Acholeplasma laidlawii B cells, highly enriched in saturated C14 and C15 fatty acids, have been grown at several temperatures. Conformational order in the acyl chains of live cell membranes has been monitored with Fourier-transform infrared (FT-IR) spectroscopy. Coupled CH2 wagging progressions, characteristic of the all-trans conformation, have been used to quantitatively monitor the extent of transgauche isomerization. A simple model relating the changes in the progression intensity to the introduction of gauche rotamers into the acyl chains suggests that at the growth temperature (25 or 37 °C for the C14-enriched cells, 30 or 37 °C for the C15-enriched cells) the cell membrane contains about 1.5 gauche bonds per acyl chain. The thermotropic response of the CH2 stretching frequencies near 2850 cm−1 reveals that the gel-liquid-crystal phase transition is shifted toward lower temperatures when the growth temperature is reduced. In addition, at any given temperature, the cells grown at lower temperature are more conformationally disordered than their counterparts grown at higher temperature. This behavior is consistent with the quantitative results from the CH2 wagging measurements. The data reveal that A. laidlawii B cells can control the overall conformational state of their membranes and that the observed degree of disorder (1.5 ± 0.3 gauche bonds/acyl chain), when achievable, provides optimum cell viability. The ability of this microorganism to control the degree of membrane disorder delineates one possible pathway for homeoviscous adaptation.
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