THE USE OF DODECYLPHOSPHOCHOLINE MICELLES IN SOLUTION NMR

Dodecylphosphocholine (DPC) micelles are useful as a model membrane sy stem for solution NMR. Several new observations on dodecylphosphocholi ne micelles and their interactions with opioid peptides are described. The optimal lipid concentration has been investigated for small pepti de NMR studies in DPC micelles for two opioid peptides, a 5-mer and a 17-mer. In contrast to reports in the literature, identical 2D spectra have been observed at low and high lipid concentrations. The chemical shift of resolved peptide proton resonances has been followed as a fu nction of added lipid and indicates that there are changes in the chem ical shifts above the critical micelle concentration and up to a ratio of 7:1 (lipid:peptide) for the 17-mer, and 9.6:1 for the 5-mer. These results suggest that conformational changes occur in the peptide sign ificantly above the critical micelle concentration, up to a lipid:pept ide ratio which is dependent upon the peptide, here ranging from 7:1 t o 9.6:1. To address the stoichiometry more directly, the diffusion coe fficients of the lipid alone and the lipid with peptide have been meas ured using pulsed-field gradient spin-echo NMR experiments. These data have been used to calculate the hydrodynamic radius and the aggregati on number of the micelle with and without peptide and show that the ag gregation number of the peptide-lipid complex increases at high lipid concentrations without a concomitant change in the peptide conformatio n. Last, several protonated impurities have been observed in the comme rcial preparation of DPC which resonate in the amide proton region of the NMR spectrum. These results are significant for researchers using DPC micelles and illustrate that both care in sample preparation and t he stoichiometry are important issues with the use of DPC as a model m embrane. (C) 1995 Academic Press, Inc.