Attenuated total reflection (ATR) Fourier transform infrared spectroscopy of dimyristoyl phosphatidylserine-cholesterol mixtures.

Mixtures of cholesterol with dimyristoyl phosphatidylserine or deuterated dimyristoyl phosphatidylserine were investigated by polarized and non polarized attenuated total reflection (ATR) Fourier transform infrared (FTIR) Spectroscopy. From polarized spectra the dichroic ratios of various vibrations as a function of cholesterol were calculated. Dichroic ratios of methylene vibration (CH(2)) 2934 cm(-1) of cholesterol decreases with increase of cholesterol concentration leveling off in the region where cholesterol phase separation takes place. The orientation of deuterated methylene (CD(2)) symmetric and asymmetric bands of the deuterated dimyristoyl phosphatidylserine is influenced little by cholesterol. In the polar region of dimyristoyl phosphatidylserine no effect of cholesterol on the dichroic ratios of carbonyl (C==O) and asymmetric phosphate (PO(2)(-)) vibrations were detected. For nonpolarized spectra the broad bands in the polar region of the phospholipid were deconvoluted. The carbonyl band (C==O) in pure dimyristoyl phosphatidylserine is composed of five bands; in the presence of increasing concentrations of cholesterol conformational change of these vibrations takes place evolving into one predominant band. Similar conformational change takes place in the presence of 75 molecules water/molecule DMPS. For the asymmetric phosphate band very small shifts due to interaction with cholesterol were detected.