Translational diffusion of lipid monomers determined by spin label electron spin resonance

Abstract

The collision rates between spin-labelled valeric acid in water, and between the corresponding mixed-chain, spin-labelled phosphatidylcholine in water-methanol mixtures, and also between spin-labelled phosphatidylcholine monomers and micelles in water have been determined from the spin-spin broadening of the electron spin resonance spectrum. In each case the second order rate constants are consistent with a diffusion-controlled process. For spin-labelled valeric acid in water the translational diffusion coefficient at 20°C is 3.4 · 10 −6 cm 2 · s −1, and for spin-labelled phosphatidylcholine varies between 2.3 · 10 −6 and 3.8 · 10 −6 cm 2 · s −1 within the range 44 to 88 wt% methanol. The spin-labelled phosphatidylcholine monomer diffusion coefficient in water at 20°C is 2.4 · 10 −6 cm 2 · s −1, deduced from the monomer-micelle association rate, with an activation energy of 4.0 kcal · mol −1. The much slower on-rates for association of lipid monomers with phospholipid bilayer vesicles reported in the literature, therefore indicate that incorporation into bilayers is not a diffusion-controlled process.