STRUCTURE EFFECTS OF DOUBLE D-AMINO-ACID REPLACEMENTS - A NUCLEAR-MAGNETIC-RESONANCE AND CIRCULAR-DICHROISM STUDY USING AMPHIPATHIC MODEL HELICES

D-Amino acid replacements and the determination of resulting structura l changes are a useful tool to recognize amphipathic helices in biolog ically active peptides such as neuropeptide Y and corticotropin-releas ing factor. In this paper the secondary structures of one amphipathic alpha-helical peptide and its double D-amino acid analog have been det ermined by means of H-1 NMR and CD spectroscopies under equivalent con ditions. The chemical shifts (NH and C alpha H) and the analysis of nu clear Overhauser effects show a split of the continuous helix for the all-L peptide into two helices at the position of double D-amino acid replacement. Hydrogen exchange rates correlate with water accessibilit ies in the hydrophobic/hydrophilic face and confirm the amphipathic he lical structure in the all-L peptide as well as in its double D-amino acid analog, A significantly accelerated hydrogen isotope exchange rat e is observed for the D-Ala(9) backbone proton, implying an increased flexibility at that position, These results show that the incorporatio n of an adjacent pair of D-amino acids only causes a local change in s tructure and flexibility, which makes the double D replacement interes ting as a tool for specific helix-disturbing modifications to search f or helical conformations in biologically active peptides.