α-proton Chemical Shift Index and Amide Proton Chemical Shift Temperature Coefficient of Melittin in Methanol: Indicators for a Helix Structure and an Intra-Molecular Hydrogen Bond?

The methods of the α-proton chemical shift index (CSI) and the amide proton (NH) chemical shift temperature coefficient (Δδ/ΔT) were found experimentally by a number of studies on proton NMR chemical shifts of peptides and proteins in an aqueous solution, and have been widely accepted. They provide an insight into secondary structures and intra-molecular hydrogen bonds in peptides and proteins without complex calculation. Our question is whether the methods are applicable to helical peptides in methanol. Melittin, a peptide of 26 amino acid residues found in bee venom, has been known to consist of two helices (the residues 2–11 and 13–26) connected by a kink section (the residues 11–13). Employing the methods for melittin in methanol, we have shown that most of the CSI’s for the residues 3–10 and 14–26 are − 1, and the Δδ/ΔT’s for the residues 5–26 except 6 and 14 are more positive than − 6 ppb/℃. If the methods are applicable to melittin in methanol, these results indicate that helix structures are formed in the regions of the residues 3–10 and 14–26 and NH’s in the residues 5–26 except 6 and 14 are involved in intra-molecular hydrogen bonds. The helical structure evaluated from the methods, hence, agrees nearly with the known helix structure. We also apply the methods to D-Pro14 melittin (synthetic melittin) and alamethicin (a peptide of 20 amino acid residues) in methanol, and show that they virtually work well.