Peptide models of the helical hydrophobic transmembrane segments of membrane proteins: Interactions of acetyl-K-2-(LA)(12)-K-2-amide with phosphatidylethanolamine bilayer membranes

High-sensitivity differential scanning calorimetry (DSC) and Fourier transf orm infrared (FTIR) spectroscopy were used to study the interaction of a sy nthetic alpha -helical hydrophobic transmembrane peptide, acetyl-Lys(2)-(Le u-ala)(12)-Lys(2)-amide [(LA)(12)], and members of a homologous series of n -saturated diacylphosphatidylethanolamines (PEs). Ln the lower range of pep tide mole fractions, the DSC endotherms exhibited by the lipid/peptide mixt ures consist of two components. The temperature and cooperativity of the sh arper, higher temperature component are very similar to those of pure PE bi layers and are almost unaffected by variations in the protein/lipid ratio. However, the fractional contribution of this component to the total enthalp y changes decreases with increases in peptide concentration, and this compo nent completely disappears at higher protein mole fractions. The other comp onent, which is less cooperative and occurs at a lower temperature, predomi nates at higher protein concentrations. These two components of the DSC end otherm have been assigned to the chain-melting phase transitions of peptide -nonassociated and peptide-associated PE molecules, respectively. Although the temperature at which the peptide-associated PE molecules melt is progre ssively decreased by increases in (LA)12 concentration, the magnitude of th is downward shift is progressively greater as the length of the PE hydrocar bon chain decreases. As well, mixtures of (LA)(12) with the longer chain PE s exhibit unusual biomodal enthalpy variations, suggesting peptide immiscib ility in thicker gel state bilayers. Moreover, the enthalpy of the chain-me lting transition of the peptide-associated PE does not decrease to zero eve n at high peptide concentrations, indicating that (LA)(12) attenuates but d oes not abolish the cooperative gel/liquid-crystalline phase transition of the lipids with which it is in contact. Our FTIR spectroscopic data indicat e that (LA)(12) remains in a predominantly alpha -helical conformation in l iquid-crystalline PE bilayers of various hydrophobic thickness but that the helical conformation is altered in gel-state PE bilayers generally, probab ly due to peptide lateral aggregation. These data also suggest that (LA)(12 ) significantly disorders the hydrocarbon chains of adjacent PE molecules i n both the gel and liquid-crystalline states, relatively independently of l ipid hydrocarbon chain Length. Many aspects of PE/(LA)(12) interactions exh ibit a different dependence on the hydrophobic thickness of the host bilaye r than was observed in our previous study of (LA)(12)-phosphatidylcholine ( PC) model membranes [Zhang et al. (1995) Biochemistry 34, 2362-2371]. The d iffering effects of (LA)(12) incorporation On PE and PC bilayers is ascribe d primarily to the much stronger lipid polar headgroup interactions charact eristic of the former system. Finally, the considerable differences observe d in the behavior of (LA)(12) and the related polyleucine-based peptide P-2 4 in both PC and PE bilayers indicate that the structure of the hydrophobic core of alpha -helical transmembrane peptides can affect their conformatio nal plasticity and state of aggregation and thus the nature of their intera ctions with different phospholipid bilayers.