Design of specific structures using alpha,beta-dehydro-phenylalanine residues: synthesis, crystal structure, and molecular conformation of Boc-L-Val-delta Phe-delta Phe-L-Val-delta Phe-delta Phe-L-Val-OCH3, a 3(10)-helical heptapeptide.

Abstract

The peptide design using alpha,beta-dehydro-residues has wide applications. To design an extensive 3(10)-helical conformation, a heptapeptide Boc-L-Val-delta Phe-delta Phe-L-Val-delta Phe-delta Phe-L-Val-OCH3, with a repeat of two consecutive delta Phe residues has been synthesized using an azlactone method in solution phase. This is the first design using a repeat of two consecutive delta Phe residues. It is observed that the delta Phe in a sequence of two consecutive delta Phe residues, adopts only one set of phi, psi values, i.e., +/- 60 degrees, +/- 30 degrees, thus making it a specific design tool. The peptide crystallized from its solution in a methanol-water mixture in the space group P2(1) with a = 10.159(5)A, b = 20.057(2)A, c = 14.448(3)A, beta = 99.41(2)degrees, V = 2904(2)A3. The structure has been determined by direct methods and refined to an R value of 0.048 for 5404 observed [I > or = 3 sigma(I)] reflections. The structure consists of a heptapeptide Boc-L-Val-delta Phe-delta Phe-L-Val-delta Phe-delta Phe-L-Val-OCH3 and a solvent methanol molecule in the asymmetric unit. All peptide units in the structure are trans. As a result of six overlapping type III beta-turns formed involving seven residues and five intramolecular 4-->1 hydrogen bonds, the peptide adopts a right-handed 3(10)-helical conformation with more than two complete helical turns. It is noteworthy that starting from the Boc group to the C-terminal residue of Val, the 3(10)-helical structure is maintained well. The carbonyl oxygen atom of the Boc group is the first acceptor whereas the carbonyl oxygen atom of Val4 is the last acceptor in the helical structure of the peptide. The side chains of four delta Phe residues in this helical arrangement exist in a slightly staggered arrangement. The solvent methanol molecule interacts through its hydroxyl group and forms two intermolecular hydrogen bonds, one as a donor with a C-terminal CO group of delta Phe6 and second as an acceptor with the NH group of delta Phe2 from the N-terminal region of the peptide. Thus the solvent molecule plays a significant role in promoting a head-to-tail packing of 3(10)-helices of the peptide. There are no lateral hydrogen bonds between the helices, but there exist several van der Waals interactions involving the hydrophobic side chains of peptide molecules.