NONSTANDARD AMINO-ACIDS IN CONFORMATIONAL DESIGN OF PEPTIDES - HELICAL STRUCTURES IN CRYSTALS OF 5-10-RESIDUE PEPTIDES CONTAINING DIPROPYLGLYCINE AND DIBUTYLGLYCINE

Nonstandard amino acids dipropylglycine (Dpg) and dibutylglycine (Dbg) have been incorporated into penta- to decapeptides in order to impose local restrictions on the polypeptide chain stereochemistry. In each case, the Dpg and Dbg residues showed similar helix-forming propensity as the Aib (alpha-aminoisobutyric) residue. Further, the 3(10)- and alpha-helices containing the Dbg and Dpg residues had crystal packing motifs quite similar to those found for Aib-containing peptides. Crystal structure analyses are presented for Boc-Aib-Ala-Leu-Ala-Leu-Dpg-Leu-Ala-Leu-Aib-OMe (I), space group P2(1) with a = 11.313(3) Angstrom, b = 28.756(5) Angstrom, c = 11.884 A, beta = 103.74(1)degrees; Boc-Leu-Dpg-Leu-Ala-Leu-Aib-OMe, polymorph a (IIa), space group P1 with a = 10.205(5) Angstrom, b = 10.996(5) Angstrom, c = 21.393(9) Angstrom, = 81.92(3)degrees, beta = 88.20(3)degrees, gamma = 89.74(3)degrees and polymorph b (IIb), space group P2(1)2(1)2(1) with a = 9.291(1) Angstrom, b = 23.003(5) Angstrom, c = 23.085(6) Angstrom; and Boc-Leu-Dbg-Val-Ala-Leu-OMe (III), space group P2(1) with a 9.907(2) Angstrom, b = 16.078(3) Angstrom, c 13.543(3) Angstrom, beta = 104.48(2)degrees. The observation of helical conformations at all Dpg/Dbg residues is not entirely expected on the basis of conformational energy calculations and crystal structure observations on small homooligopeptides.