Proton NMR conformational study of an annexin I fragment: influence of a phospholipidic micellar environment.

A 32 residue peptide, Ac-AQWDADELRAAMKGLGTDEDTLIELASRTNK, spanning the first helix-loop-helix motif of the second repeat of human annexin I, was synthesized and studied by standard 2D proton NMR and molecular modeling. The peptide was solubilized either in aqueous solution, in TFE-H2O mixtures or in aqueous phospholipidic micellar solution. In pure aqueous solution, elements of helix secondary structure were observed. Addition of TFE led to a dramatic cooperative effect on the secondary structure with a very low transition midpoint indicative of the strong tendency of the peptide to form alpha helices. Only in the aqueous micellar solution was the full helix-loop-helix motif obtained, showing again the potency of a membrane-like micellar environment to initiate peptide secondary structures and even elements of tertiary structure. There were sufficient NMR data to perform molecular modeling of the structure of the annexin fragment solubilized in the presence of micelles. However, this structure showed a relatively high degree of flexibility, especially around the T17-D18 hinge at the end of the loop.