Synthesis, Crystal and Molecular Structure of Boc-Pro- \bigtriangleup Phe- Ala- \bigtriangleup Phe- Ala-OMe ; A Pentapeptide with a Novel \beta -Bend Ribbon Structure

\alpha, \beta -Dehydroamino acid residues (2,3-didehydroamino acid residues) are found in several naturally occurring peptides from microbial sources as well as in some proteins;[1] their presence in a peptide chain produces remarkable conformational consequences.[2] In addition, dehydropeptides show enhanced resistance to enzymatic degradation.[3] Thus, introduction of \alpha, \beta -de-hydroamino acid residues into bioactive peptide sequences has become an important tool to study structure-function relationships, and to provide analogues of peptide hormones with improved bioactivity.[4