THERMODYNAMICS OF INTERDIGITATED PHASES OF PHOSPHATIDYLCHOLINE IN GLYCEROL

Comparison of the electron spin resonance spectra of phosphatidylcholi nes spin-labeled in the sn-2 chain at a position close to the polar re gion and close to the methyl terminus indicate that symmetrical satura ted diacyl phosphatidylcholines with odd and even chain lengths from 1 3 to 20 C-atoms (and probably also 12 C-atoms) have gel phases in whic h the chains are interdigitated when dispersed in glycerol. The chain- length dependences of the chain-melting transition enthalpies and entr opies are similar for phosphatidylcholines dispersed in glycerol and i n water, but: the negative end contributions are smaller for phosphati dylcholines dispersed in glycerol than for those dispersed in water: d Delta H-t/dCH(2) = 1.48 (1.43) kcal . mol(-1), d Delta S-t/dCH(2) = 3 .9 (4.0) cal . mol(-1)K(-1), and Delta H-o = -12.9(-15.0) kcal . mol(- 1), Delta S-o = -29(-40) cal . mol(-1)K(-1), respectively, for dispers ions in glycerol (water). These differences reflect the interfacial en ergetics in glycerol and in water, and the different structure of the interdigitated gel phase.