The square conformation of phenylglycine-incorporated ascidiacyclamide is stabilized by CH/π interactions between amino acid side chains

Electronic substituent effect on the conformation of a phenylalanine-incorporated cyclic peptide

The Phe-incorporated cyclic peptide [cyclo(-Phe1-oxazoline2-d-Val3-thiazole4-Ile5-oxazoline6-d-Val7-thiazole8-)] is in a conformational equilibrium between square and folded forms in solution. In the folded form, a CH⋯π interaction between the Phe1 aromatic ring and the Oxz2 methyl group is observed. We endeavored to control the local conformation and thus modulate the CH⋯π interaction and flexibility of the Phe1 side chain by controlling the electronic substituent effects at the 4-position of the aromatic ring of the Phe1 residue. The effect of the 4-substituent on the global conformation was indicated by the linear relationship between the conformational free energies (ΔGo) determined through NMR-based quantification and the Hammett constants (σ). Electron-donating substituents, which had relatively strong CH⋯π interactions, promoted peptide folding by restraining the loss in entropy. Local control by the 4-substituent effects suggested that the Phe side chain exerts an entropic influence on the folding of these cyclic peptides.

Experimental evidence for CH⋯π interaction-mediated stabilization of the square form in phenylglycine-incorporated ascidiacyclamide

Ascidiacyclamide [cyclo(-Ile-oxazoline-D-Val-thiazole-)₂] is a cytotoxic cyclic peptide from ascidian. We examined the potential of the CH⋯π interaction at the diagonal position of ascidiacyclamide by comparing the interactions of Ile, Val, Abu (2-aminobutyric acid) or Ala with Ile, Chg (cyclohexylglycine) or Phg (phenylglycine). In solution, ascidiacyclamides are in a conformational equilibrium between square and folded forms. The CH⋯π interaction is expected to contribute to stabilization of the square form, which enhances the peptides' cytotoxicity. The distances between the alkyl side chain of Xaa and the π-plane of Phg were estimated from the crystal structures. The conformational free energies (ΔG°) determined through NMR-based quantitation indicated remarkable stabilization of the square form upon incorporation of Phg. These observations were consistent with the circular dichroism (CD) spectral measurements. Chemical shift perturbation studies suggested that stabilization of the square form of Phg-incorporated peptides was due to the CH⋯π interaction with the alkyl side chain of Xaa. Greater enthalpic losses were caused during the folding process of Phg-incorporated peptides than Ile- or Chg-incorporated peptides. It is suggested that these enthalpic losses are relevant to the CH⋯π interaction energies, which must be disrupted during folding. In addition, the CH⋯π interactions in the Phg-incorporated peptides increased cytotoxicity.

Effect of the powerful plasticity of the tert-butyl side chain on the conformational equilibrium of ascidiacyclamides

Ascidiacyclamide [cyclo(-Ile^1,5 -oxazoline^2,6 -d-Val^3,7 -thiazole^4,8 -)₂ ] is a cytotoxic cyclic peptide from ascidian. Through structural analyses using monosubstituted analogues (Xaa¹ : Ala, 2-aminobutyric acid, Val, cyclohexylglycine, and phenylglycine), we previously demonstrated the conformational equilibrium between its square and folded forms. As the bulkiness of the Xaa¹ residue side chain was reduced, spontaneous folding was promoted, and the cytotoxicity decreased accordingly. In the present study, five disubstituted analogues in which a tert-leucine residue (Tle) was incorporated at the 5-position of the abovementioned monosubstituted analogues were synthesized, after which their structures were analyzed using X-ray diffraction, circular dichroism (CD) spectral measurements, and ¹ H NMR-based quantitative analysis. The side chains of the Tle and Ile residues are structural isomers of one another, and the Tle residue bearing the tert-butyl group can be expected to play a role as a building block. In fact, peptides incorporating Tle⁵ exhibited much less spontaneous folding than their Ile⁵ counterparts in both crystal and solution. Increases in enthalpy and entropy due to the tert-butyl group during the folding process resulted in increased conformational free energy (ΔG°). The powerful plasticity of the tert-butyl group would stabilize the square form relating with cytotoxicity.

NMR-based quantitative studies of the conformational equilibrium between their square and folded forms of ascidiacyclamide and its analogues

Ascidiacyclamide [cyclo(-Ile^1,5-oxazoline^2,6-D-Val^3,7-thiazole^4,8-)] (1) is a cytotoxic cyclic peptide from the ascidian, or sea squirt. Through structural analyses using asymmetric analogues [Xxx¹: Ala (2), Val (3), Leu (4), Phe (5), cyclohexylalanine (6) and phenylglycine (7)], we previously showed 1 to exist in a conformational equilibrium between square and folded forms. In the present study, five new asymmetric analogues [Xxx¹: 2-aminobutyric acid (8), 2-aminopentyric acid (9), tert-butylalanine (10), cyclohexylglycine (11) and tert-leucine (12)] were synthesized, and their structures were analyzed with X-ray diffraction and CD spectral measurements. Variable temperature ¹H NMR measurements were performed to determine their equilibrium constants and their thermodynamic parameters. The use of two reference peptides made these quantitative studies possible. T3ASC, which contains three thiazole rings as a result of replacing oxazoline² with thiazole, and dASC, in which the two oxazoline rings were deleted, were respectively used as square and folded reference peptides. The estimated parameters enabled more detailed discussion of the relationship between the bulkiness of substituents and the conformational free energies (ΔG°) of the peptides as well as the relationship between structure and cytotoxicity. The ΔG° values for peptides 1, 2, 3, 8, 9 and 11 decreased with decreases in the bulkiness of their substituents. We suggest that spontaneous folding is promoted as the bulkiness of substituents decreases. Peptides 7 and 12, which have large positive ΔG° values independently of temperature, did not exhibit spontaneous folding at any temperature; that is, their conformations were very stable in the square form. Peptides 4, 5, 6 and 10 had negative ΔG° values, despite their bulky substituents. Peptides with a positive ΔG° value showed cytotoxicity, and peptides with a negative ΔG° value showed reduced or no cytotoxicity. However, peptides 5 and 6 showed cytotoxicity equal to or stronger than 1. Those ten peptides except for 5 and 6 showed a clear structure–cytotoxicity relationship based on ΔG° values.

The estimated thermodynamic parameters by NMR-based experiments allowed for a detailed discussion of the conformational equilibrium of ascidiacyclamide.

Incorporation of β-amino acids into ascidiacyclamides: Effects on conformation, cytotoxicity and interaction with copper (II) ion

Seven ascidiacyclamide [cyclo(-Ile-oxazoline-d-Val-thiazole-)₂ ] (ASC) analogues incorporating the β-amino acids βIle, βoxazoline, and/or d-βVal were synthesized. We then investigated the effects of the position and number of incorporated β-amino acids on the structure, cytotoxicity, and copper binding by these seven analogues. The structural analyses revealed that both βIle and d-βVal favor a gauche-type θ torsion angles, while βoxazoline favors a trans-type θ torsion angle. Expansion of the macrocycle by incorporation of βIle or d-βVal readily induced molecular folding. On the other hand, the incorporation of two βoxazoline residues strongly extended the peptide conformation, and the incorporation of one was sufficient for the moderate restriction important for conformational equilibrium and cytotoxicity. Despite expansion of the macrocycles, the structure-cytotoxicity relationships were largely maintained. In studies of complexation of the analogues with Cu (II) ion, the position and number of incorporated β-amino acids had a large impact on the structure of the metal complex and may contribute to its stabilization.

A bis-copper(II)-[D-βVal3,7]ascidiacyclamide complex enveloping two square pyramids and sharing an apex atom from a carbonate anion

The four azole rings place structural restrictions on ascidiacyclamide (ASC). As a result, the structure of ASC exists in an equilibrium between two major forms (i.e. folded and square). [D-βVal^3,7]Ascidiacyclamide (βASC) was synthesized by replacing two D-Val-Thz (Val is valine and Thz is thiazole) blocks with D-β-Valine (D-βVal-Thz). This modification expands the peptide ring; the original 24-membered macrocycle of ASC becomes a 26-membered ring. Circular dichroism (CD) spectra showed that, in solution, the structural equilibrium is maintained with βASC, but the folded form is dominant. A copper complex was prepared, namely [[D-βVal^3,7]ascidiacyclamide(2-)]aqua-μ-carbonato-dicopper(II) monohydrate, [Cu₂(C₃₈H₅₄N₈O₆S₂)(CO₃)(H₂O)]·H₂O, to determine the effect of the change in ring size on the coordinated structure. The obtained bis-Cu^II-βASC complex contains two water molecules and a carbonate anion. Two Cu^II ions are chelated by three N-donor atoms of two Thz-Ile-Oxz (Ile is isoleucine and Oxz is oxazoline) units. An O atom of the carbonate anion bridges two Cu^II ions, forming two square pyramids. These features are similar to the previously reported structure of the Cu^II-ASC complex, but the two pyramids are enveloped inside the peptide and share one apex. In the Cu^II-ASC complex, the apex of each square pyramid is an O atom of a water molecule, and the two pyramids are oriented toward the outside of the peptide. The incorporated β-amino acids of βASC make the space inside the peptide large enough to envelop the two square pyramids. The observed structural changes in the bis-Cu^II-βASC complex arising from ring expansion are particularly interesting in the context of the previously reported structure of the Cu^II-ASC complex.

Ascidiacyclamides containing oxazoline and thiazole motifs assume square conformations and show high cytotoxicity

Four cyclic octapeptides were designed from ascidiacyclamide [cyclo(-Ile-Oxz-D-Val- Thz-)₂ ] (ASC, 1) to investigate the effects of oxazoline (Oxz) and thiazole (Thz) rings on the structures and cytotoxicities of the peptides. cyclo(-Ile-Thz-D-Val-Oxz-)₂ (2) had the same number of Oxz and Thz rings as ASC, but the ring positions were switched. cyclo(-Ile-Oxz-D-Val-Thz-Ile-Thz-D-Val-Thz-) (3) and cyclo(-Ile-Thz-D-Val-Oxz-Ile-Thz-D-Val-Thz-) (4) contained one Oxz and three Thz rings within the molecule. All Oxz rings were substituted with Thz in cyclo(-Ile-Thz-D-Val-Thz-)₂ (5). These analogues had new Oxz and Thz blocks forming the 24-membered ring. Based on CD spectra and X-ray diffraction analyses, the structures of all four analogues were classified as square ASC forms. But the structures of 2 and 5 differed from the original square form of 1, and they showed no cytotoxicity. The structure of 3 was very similar to that of 1, and 3 showed 10 times greater cytotoxicity than 1. Although no definite structure of 4 was obtained, it showed three times greater cytotoxicity than 1. It appears that the position and number of Oxz residues are essential determinants in the structure-cytotoxicity relationship of ASC analogues.