INTERACTION OF CLASS-A AMPHIPATHIC HELICAL PEPTIDES WITH PHOSPHOLIPIDUNILAMELLAR VESICLES

The exchangeable apolipoproteins are important in determining the stru cture/function properties of lipoproteins. These proteins typically co ntain varying amounts of amphipathic helices. Five model peptides, 18A , Ac-18A-NH2, Ac-18R-NH2, 37pA, and 37aA, have been designed to invest igate variations of tile amphipathic alpha-helix structural motif on t heir lipid-binding propel-ties. These include the 18-residue peptides, 18A and Ac-18A-NH2, examples of class it helices, and Ac-18K-NH2, whi ch has the positions of acidic and basic residues interchanged relativ e to 18A. Three larger peptides were also studied: 36A, a dimer of 18A , 37pA and 37aA, dimers of 18A coupled by Pro (18A-Pro-18A) and hla (1 8A-Ala-18A), respectively. We report here the results of a thermodynam ic characterization of the binding properties of these peptides to sma ll unilamellar vesicles of POPC. Partition coefficients, K-p, were det ermined by fluorescence spectroscopy and binding enthalpies, Delta H, by titration calorimetry. These parameters were used to obtain the fre e energies, Delta G(0), and entropies, Delta S degrees, of binding. Th e results of this study indicate k(p) values on the order of 10(5), wi th interactions being enthalpically but not entropically favored in al l cases. The presence of positively charged residues at the interface (18A and Ac-18A-NH3) enhances binding but has little effect on the ext ent of bilayer penetration. The presence of tandem repeats decreases l ipid affinities for these small, highly curried bilayers. Our results are consistent with the idea that interaction appears to be confined l argely to the surface, with some degree of penetration of the hydropho bic face of the helix into the interior of the bilayer.