FLUCTUATIONS IN IR SPECTRAL PARAMETERS DETECTED IN MIXED ACYL-CHAIN MEMBRANES OF ACHOLEPLASMN-LAIDLAWII B

Acholeplasma laidlawii B cells were grown at 37 degrees C on three bin ary 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 lipi d 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 micr oorganism. Membrane conformational order for the deuterated and protea ted acyl chains in intact cells was monitored by FT-IR spectroscopy th rough the thermotropic response of the acyl chain CD2 and CH2 stretchi ng frequencies. Intact cells and isolated membranes revealed broad pha se 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 T-m was close to the growth temperature. Fluctuati ons in IR spectral parameters from the liquid crystalline phases were detected in ten separate samples of cells grown on a 2:3 mixture (fina l mol ratio 0.58:1) of C16:0-d(31)/C18:1, and in no other cell prepara tion. These were manifest by reduced precision in the measurement of C H2 and CD2 stretching frequencies and are attributed to fluctuations i n the membrane conformational order. In addition to conformational Ord er fluctuations in intact cells, similar behavior was noted for the si mple binary phosphatidylcholine (PC) mixture, DOPC/1-C16:0-d(31), 2-C1 8:1 PC (2:1 molar ratio). In this instance, the fluctuations were also detected through the temporal and thermotropic response of the relati ve 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.