A direct simulation of EPR slow-motion spectra of spin labelled phospholipids in liquid crystalline bilayers based on a molecular dynamics simulationof the lipid dynamics

EPR line shapes can be calculated from the stochastic Liouville equation as suming a stochastic model for the reorientation of the spin probe. Here we use instead and for the first time a detailed molecular dynamics (MD) simul ation to generate the stochastic input to the Langevin form of the Liouvill e equation. A 0.1 mus MD simulation at T = 50 degreesC of a small lipid bil ayer formed by 64 dipalmitoylphosphatidylcholine (DPPC) molecules at the wa ter content of 23 water molecules per lipid was used. In addition, a 10 ns simulation of a 16 times larger system consisting of 32 DPPC molecules with a nitroxide spin moiety attached at the sixth position of the sn2 chain an d 992 ordinary DPPC molecules, was used to investigate the extent of the pe rturbation caused by the spin probe. Order parameters, reorientational dyna mics and the EPR FID curve were calculated for spin probe molecules and ord inary DPPC molecules. The timescale of the electron spin relaxation for a s pin-moiety attached at the sixth carbon position of a DPPC lipid molecule i s 11.9 x 10(7) rad s(-1) and for an unperturbed DPPC molecule it is 3.5 x 1 0(7) rad s(-1).