CH₂ wagging progressions as IR probes of slightly disordered phospholipid acyl chain states

CH₂ wagging progressions have been measured in the IR spectra for a series of saturated gel phase phosphatidylcholines (PC's) over the range of chain lengths diC₁₃PC to diC₂₀PC. Band assignments have been confirmed from a dispersion curve which closely parallels that constructed by Snyder for solid alkanes. The intensity of the progression has been used to monitor the all-trans conformational state in phospholipid aqueous phases and in various regions of the 1,2-dipalmitoylphosphatidylcholine (DPPC)/cholesterol phase diagram. With the assumption that the intensity of the progression arises only from all-trans chains and is thus a highly nonlinear function of the number of gauche rotamers, the data suggest that about 1 gauche bond/chain exists in the "liquid-ordered" phase (33 mol % cholesterol) as compared with 3.6-4.2 gauche rotamers for the L_α phase of DPPC alone. The strong ordering effect of cholesterol on the DPPC acyl chains is thus evident. At 20 mol % cholesterol, a phase separation region is evident from 39 to 45°C. The conversion of the "liquid-ordered" to the "liquid-disordered" phase is accompanied by the formation of 0.15 double gauche conformers/chain, as monitored from localized CH₂ wagging vibrations. The role of the eight-carbon side chain of cholesterol in the ordering process at 33 mol % is revealed through studies of DPPC with 5-androsten-3-β-ol (androsten). Substantially more disorder is observed (∼2-2.5 gauche rotamers/chain at 50°C). The various IR measurements of acyl chain conformational order are compared.