Fluctuations in IR spectral parameters detected in mixed acyl chain membranes of Acholeplasma laidlawii B.

Acholeplasma laidlawii B cells were grown at 37 degrees C on three binary C16:0-d(31)/C18:1 fatty acid mixtures at initial mol ratios of 3:2, 1:1, and 2:3. These mol ratios produced final C16:0-d(31)/C18:1 lipid acyl chain mol ratios of 1.66 +/- 0.23 (n=6), 1.3 +/- 0.20 (n=6) and 0.58 +/- 0.09 (n=10), respectively, in the membrane of the microorganism. Membrane conformational order for the deuterated and proteated acyl chains in intact cells was monitored by FT-IR spectroscopy through the thermotropic response of the acyl chain CD2 and CH2 stretching frequencies. Intact cells and isolated membranes revealed broad phase transitions centered well below the growth temperature. This result differs from previous studies (Moore, D.J. and Mendelsohn, R. (1994) Biochemistry 33, 4080-4085) of cells grown on a single saturated fatty acid source, where Tm was close to the growth temperature. Fluctuations in IR spectral parameters from the liquid crystalline phases were detected in ten separate samples of cells grown on a 2:3 mixture (final mol ratio 0.58:1) of C16:0-d(31)/C18:1, and in no other cell preparation. These were manifest by reduced precision in the measurement of CH2 and CD2 stretching frequencies and are attributed to fluctuations in the membrane conformational order. In addition to conformational order fluctuations in intact cells, similar behavior was noted for the simple binary phosphatidylcholine (PC) mixture, DOPC/1-C16:0-d(31),2-C18:1 PC (2:1 molar ratio). In this instance, the fluctuations were also detected through the temporal and thermotropic response of the relative intensity of the 1341 cm(-1) band assigned to end-gauche conformers about the penultimate C-C bond in the oleoyl chains. The relationship of these observations to the Raman spectroscopic detection of packing fluctuations in highly unsaturated PC's (Litman, B.J., Lewis, N., and Levin, I.W. (1991) Biochemistry 30, 313-319) is considered.