A major difficulty in the study of nonbonded effects, particularly hydration, upon the phosphate group frequencies in phospholipids and other biologically important systems lays in the lack of reference for the nonbonded species, i.e. inaccessiblity to measurement of isolated, ionic phosphodiesters. We have therefore ab-initio calculated (3-21 G* basis set) the frequencies of the isolated dimethylphosphate (DMP), its mono- and dihydrates and its sodium, ammonium and tetramethylammonium salts. The computed results (unscaled frequencies, PED) will be compared with experimental ones obtained with stepwise hydrated lecithins and dialkylphosphate salts. The antisymmetric N(CH3)3frequencies turned out to be useful in interpreting the observed phosphate frequency trends upon hydration. The discussion of the results will be centered on the behaviour and vibrational character of the PO2-frequencies. A molecular representation of phophatidylcholine hydration is proposed. © 1992.
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