Study of Ionic Structures in Phospholipids by Infrared Spectra.

The infrared absorption spectra of lipids have been studied by numerous investigators chiefly interested in using the spectra to identify the various substances studied or for quantitative estimations. However, the spectra can be further used to obtain an understanding of the ionic groups that are present in the molecule and to identify changes in ionic structures arising as the lipid reacts with an ionic medium. Kimura and Nagai (2) in a study of the spectra of phosphatidylserine pointed out changes that were evident as this lipid was treated with acid or base, and attributed some of these to changes in the ionization of the carboxylic acid group. Abramson, Katzman, and Gregor (3), in agreement with this work, showed that the salt forms of phosphatidylserine produce spectra with strong absorbance at 6.1 p which does not exist in the acid form. This is attributed to the -COOresonance. This recognition of the changes in the spectra of the carboxyl group was valuable in elucidating the structure of the molecule. In this communication the changes occurring in the ionization of the phosphate group of several phospholipids are described. It is shown that the infrared spect,ra can be used to study the changes in the ionization of these compounds and related esters of orthophosphoric acid. Adlthough various authors have assigned absorption frequency ranges to the ionized and unionized phosphate groups (4-7), these ranges are broad and overlap. These authors also disagree in their assignment of specific absorption bands to definite chemical groups. An attempt is made in this paper to identify these bands more closely for phospholipids and to use them for the recognition of structural characteristics of these compounds. To aid in the understanding of the more complex phospholipid molecules a study was made of octadecyl dihydrogen phosphate, dioctadecyl hydrogen phosphate, and the sodium salts of both these esters. Phosphatidic acid and its salt resemble octadecyl dihydrogen phosphate; while phosphatidylserine, lecithin, sphingomyelin, and phosphatidylethanolamine, diesters of orthophosphoric acid, are similar in a limited way to dioctadecyl hydrogen phosphate.