QUANTITATIVE IR STUDIES OF ACYL-CHAIN CONFORMATIONAL ORDER IN FATTY-ACID HOMOGENEOUS MEMBRANES OF LIVE CELLS OF ACHOLEPLASMA-LAIDLAWII B

Acholeplasma laidlawii B has been grown highly enriched in myristic, p entadecanoic, and palmitic acids. The conformational order in the acyl chains of living cell membranes has been compared with that in the me mbranes of lysed cells and the lipids extracted therefrom, using Fouri er-transform infrared (FT-IR) spectroscopic techniques. A gel-liquid-c rystal-phase transition (25-45-degrees-C) was noted for both the live cells and the membranes. Surprisingly, CH2 wagging progessions, charac teristic of coupled oscillators from acyl chains in the all-trans conf ormation, 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 g rowth temperature (37-degrees-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-degrees-C), as measured by the thermotro pic responses of the k = 1 and k = 2 components of the wagging mode pr ogression. This result was confirmed by studies of the thermotropic re sponse of the symmetric CH2 stretching vibrations, a qualitative index of acyl chain order. In contrast, the membrane lipid extracts (i) sho w much more conformational disorder from 5 to 25-degrees-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 chan ge in the symmetric CH2 stretching frequency, and (iii) demonstrate a second transition centered at 50-degrees-C, which is detected by a lar ge increase in the localized CH2 wagging mode (1368 cm-1) that arises from conformationally disordered kink + gtg states. The current result s are in good accord with some prior H-2 NMR and differential scanning calorimetry studies, but are in substantial disagreement with prior F T-IR investigations of similar systems. Possible reasons for the discr epancies are suggested.