Ordered-fluid phase transitions of methylphosphatidic and phosphatidic acid bilayers are discussed theoretically to explain comprehensively various observed data in the presence of Ca2+ or protons. It is shown that the observed data can be explained reasonably by taking account of the interaction among neighboring head-groups with the aid of Ca2+ or protons. As a result, a quantitative explanation is given for the dependence of the phase-transition temperature on pH as well as Ca2+ concentration with the coexistence of monovalent cations. The dependence of the cooperativity of the transition on pH and monovalent cation concentration is also well explained. It is also pointed out that in the ordered-fluid phase transitions, a hysteresis metastable state and phase separation can be expected to appear under some conditions.
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