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Acid dissociation constants of diphytanylglycerolphosphorylglycerol-methylphosphate, and diphytanylglycerolphosphorylglycerophosphate and its deoxy analog |
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| Authors: | Maria Tomoaia-Cotisel Laura C. Stewart Morris Kates Janos Zsako Emil Chifu Aurora Mocanu Petre T. Frangopol Lewis J. Noe Peter J. Quinn |
| Affiliation: | a Molecular/Structural Biotechnologies, Inc., Rollins Avenue 143, Rockville, MD 20847-2539, USA b Department of Biochemistry, University of Ottawa, Ottawa K1N 6N5, Canada c Department of Physical Chemistry, University of Cluj-Napoca, 3400 Cluj-Napoca, Romania d Romanian Academy of Sciences, Calea Victoriei 125, 71102 Bucharest 22, Romania e Department of Chemistry, University of Wyoming, Laramie, WY 82071, USA f Department of Biochemistry, King’s College London, Campden Hill, London W8 7AH, UK |
| Abstract: | Acid dissociation constants of 2,3-diphytanyl-sn-glycero-1-phosphoryl-sn-3′-glycero-1′-methylphosphate (PGP-Me), the major phospholipid in extreme halophiles (Halobacteriaceae), and of the demethylated 2,3-diphytanyl-sn-glycero-1-phosphoryl-sn-3′-glycero-1′-phosphate (PGP) and its deoxy analog 2,3-diphytanyl-sn-glycero-1-phosphoryl-1′-(1′,3′-propanediol-3′-phosphate) (dPGP), were calculated by an original mathematical procedure from potentiometric titration data in methanol/water (1:1, v/v) and found to be as follows: for PGP-Me (and presumably PGP), pK1=3.00 and pK2=3.61; for PGP, pK3=11.12; and for dPGP, pK1=2.72, pK2=4.09, and pK3=8.43. High-resolution 31P NMR spectra of intact PGP-Me in benzene/methanol or in aqueous dispersion showed two resonances corresponding to the two P-OH groups, each of which exhibited a chemical shift change in the pH range 2.0–4.5, corresponding to acid dissociation constants pK1=pK2=3.2; no further ionization (pK3) was detected at higher pH values in the range 5–12. The present results show that PGP-Me titrates as a dibasic acid in the pH range 2–8, but above pH 8, it titrates as a tribasic acid, presumably PGP, formed by hydrolysis of the methyl group during the titration with KOH. Calculation of the concentrations of the ionic molecular species of PGP-Me, PGP and dPGP as a function of pH showed that the dianionic species predominate in the pH range 5–9, covering the optimal pH for growth of Halobacteriaceae. The results are consistent with the concept that the hydroxyl group of the central glycerol moiety in PGP-Me and PGP is involved in the formation of an intramolecular hydrogen-bonded cyclic structure of the polar headgroup, which imparts greater stability to the dianionic form of PGP-Me and PGP in the pH range 5–9 and facilitates lateral proton conduction by a process of diffusion along the membrane surface of halobacterial cells. |
| Keywords: | Acid dissociation constants Deoxy analogue of diphytanylglycerophosphorylglycerophosphate Diphytanylglycerolphosphorylglycerolmethylphosphate Diphytanylglycerophosphorylglycerophosphate Polar headgroup conformation Proton conductance |
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