CONFORMATIONAL PREFERENCES OF OLIGOPEPTIDES RICH IN ALPHA-AMINOISOBUTYRIC-ACID .3. DESIGN, SYNTHESIS AND HYDROGEN-BONDING IN 3(10)-HELICES

Two sterically constrained peptides {(i)Boc-Aib-Aib-Aib-DkNap-Leu-Qx-A la-Aib-Aib-Fl, (Dk(4)Qx(6)[7/9]) and (i)Boc-Aib-Aib-Aib-DkNap-Leu-Aib- Ala-Aib-Aib-Fl, (Dk(4)[7/9])} containing cc-aminoisobutyric acid (Aib) and Aib-class amino acids in conjunction with selected mono-alpha-alk yl amino acids were synthesized by an optimized TBTU/HOBt procedure. T he use of Aib-class amino acids (e.g. DkNap and Qx), defined and discu ssed here, gives rise to the same overwhelmingly 3(10)-helical backbon e conformation as that provided by simpler Aib-rich peptides and homop eptides. The synthetic alpha,alpha-dialkylamino acids (DkNap, Qx) are aromatic homologues of the known alicyclic variants of Aib, the Ac(5)c and Ac(6)c amino acids. Two new organic solubilizing groups for pepti des (i)Boc and 2-methoxyethylamine, are introduced. The H-1 nuclear ma gnetic resonance analyses of the DK4[7/9] and Dk(4)Qx(6)[7/9] peptides demonstrate the unambiguous 3(10)-helical hydrogen bonding pattern of these peptides, confirming the design objective of these sequence pat terns containing greater than 50% Aib and Aib-class composition. (C) M unksgaard 1994.