2 ALTERNATIVE CONFORMATIONAL STATES OF ALPHA,ALPHA-DIALKYLGLYCYL-L-PROLYL SEQUENCES GOVERNED BY PRESENCE ABSENCE OF AN NH GROUP DIRECTLY FOLLOWING THE PROLINE RESIDUE, X-RAY CRYSTAL AND MOLECULAR-STRUCTURES OFBOC-D-IVA-L-PRO-NHBZL AND BOC-L-IVA-PRO-NHBZL

The crystal structures of the isovaline-containing dipeptides, Boc-D-I va-L-Pro-NHBzl 4 and Boc-L-Iva-L-Pro-NHBzl 5 were determined by X-ray diffraction. The diastereoisomeric peptides adopt intramolecular hydro gen-bonded beta-turn conformations closely similar to each other (4:ph i(Iva) -51 degrees, psi(Iva) -38 degrees, phi(Pro) - 70 degrees and ps i(Pro) -17 degrees and 5:phi(Iva) -53 degrees, psi(Iva) -35 degrees, p hi(Pro) -72 degrees and psi(Pro) -14 degrees). The Pro ring of each pe ptide is in CY-exo conformation. These conformations are essentially t he same as those in the reported crystal structures of the Aib-L-Pro s equence possessing an NH group directly attached to the carbonyl of th e L-Pro, indicating that replacement of either one of the two methyl g roups of the Aib moiety with an ethyl group does not cause any signifi cant change in the beta-turn conformation of the Aib-L-Pro sequence in the crystalline state. CD spectral analysis of the terminal chromopho ric group-carrying peptides Dnp-Gly-X-L-Pro-Gly-pNA (X = Aib 6 and D/L -Iva 7/8) has shown that these three tetrapeptides in CHCl3 and THF so lutions also adopt a beta-turn-type conformation. CD spectra of glycol ic acid residue-containing analogues in place of the fourth Gly residu e revealed a lack of beta-turn tendency in these analogues, indicating the importance of intramolecular hydrogen bonding for the beta-turn c onformation of the central dipeptide moieties. The results are consist ent with the reported unturned crystal structures of Aib-L-Pro and D/L -Iva-L-Pro sequence-containing peptides tacking the NH group which dir ectly follows the Pro residue available for intramolecular hydrogen bo nding.