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NMR Study on the Binding of Neuropeptide Achatin-I to Phospholipid Bilayer: The Equilibrium, Location, and Peptide Conformation

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

Correspondence: Address reprint requests to Masaru Nakahara, Tel./ Fax: 81-774-38-3070; E-mail: nakahara{at}scl.kyoto-u.ac.jp.

Molecular mechanism of the binding of neuropeptide achatin-I (Gly-D-Phe-Ala-Asp) to large unilamellar vesicles of zwitterionic egg-yolk phosphatidylcholine (EPC) was investigated by means of natural-abundance ¹³C and high-resolution (of 0.01 Hz order) ¹H NMR spectroscopy. The binding equilibrium was found to be sensitive to the ionization state of the N-terminal group in achatin-I; the de-ionization of decreases the bound fraction of the peptide from 15% to nearly none. The electrostatic attraction between the N-terminal positive group and the negative group in the EPC headgroup plays an important role in controlling the equilibrium. Analysis of the ¹³C chemical shifts () of EPC showed that the binding location of the peptide within the bilayer is the polar region between the glycerol and ester groups. The binding caused upfield changes of the ¹³C resonance for almost all the carbon sites in achatin-I. The changes for the ionic Asp at the C-terminus are more than five times as large as those for the other residues. The drastic changes for Asp result from the dehydration of the ionic groups, which are strongly hydrated by electrostatic interactions in bulk water. The side-chain conformational equilibria of the aromatic D-Phe and ionic Asp residues were both affected by the binding, and the induced changes in the equilibria appear to reflect the peptide-lipid hydrophobic interactions.