Lateral proton conduction in monolayers of phospholipids from extreme halophiles

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Abstract
Studies have been carried out on the lateral proton conductance properties of monolayers of the major and minor phospholipids of extremely halophilic archaebacteria, 2,3-diphytanyl-sn-glycero-1-phospho-3''-sn-glycerol 1''-phosphate (PGP) and 2,3-diphytanyl-sn-glycero-1-phospho-3''-sn-glycerol (PG), respectively, as well as on their respective deoxy analogues: 2,3-diphytanyl-sn-glycero-1-phospho-1''-propanediol 3''-phosphate (dPGP), 2,3-diphytanyl-sn-glycerol-1-phospho-1''-1''-3''-propanediol (dPG), and 2,3-diphytanyl-sn-glycero-1-phospho-1''-propanol (ddPG). Lateral proton conduction was found to occur with monolayers of all ether phospholipids examined at reduced surface pressure (.pi. > 25 mN/m) on subphases of low (1 mM) and high (4 M) ionic strength. Proton conduction was also detected in highly condensed monolayers (> 35 mN/m) of the naturally occurring phospholipids (PGP, PG) but was abruptly terminated in tightly packed monolayers (> 35 mN/m) of the corresponding deoxy compounds (dPGP, dPG, ddPG) on subphases with low ionic strength. Conduction did occur, however, along monolayers of the deoxy compounds at high surface pressure when spread on a subphase of high ionic strength (4 M). The abrupt termination of conduction with monolayers of the deoxy compounds at low ionic strength cannot be attributed to a lipid phase transition or to changes in the lateral fluidity of the monolayers, nor was the pK of the fluorescent interfacial proton indicator affected at high surface pressures. Our data are consistent with the occurrence of a conformational change in the polar headgroup region of the deoxy compounds under high compression of the monolayers but not in that of the naturally occurring phospholipids. The most likely change in the polar headgroup of the deoxy compounds would be the formation of internal hydrogen bonding only under high surface pressure, which would expel water molecules and result in disruption of the conducting network. In the natural phospholipids PGP and PG, the presence of the central hydroxyl group would facilitate the formation of stable intramolecular bondings which would render the molecular structure of their headgroups insensitive to high surface pressure and hence preserve the proton conduction network.

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