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Understanding The Interfacial Structure Of Aqueous Phospholipid Monolayer Films Via External Reflection FT-IR Spectroscopy.

Monolayer films of dimyristoyl-phosphatidic-acid (DMPA) at neutral and basic pH exhibit first-order phase transitions in their pressure-area curves. In situ external reflection FT-IR studies in the CH, stretching bands over this phase transition region exhibit a --6 cm-1 shift similar to that observed in previous studies of dipalmitoyl-phosphotidylcholine (DPPC)1. The acid form of DMPA at pH 3.0 does not exhibit the first order phase transition, but a ~1cm-1 frequency shift is observed in the liquid condensed phase and is also present in the neutral pH form. A solid-solid phase transition is proposed. Examination of the polar headgroup region (1300-960 cm-1)for acidic, neutral, and basic forms of DMPA give characteristic bands of each protonation state of PO3.