Conformational preferences of oligopeptides rich in α‐aminoisobutyric acid. III. Design, synthesis and hydrogen bonding in 310‐helices

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Abstract
Two sterically constrained peptides {iBoc‐Aib‐Aib‐Aib‐DkNap‐Leu‐Qx‐Ala‐Aib‐Aib‐F1, (Dk4Qx6[7/9]) and iBoc‐Aib‐Aib‐Aib‐DkNap‐Leu‐Aib‐Ala‐Aib‐Aib‐Fl, (Dk47/9)} containing α‐aminoisobutyric acid (Aib) and Aib‐class amino acids in conjunction with selected mono‐α‐alkyl amino acids were synthesized by an optimized TBTU/HOBt procedure. The use of Aib‐class amino acids (e.g. DkNap and Qx), defined and discussed here, gives rise to the same overwhelmingly 310‐helical backbone conformation as that provided by simpler Aib‐rich peptides and homopeptides. The synthetic α,α‐dialkylamino acids (DkNap, Qx) are aromatic homologues of the known alicyclic variants of Aib, the Ac5c and Ac6c amino acids. Two new organic solubilizing groups for peptides, iBoc and 2‐methoxyethylamine, are introduced. The 1H nuclear magnetic resonance analyses of the Dk4s/p[7/9] and Dk4Qx6[7/9] peptides demonstrate the unambiguous 310s/b‐helical hydrogen bonding pattern of these peptides, confirming the design objective of these sequence patterns containing greater than 50% Aib and Aib‐class composition. © Munksgaard 1994.

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