Parallel packing of α-helices in crystals of the zervamicin IIA analog Boc-Trp-Ile-Ala-Aib-Ile-Val-Aib-Leu-Aib-Pro-OMe.2H₂0

Abstract

An apolar synthetic analog of the first 10 residues at the NH₂-terminal end of zervamicin HA crystallizes in the triclinic space group P1 with cell dimensions a = 10.206 ± 0.002 A, b = 12.244 ± 0.002 A, c = 15.049 ± 0.002 A, α = 93.94 ± 0.01°, β = 95.10 ± 0.01°, γ = 104.56 ± 0.01°, Z = 1, C₆₀H₉₇N₁₁0₃.2H₂O. Despite the relatively few a-aminoisobutyric acid residues, the peptide maintains a helical form. The first intrahelical hydrogen bond is of the 3₁₀ type between N(3) and 0(0), followed by five α-helix-type hydrogen bonds. Solution ¹H NMR studies in chloroform also favor a helical conformation, with seven solvent-shielded NH groups. Continuous columns are formed by head-to-tail hydrogen bonds between the helical molecules along the helix axis. The absence of polar side chains precludes any lateral hydrogen bonds. Since the peptide crystallizes with one molecule in a trilinic space group, aggregation of the helical columns must necessarily be parallel rather than antiparallel. The packdng of the columns is rather inefficient, as indicated by very few good van der Waals' contacts and the occurrence of voids between the molecules.