1064 nm FT‐Raman spectra of hog kidney mitochondrial outer membranes: effects of NaCl and temperature on 1100 cm ‐1 region spectra

In order to obtain the experimental basis for the cause of the previously reported intensity decrease of the 1080 cm ‐1 band in the presence of NaCl, FT‐Raman spectra were measured with 1064 nm excitation for aqueous dispersions of hog kidney mitochondrial outer membrane preparations in phosphate and Tris–HCl buffer (pH 7.5) in the absence and presence of NaCl. It was found that the relative intensity of the ∽1084 cm ‐1 Raman band with respect to the ∽1064 cm ‐1 band decreased considerably in the presence of NaCl in phosphate buffer and that the intensity decrease in the presence of NaCl was not appreciable in the spectrum in Tris–HCl buffer. It was also found that the presence of NaCl decreased the relative intensities of the ∽1126 cm ‐1 band with respect to the ∽1064 and ∽1134 cm ‐1 bands in both phosphate and Tris–HCl buffer. The spectral changes in the same region due to the temperature increase were studied for the membrane preparation of ultracentrifuged pellets for the elucidation of the vibrational assignments. It was found that the intensity ratio of the 1084 cm ‐1 band with respect to the ∽1064 cm ‐1 band was increased with a temperature increase from 8 to 20°C. An unexpected increase was observed in the relative intensity of the ∽1126 cm ‐1 band with respect to the ∽1064 cm ‐1 band with temperature elevation. It was concluded that the decrease in I 1084 / I 1064 in phosphate buffer (pH 7.5) in the presence of NaCl could be attributed to the decrease in the gauche conformation of phospholipid acyl chains as predicted previously. The NaCl‐caused decrease of the gauche conformation was found to be small in Tris–HCl buffer (pH 7.5). The increase in I ∽1126 / I ∽1064 in the temperature increase experiment was interpreted in terms of a fairly large contribution of another chain conformation containing gauche bonds, the so‐called almost fully extended form against the fully extended form, i.e. all‐ trans conformation, to the ∽1126 cm ‐1 Raman band in the fluid state, in which the usual natural biomembranes exist. The observed decrease in I ∽1126 / I ∽1064 and I ∽1126 / I ∽1134 in the presence of NaCl in both phosphate and Tris–HCl buffers could result from a decrease in this conformation in the acyl chains due to the interchain tighter packing induced by the non‐specific screening effect on the membrane surface charges in the presence of NaCl. © 1997 John Wiley & Sons, Ltd.