operating in positive ion ESI mode, coupled to a Thermo Fisher Accela ultra-high-performance liquid chromatograph (UHPLC). HRMS measurements were made using an Agilent 6230 TOFMS under positive ion ESI TOFMS conditions and provided by the UCSD Small Molecule MS Facility and the Torrey Pines Scripps Research Institute, La Jolla, California. Semipreparative HPLC was carried out using an Agilent 1100 HPLC under specified columns and gradient conditions.

Animal Material, Extraction, and Isolation

A sample of Stylissa caribica (accession 08-08-045) was collected at Sweetings Cay, Bahamas (49.646′ S, 77°54.054′ W) using scuba from a depth of −27 m in June 2008 and kept frozen (−20 °C) until required. The frozen sponge (wet wt 107.1 g) was cut into ∼3 cm cubes and extracted with 1:1 CH2Cl2–MeOH with stirring (rt, overnight, 2 × 500 mL). Following removal of the solvent under reduced pressure, the residue was partitioned between hexanes (3 × 250 mL) and 9:1 MeOH–H2O (250 mL). The aqueous MeOH layer was concentrated and further partitioned between n-BuOH (3 × 250 mL) and H2O (250 mL). Removal of the volatile solvent from the upper layer gave a brown semisolid (1.68 g), a portion of which was adsorbed onto a solid-phase extraction cartridge (C18) and eluted with MeOH. The MeOH eluate was concentrated under reduced pressure to give a solid (870 mg), which was separated by semipreparative reversed-phase HPLC (C18, gradient from 10:90 to 60:40 CH3CN–H2O + 0.1% TFA) to give 10 fractions. A portion (8.6 mg) of the eighth fraction (36.8 mg) was further purified by semipreparative HPLC (C18, gradient 34:8:58 CH3CN–iPrOH–H2O + 0.1% TFA) to give didebromonagelamide A[11] (0.4 mg). The tenth fraction (72.3 mg) was found to be cytotoxic against HCT-116 cells (IC50 = 5.5 μg.mL–1). A portion of the tenth fraction (22 mg) was further purified by reversed-phase HPLC (C18 Phenomenex 250 × 10 mm 5 μm, 100 Å column, 3.0 mL·min–1 under the following gradient profile (A = H2O–0.1% CF3COOH, B = CH3CN; 40% B, t = 0 min; 40% B, t = 5 min; 60% B, t = 20 min) and gave stylissamide H (tR = 12.6 min, 0.50 mg, 1h, 0.015% wet wt), stylissamide G (1g, tR = 15.1 min, 1.6 mg, 0.049% wet wt), and known compounds stylisin-1 (5, tR = 9.78 min), stylissamide A (1a, tR = 18.5 min), and stylissamide C (1c, tR = 18.5 min) as colorless solids. See Supporting Information for complete structures.

Stylissamide G (1g):

colorless solid; UV (CH3CN, 23 °C) λmax (log ε) 258 (2.88), 264 (2.87) nm; CD (0.322 mM, CH3CN, 23 °C) λmax (Δε) 196 (+23.2), 223 (−7.2) nm, see also Figures 3 and 4; 1H and 13C NMR, see Table 1; HRESIMS m/z 812.4704 [M + H]+, 834.4522 [M + Na]+ (calcd for C45H62N7O7+ 812.4705; calcd for C45H61N7NaO7+, 834.4525).

Stylissamide H (1h):

colorless solid; UV (CH3CN, 23 °C) λmax (log ε) 272 (3.39), 289 (3.25) nm; CD (0.189 mM, CH3CN, 23 °C) λmax (Δε) 203 (−3.2), 221 (−7.8) nm, see also Figures 3 and 4; 1H and 13C NMR, see Table 2; HRESIMS m/z 827.4417 [M + H]+ (calcd for C44H59N8O8+ 827.4456).

Band-Selective Constant Time HMBC of Stylissamide G (1g) (ref (14))

A band-selective constant-time HMBC spectrum of a sample of 1g (∼1.0 mg) in CDCl3 was recorded under the following conditions: the band-selective pulse was a G3 Gaussian cascade of 500 μs duration centered at δ 172 ppm. The 13C spectral width was set to 20 ppm, and 1H-detected FIDs (64 t1 increments) were collected at 600 MHz for a total experiment time of 84 min. The delay for selection of long-range coupling was set to J = 8 Hz, and the 2-fold low-pass J filter was optimized to suppress one-bond couplings between J = 120 and 180 Hz.