Molecular and component volumes of N,N-dimethyl-N-alkylamine N-oxides in DOPC bilayers

The volumetric properties of fluid bilayers formed of dioleoylphosphatidylcholine (DOPC) with incorporated N,N-dimethyl-N-alkylamine N-oxides (CnNO, n = 6, 10-18 is the even number of carbons in alkyl chain) were studied by vibrating tube densitometry in the temperature interval from 20 °C to 50 °C. It was found that the DOPC and CnNO mixed ideally in the investigated composition range and hence the molecular volumes of DOPC (VDOPC) and incorporated CnNO (VCnNO) were constant and additive within error limits. From the temperature dependencies of the molecular volumes of DOPC and CnNO their coefficients of isobaric thermal expansivities in the investigated temperature interval were obtained. The VCnNO volumes of CnNO incorporated into DOPC bilayers showed linear dependencies on the CnNO alkyl chain length at all measured temperatures. This allowed to calculate the component volume of the CnNO methylene group (VCH2) at several temperatures and its coefficient of isobaric thermal expansivity. Using the assumption that the component volume of the CnNO methyl group VCH3=2VCH2 we also calculated the component volume and the coefficient of isobaric thermal expansivity of the hydrophilic group of CnNO (VNO). We found that the VCH2 volume increases in the whole temperature interval whereas the VNO volume decreases.