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.