Lateral proton conduction in monolayers of phospholipids from extreme halophiles

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-glycero-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 greater than 25 mN/m) on subphases of low (1 mM) and high (4 M) ionic strength. Proton conduction was also detected in highly condensed monolayers (greater than 35 mN/m) of the naturally occurring phospholipids (PGP, PG) but was abruptly terminated in tightly packed monolayers (greater than 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.(ABSTRACT TRUNCATED AT 250 WORDS)