Acyl chain organization and protein secondary structure in cholesterol-modified erythrocyte membranes.

Fourier transform infrared and Raman spectroscopies are used to study the effects of cholesterol on human erythrocyte membrane acyl chain organization (mobility and conformation) and protein secondary structure. Compared to normal red cell membranes (approximately equal to 0.8 mol of cholesterol/mol of phospholipid) (C/P), acyl chain mobility is greater for the depleted (C/P approximately equal to 0.6) and less for the enriched (C/P approximately equal to 1.2) membranes as monitored by shifts of the IR symmetric methylene C-H stretching band (2852 wave numbers, cm-1) over the temperature range 5 to 40 degrees C. There is a continuous first order trend to the IR shifts, but no evidence of a phase change for any of the three cholesterol contents. Raman scattering of C-C stretching vibrations (1065-1130 cm-1) revealed that acyl chain conformation in the three membrane preparations is in a similar state of high disorder; however, compared to depleted and control membranes, the enriched membrane acyl chains display higher order lattice packing. The alpha-helical content of membrane proteins is correlated with the relative intensity of the Raman peptide backbone C-C stretching band (940 cm-1). Spectra of cholesterol-enriched erythrocyte membranes indicate a substantial increase in protein helical structure compared to those of the cholesterol-depleted membranes.