The use of FT-IR for quantitative studies of the apparent pKa of lipid carboxyl groups and the dehydration degree of the phosphate group of phospholipids.

Fourier-transform infrared spectroscopy (FT-IR) has been applied to the quantitative study of the dehydration of the phosphatidylserine phosphate group in the presence of Ca2+ exerted by different molecules, such as diacylglycerol, sphingosine and stearylarnine, by using a partial least-squares statistical procedure. By using this method it was observed that diacylglycerol enhanced the dehydration of this PO2- group produced by Ca2+ whereas the amino-bases sphingosine and stearylamine protected the phosphate group from the dehydration produced by Ca2+ due to the very strong electrostatic interaction established. The apparent pKa of lipid carboxyl groups can also be estimated by using FTIR. The method consisted in quantifying the absorbance intensities due to the protonated and the unprotonated forms of the specific group being studied. The pKa of the carboxyl group of [1-13C]-palmitic acid included in dipalmitoylphosphatidylcholine membranes was found to be 8.7, a value much higher than that estimated from a molecular solution of the fatty acid. It was observed using the same method that the pKa of free fatty acids in model stratum corneum lipid mixtures was in the range 6.2-7.3 increasing with the preponderance of oleic acid over palmitic acid. Finally the pKa of the carboxyl group of phosphatidylserine was shifted from 4.6 in the pure phospholipid to 2.1 and 2.2 in the presence of equimolar sphingosine and stearylamine, respectively, as a consequence of electrostatic interactions.