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A differential scanning calorimetric and 31P NMR spectroscopic study of the effect of transmembrane a-helical peptides on the lamellar-reversed hexagonal phase transition of phosphatidylethanolamine model membranes



A differential scanning calorimetric and 31P NMR spectroscopic study of the effect of transmembrane a-helical peptides on the lamellar-reversed hexagonal phase transition of phosphatidylethanolamine model membranes



Biochemistry (American Chemical Society) 40(3): 0-8



We have investigated the effects of the model a-helical transmembrane peptide Ac-K2L24K2-amide (L24) on the thermotropic phase behavior of aqueous dispersions of 1,2-dielaidoylphosphatidyle-thanolamine (DEPE) to understand better the interactions between lipid bilayers and the membrane-spanning segments of integral membrane proteins. We studied in particular the effect of L24 and three derivatives thereof on the liquid-crystalline lamellar (La)-reversed hexagonal (HII) phase transition of DEPE model membranes by differential scanning calorimetry and 31P nuclear magnetic resonance spectroscopy. We found that the incorporation of L24 progressively decreases the temperature, enthalpy, and cooperativity of the La-HII phase transition, as well as induces the formation of an inverted cubic phase, indicating that this transmembrane peptide promotes the formation of inverted nonlamellar phases, despite the fact that the hydrophobic length of this peptide exceeds the hydrophobic thickness of the host lipid bilayer. These characteristic effects are not altered by truncation of the side chains of the terminal lysine residues or by replacing each of the leucine residues at the end of the polyleucine core of L24 with a tryptophan residue. Thus, the characteristic effects of these transmembrane peptides on DEPE thermotropic phase behavior are independent of their detailed chemical tructure. Importantly, significantly shortening the polyleucine core of L24 results in a smaller decrease in the La--HII phase transition temperature of the DEPE matrix into which it is incorporated, and reducing the thickness of the host phosphatidylethanolamine bilayer results in a larger reduction in the La-HII phase transition temperature. These results are not those predicted by hydrophobic mismatch considerations or reported in previous studies of other transmembrane a-helical peptides containing a core of an alternating sequence of leucine and alanine residues. We thus conclude that the hydrophobicity and conformational flexibility of transmembrane peptides can affect their propensity to induce the formation of inverted nonlamellar phases by mechanisms not primarily dependent on lipid--peptide hydrophobic mismatch. Reprinted by permission of the publisher.

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