Molecular design of peptides. Synthesis, molecular structure and beta-turn II' formation of N-Boc-L-Phe-dehydro-Abu-NH-CH3 in crystals.

The peptide N-Boc-L-Phe-dehydro-Abu-NH-CH3 was synthesized by the usual workup procedure. The crystals grown from methanol at 4 degrees C belong to the space group P2(1)2(1)2(1) with a = 7.589(2), b = 13.690(4), c = 21.897(6) A, Z = 4 and dc = 1.149(5) g cm-3 for C19H29N3O5.CH3OH. The peptide crystals were highly sensitive to radiation. The final agreement factor R was 0.055 for 1109 observed reflections (I > or = 2 sigma) with data extending to a 2 theta value of 103 degrees. The methanol oxygen atom is split into two occupancies. Both sites are involved in identical hydrogen bonding. As a result of substitution of a dehydro-Abu residue at the (i + 2) position the peptide adopts an ideal beta-turn II' conformation with torsion angles of corner residues as phi 1 = 63(1) degrees, psi 1 = -127(1) degrees, phi 2 = -66(1) degrees and psi 2 = -10(1) degrees, and an intramolecular hydrogen bond N-H...O of length 3.01(1) A. This shows that the conformational constraints produced by dehydro-Abu are similar in nature to but different in magnitude than those produced by dehydro-Phe and dehydro-Leu. The methanol-peptide interactions show characteristic features of multiple hydrogen-bond formations involving polar sites of participating peptide and methanol molecules. The packing of the molecules in the unit cell is stabilized by interactions through methanol molecules with the help of several hydrogen bonds.