Abstract
An interaction of dipalmitoylphosphatidylcholine (PC) and phosphatidylserine (PS) with manganous ions has been investigated by measuring the effect of bound manganese upon the longitudinal relaxation rate. 1/ T 1, of the solvent water protons and evaluating the enhancement factor ϵ b. The observed enhancement values were used to determine the number of interacting sites per polar head group, n. and the values of association constants, K A, of manganese to PC and PS. Changes in ϵ b correlate with structural changes at the interacting site. By increasing the temperature one can see an abrupt decrease in ϵ b within the temperature interval from 40 to 50° C indicating the thermal phase transition of PC as established by calorimetry, fluorescence and high-resolution NMR measurements. That an enhancement of 1/ T 1 of the solvent-watet protons occurs at all is explained by assuming a restricted rotation of the Mn 2+—aquo complex in the bound state. In addition we suppose that the rotation of the Mn 2+-aquo complex is the mechanism which dominates the relaxation of the waier protons in the bulk solvent when phospholipids are present.
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