The stability of two-component liposomes composed of the polymerizable 1,2-bis-[10-(2',4'-hexadienoyloxy)decanoyl]-sn-glycero-3-phosphati dylcholine (SorbPC) and either a phosphatidylethanolamine (PE) or a phosphatidylcholine (PC) were examined via fluorescence leakage assays. Ultraviolet light exposure of SorbPC-containing liposomes forms poly-SorbPC, which phase separates from the remaining monomeric lipids. If the nonpolymerizable lipids are PE's, then the photoinduced polymerization destabilizes the liposome with loss of aqueous contents. The permeability of the control dioleoylPC/SorbPC membranes was not affected by photopolymerization of SorbPC. The photodestabilization of dioleoylPE/SorbPC (3:1) liposomes required the presence of oligolamellar liposomes. NMR spectroscopy of extended bilayers of dioleoylPE/SorbPC (3:1) showed that the photopolymerization lowers the temperature for the appearance of 31P NMR signals due to the formation of isotropically symmetric lipid structures. These observations suggest the following model for the photoinduced destabilization of liposomes composed of PE/SorbPC; photopolymerization induced phase separation with the formation of enriched domains of PE, which allows the close approach of apposed regions of enriched PE lamellae and permits the formation of an isotropically symmetric structure between the lamellae. The formation of such an interlamellar attachment (ILA) between the lamellae of an oligolamellar liposome provides a permeability pathway for the light-stimulated leakage of entrapped water-soluble reagents.

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Biphalin, (Tyr-D-Ala-Gly-Phe-NH)2, is a highly potent dimeric analog of enkephalin. Its analgesic efficacy is due in part to its ability to permeate the blood-brain barrier. To aid in understanding the mechanism of the transmembrane movement we determined and analyzed the permeability and partition coefficients of biphalin and a series of analogues where F, Cl, I, NO2, or NH2 were placed in the para position of the aromatic rings of Phe4,4'. Liposomes composed of neutral phospholipids and cholesterol were used as the model membrane. The overall good correlation between permeability and water-membrane partition coefficients suggests that the movement of biphalins across the model membrane is controlled by diffusion and depends on the water-membrane partition coefficient. To explain the observed correlation between permeability and the electron withdrawing/donating character of the substituents in the phenylalanine ring, we examined various folding patterns of Leu-enkephalin, an endogenous pentapeptide that exhibits affinities toward the same classes of opioid receptors (delta and mu). The observed permeabilities and partition coefficients of biphalin and analogues, as well as the tyrosine side chain accessibility, are consistent with the presence of the type of folding where the tyrosine and phenylalanine side chains are in a close contact. We propose that the aromatic ring interaction can promote the peptide permeability by stabilizing a more compact structure of biphalin that would minimize the number of hydrogen bonds with water and therefore enhances partitioning into the model membrane.

NMR spectroscopic, peptide-membrane conformational studies on [D-Pen2,D-Pen5]-enkephalin (DPDPE), an opioid receptor selective peptide, and an acyclic analog of DPDPE (DPDPE reduced at the disulfide bond) were conducted. The NMR method of transferred nuclear Overhauser effect (TRNOE) was used to obtain NOE profiles of the free and membrane bound forms of DPDPE and acyclic DPDPE. After comparison of the profiles of both peptides in the free and membrane-bound states, we hypothesize that the cyclic DPDPE undergoes little if any conformational change upon interaction with the membrane. However, for the acyclic analog, large changes in the NOE profile associated with backbone and side-chain groups were observed after interaction with the membrane. Results of computerized molecular modeling studies also were consistent with our theory that the free and membrane-bound forms of cyclic DPDPE have very similar free and membrane-bound states. The free acyclic DPDPE has a reverse turn conformation with sidechains situated so that hydrophobic surface exposure to aqueous solution is minimized. After membrane interaction, the acyclic DPDPE has an extended conformation near the carboxy terminus with aromatic sidechains widely separated. We propose that the interaction of the acyclic DPDPE with the membrane surface is mediated by the amino terminus. We further propose that the interaction of the cyclic DPDPE with the membrane surface is limited because the D-Pen2 side chain is covalently bonded and the aromatic side chains and backbone are only slightly altered after membrane contact. Permeability studies by Ramaswami et al. [(1992) Biochim. Biophys. Acta 1109(2), 195-202] demonstrated that the acyclic DPDPE permeated through membranes at a rate 4 times greater than cyclic DPDPE. We conclude that conformational and topographical flexibility may be critical factors in peptide-membrane interactions and permeability of bilayer membranes to opioid peptides.

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