The use of FT-IR for quantitative studies of the apparent pK(a) 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 qua ntitative study of the dehydration of the phosphatidylserine phosphate grou p in the presence of Ca2+ exerted by different molecules, such as diacylgly cerol, sphingosine and stearylarnine, by using a partial least-squares stat istical procedure. By using this method it was observed that diacylglycerol enhanced the dehydration of this PO2- group produced by Ca2+ whereas the a mino-bases sphingosine and stearylamine protected the phosphate group from the dehydration produced by Ca2+ due to the very strong electrostatic inter action established. The apparent pK(a) of lipid carboxyl groups can also be estimated by using FTIR. The method consisted in quantifying the absorbanc e intensities due to the protonated and the unprotonated forms of the speci fic group being studied. The pK(a) of the carboxyl group of [1-C-13]-palmit ic acid included in dipalmitoylphosphatidylcholine membranes was found to b e 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 pK(a) of fr ee 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. Fi nally the pK(a) of the carboxyl group of phosphatidylserine was shifted fro m 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 electrostat ic interactions. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved .