Acholeplasma laidlawii B has been grown highly enriched in myristic, pentadecanoic, and palmitic acids. The conformational order in the acyl chains of living cell membranes has been compared with that in the membranes of lysed cells and the lipids extracted therefrom, using Fourier-transform infrared (FT-IR) spectroscopic techniques. A gel–liquid-crystal-phase transition (25–45 °C) was noted for both the live cells and the membranes. Surprisingly, CH2 wagging progessions, characteristic of coupled oscillators from acyl chains in the all-trans conformation, were detected in the live cells and in the cell membrane. A simple model suggested the presence of 1.7 gauche bonds/chain at the growth temperature (37 °C) in each case. Conformational order in the live cells and in the membranes was virtually identical over the range of cell viability (5–40 °C), as measured by the thermotropic responses of the k = 1 and k = 2 components of the wagging mode progression. This result was confirmed by studies of the thermotropic response of the symmetric CH2 stretching vibrations, a qualitative index of acyl chain order. In contrast, the membrane lipid extracts (i) show much more conformational disorder from 5 to 25 °C than either the live cells or the membranes, (ii) undergo a gel–liquid transition over a broader temperature range and with a reduced magnitude of change in the symmetric CH2 stretching frequency, and (iii) demonstrate a second transition centered at 50 °C, which is detected by a large increase in the localized CH2 wagging mode (1368 cm−1) that arises from conformationally disordered kink + gtg states. The current results are in good accord with some prior 2H NMR and differential scanning calorimetry studies, but are in substantial disagreement with prior FT-IR investigations of similar systems. Possible reasons for the discrepancies are suggested.
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