A mono- and bilayer study of homologous branched-chain lecithins

Abstract

Abstract Electron spin resonance (E.S.R.) studies of the bulk phase and film balance measurements of the monolayers at the air-water interface of some members of homologous rac-1-acyl-2-hexadecyl-glycerophosphocholines (1-(2C m -16:0)-2-H-PC with m = length of the side chain and PC = phospholipid) are reported. The results of the E.S.R. measurements using fatty acid spin labelled near the terminal methyl group suggest that in the lamellar gel phase of the short-chain branched phospholipids (m < 9) the hydrocarbon chains are interdigitated. In the monobranched phospholipid with m = 14 a gel phase with non-interdigitated chains is assumed. The F/A isotherms (F = film pressure and A = molecular area) of four branched-chain lecithins were measured over a wide range of temperatures. A comparison of the isotherms at similar reduced temperatures indicates that both condensed and liquid-expanded states occupy increasing molecular areas with increasing m. The phospholipid with m = 8 does not form condensed films. The temperature interval of the transition region between the liquid-expanded and the condensed films which is confined by T o (lowest temperature at which the liquid-expanded film occurs) and the critical temperature T o decreases with increasing m and passes through a minimum at m ≈ 9. Although the T o-values are very similar, the T c -values are strongly dependent on m. The values of T c for the monolayer transition and T m for the main transition in bulk are identical for m = O (unbranched) and for m = 14, whereas for the phospholipids with m = 3, 4 and 8 the values of T c are very much lower than values of T m . This is discussed in terms of the interdigitation of the lecithins in the gel phase.