| Abstract | The ¹³C labeled lipid 1 [ l’-¹³C]DPPS-NH₄⁺ and its metal salts were used to unambiguously assign all carbonyl vibrations in the infrared spectrum of phosphatidylserines. It is shown that the C=O stretching band at 1741 cm⁻¹ of phosphatidylserines previously assigned to the sn-1 C=O vibration contains contributions from both the sn- 1 and the sn-2 carbonyls. The C=O stretching band at frequencies between 1715 and 1730 cm⁻¹ previously assigned to the sn-2 C=O vibration also contains contributions from both carbonyl groups. The frequency dependence observed with the ester carbonyls primarily reflects hydrogen bonding and the polarity of the immediate vicinity. Conformational changes are accounted for in terms of frequency shifts if the conformational change involves the disposition of the C=O groups and in turn the hydrogen bonding properties. The infrared spectra of phospholipids dispersed in aqueous medium in the liquid crystalline state are inconsistent with a simple phospholipid conformation, e.g., with a conformation as found in the single-crystal structure of 1,2-dimyristoyl-sn-phosphatidylcholine and 1,2-dilauroyl-rucphosphatidylethanolamine. The spectra support the hypothesis proposed earlier (Hauser et al., Biochemistry, 1988) on the basis of existing single-crystal phospholipid structures and NMR evidence. The hypothesis states that several conformations are present in liquid crystalline phospholipid dispersions. The interconversion between these conformers takes place at rates that are fast on the NMR time scale but slow on the infrared time scale giving rise to motionally averaged NMR spectra but clearly discernible component infrared spectra. Changes in the infrared spectra observed for aqueous dispersions of phosphatidylserines induced at the order-disorder phase transition and upon adding cations such as Li⁺, Mg²⁺, and Ca²⁺ are interpreted in terms of changes in hydrogen bonding and in turn as conformational changes. |
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