STUDIES OF PEPTIDES FORMING 3(10)-HELICES AND ALPHA-HELICES AND BETA-BEND RIBBON STRUCTURES IN ORGANIC SOLUTION AND IN MODEL BIOMEMBRANES BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

In order to examine the potential correlation between infrared absorption spectra and 3(10-) and alpha-helices and beta-bend ribbon structures, the secondary structures of synthetic peptides known to contain pure 3(10)-helices, mixed 3(10)/alpha-helices, and pure beta-bend ribbon structures, based upon X-ray diffraction and NMR studies, have been investigated by using FTIR spectroscopy incorporating resolution-enhancement techniques. Studies of the peptides known to contain a stable 3(10)-helix in CDCl3 show the main amide I band of fully stable 3(10)-helices occurs at 1666-1662 cm-1. Resolution-enhancement methods revealed small contributions at 1681-1678 and 1646-1644 cm-1, while the amide II band occurs at 1533-1531 cm-1. Peptides known to contain both alpha- and 3(10)-helices in their structure exhibit bands characteristic of both types of conformation. Peptides known to fold into the beta-bend ribbon structure show an amide I band maximum at 1648-1645 cm-1 with the amide II band at 1538-1536 cm-1. Incorporation of these peptides into model membrane structures, e.g., DMPC vesicles, in aqueous buffer sometimes produces changes in the peptide secondary structure. Those peptides which possess a 3(10)-helical structure in CDCl3 solution change the secondary structure in DMPC vesicles to predominantly alpha-helical, plus a contribution from short, unstable 3(10)-helix and/or beta-turns. Those peptides which contain a combination of alpha- and 3(10)-helical structures in CDCl3 solution tend to retain some 3(10)-helical structure within the lipid environment, although the overall H-bonding pattern is altered. Those peptides which form a beta-bend ribbon structure appear to be largely unaffected in the membrane environment. This study represents the first complete characterization of a pure 3(10)-helix and a pure beta-bend ribbon by FTIR spectroscopy using resolution-enhancement techniques.