Differential scanning calorimetry and 2H nuclear magnetic resonance and Fourier transform infrared spectroscopy studies of the effects of transmembrane α-helical peptides on the organization of phosphatidylcholine bilayers

Abstract

We have studied the effects of the incorporation of the α-helical transmembrane peptides Ac-K2-L24-K2-amide (L24) and Ac-K2-(L-A)12-K2-amide ((LA)12) on the thermotropic phase behavior of 1,2-dipalmitoyl-d62-sn-glycero-3-phosphocholine (DPPC-d62) and 1-palmitoyl-d31-2-oleoyl-sn-glycero-3-phosphocholine (POPC-d31) lipid bilayer model membranes by differential scanning calorimetry (DSC) and the conformational and orientational order of the phospholipid chains by Fourier transform infrared (FTIR) spectroscopy and 2H nuclear magnetic resonance (2H-NMR) spectroscopy, respectively. Our DSC and FTIR spectroscopic studies indicate that the peptides L24 and (LA)12 both decrease the temperature and enthalpy of the gel/liquid-crystalline phase transition of DPPC-d62 bilayers, with (LA)12 having the greater effect in this regard. An examination of the frequencies of the CH2 and CD2 symmetric stretching bands of the infrared spectra of liquid-crystalline states of the peptide-free and peptide-containing DPPC-d62 and POPC-d31 samples, and a comparison with the orientational order as measured by 2H-NMR spectroscopy as well as with the chain order as measured by electron spin resonance spectroscopy, lead us to conclude that the CH2 (or CD2) stretching frequencies of lipid hydrocarbon chains are not a reliable measure of chain conformational order in lipid bilayers containing significant amounts of peptides or other lipophilic inclusions. In contrast, the results of our 2H-NMR spectroscopic studies present a consistent picture in which both L24 and (LA)12 increased in a similar way the time-averaged orientational order of the lipid chains of their liquid-crystalline lipid bilayer hosts. The comparison of the effects L24 and (LA)12 on phosphatidylcholine bilayers indicates that the gel-to-liquid-crystalline phase transition appears to be more sensitive to small changes in transmembrane peptide surface topology than hydrocarbon carbon chain orientational order in the liquid-crystalline state.