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

We have studied the effects of the incorporation of the a-helical transmemb rane peptides Ac-K-2-L-24-K-2-amide (L-24) and Ac-K-2-(L-A)(12)-K-2-amide ( (LA)(12)) on the thermotropic phase behavior of 1,2-dipalmitoyl-d(62)-sn-gl ycero-3-phosphocholine (DPPC-d(62)) and 1-palmitoyl-d(31)-2-oleoyl-sn-glyce ro-3-phosphocholine (POPC-d(31)) lipid bilayer model membranes by different ial scanning calorimetry (DSC) and the conformational and orientational ord er of the phospholipid chains by Fourier transform infrared (FTIR) spectros copy and H-2 nuclear magnetic resonance (H-2-NMR) spectroscopy, respectivel y. Our DSC and FTIR spectroscopic studies indicate that the peptides L-24 a nd (LA)(12) both decrease the temperature and enthalpy of the gel/liquid-cr ystalline phase transition of DPPC-d(62) bilayers, with (LA)(12) having the greater effect in this regard. An examination of the frequencies of the CH 2 and CD2 symmetric stretching bands of the infrared spectra of liquid-crys talline states of the peptide-free and peptide-containing DPPC-d(62) and PO PC-d(31) samples, and a comparison with the orientational order as measured by H-2-NMR spectroscopy as well as with the chain order as measured by ele ctron spin resonance spectroscopy, lead us to conclude that the CH2 (or CD2 ) stretching frequencies of lipid hydrocarbon chains are not a reliable mea sure of chain conformational order in lipid bilayers containing significant amounts of peptides or other lipophilic inclusions. In contrast, the resul ts of our H-2-NMR spectroscopic studies present a consistent picture in whi ch both L-24 and (LA)(12) increased in a similar way the time-averaged orie ntational order of the lipid chains of their liquid-crystalline lipid bilay er hosts. The comparison of the effects L-24 and (LA)(12) On phosphatidylch oline bilayers indicates that the gel-to-liquid-crystalline phase transitio n appears to be more sensitive to small changes in transmembrane peptide su rface topology than hydrocarbon carbon chain orientational order in the liq uid-crystalline state. (C) 2001 Elsevier Science B.V. All rights reserved.