Component volumes of unsaturated phosphatidylcholines in fluid bilayers: a densitometric study

Abstract

The specific volumes of six 1,2-diacylphosphatidylcholines with monounsaturated acyl chains (diCn:1PC, n = 14–24 is the even number of acyl chain carbons) in fluid bilayers in multilamellar vesicles dispersed in H 2O were determined by the vibrating tube densitometry as a function of temperature. From the data obtained with diCn:1PC ( n = 14–22) vesicles in combination with the densitometric data from Tristram-Nagle et al. [Tristram-Nagle, S., Petrache, H.I., Nagle, J.F., 1998. Structure and interactions of fully hydrated dioleoylphosphatidylcholine bilayers. Biophys. J. 75, 917–925.] and Koenig and Gawrisch [Koenig, B.W., Gawrisch, K., 2005. Specific volumes of unsaturated phosphatidylcholines in the liquid crystalline lamellar phase. Biochim. Biophys. Acta 1715, 65–70.], the component volumes of phosphatidylcholines in fully hydrated fluid bilayers at 30 °C were obtained. The volume of the acyl chain CH and CH 2 group is V CH = 22.30 Å 3 and V C H 2 = Å 3, respectively. The volume of the headgroup including the glyceryl and acyl carbonyls, V H, and the ratio of acyl chain methyl and methylene group volumes, r = V C H 3 : V C H 2 are linearly interdependent: V H = a − br, where a = 434.41 Å 3 and b = −55.36 Å 3 at 30 °C. From the temperature dependencies of component volumes, their isobaric thermal expansivities ( α X = V X − 1 ( ∂ V X / ∂ T ) where X = CH 2, CH, or H were calculated: α C H 2 = 118.4 × 10 − 5 K − 1 , α CH = 71.0 × 10 −5 K −1, α H = 7.9 × 10 −5K −1 (for r = 2) and α H = 9.6 × 10 −5 K −1 (for r = 1.9). The specific volume of diC24:1PC changes at the main gel–fluid phase transition temperature, t m = 26.7 °C, by 0.0621 ml/g, its specific volume is 0.9561 and 1.02634 ml/g at 20 and 30 °C, respectively, and its isobaric thermal expansivity α = 68.7 × 10 −5 and 109.2 × 10 −5 K −1 below and above t m, respectively. The component volumes and thermal expansivities obtained can be used for the interpretation of X-ray and neutron scattering and diffraction experiments and for the guiding and testing molecular dynamics simulations of phosphatidylcholine bilayers in the fluid state.