Structure and dynamics of stearic acid spin label (n-SASL) in CHAPS micelles studied by molecular dynamics simulations

GARAY A SERGIO; HERRERA FERNANDO E; PRUVOST PAULA; BARCAROLO DAIANA; RODRIGUEZ DANIEL E

Congreso; XLIII Reunión Anual de la Sociedad Argentina de Biofísica; 2014

dimethylammonio]-1-propanesulfonate) is frequently used to extract intrinsic proteins frommammalian cells with mild perturbations of their native activity. EPR spectroscopy had beenused to characterize their micelles spin-labeled by n-SASL. We present in this work a studyof the structure and dynamics of CHAPS micelles labeled with n-SASL using MolecularDynamics simulations. The rotational dynamics of the doxyl moiety of the n-SASL moleculedetermines the EPR spectra, therefore it is relevant to investigate how it is affected by themotion of the micelle as a whole and the degrees of freedom of the labeled molecule.We performed several MD simulations of micelles of 14 CHAPS with one of the n-SASLspin labels (n=5,12,16) solvated in water. The simulations have been started from a solutionof CHAPS molecules and the n-SASL distributed uniformly in the system. Long simulations(>400ns) have been run for each n to allow for the formation of the micelles and theequilibration of their components. The arrival to the equilibrium have been followed bycalculating the temporal evolution of the gyration radii tensor, and the radial distributions ofthe CO2, n-C and C18 groups of the n-SASL referred to the center of mass of the micelle inconsecutive time intervals.The rotational dynamics of the micelle as a whole can be characterized as a normal diffusiveregime whose correlation time are in the interval 1.4ns (n=5) to 7.3ns (n=16) and correlatewell with their gyration radii. The rotational dynamics of the doxyl ring follow instead a subdiffusiveregime with a power similar for all n, but with different time scales 1.0ns (n=5),1.2ns (n=12) and 0.2 (n=16). The differences in these values can be explained by theinteractions of the moieties of the n-SASL with the CHAPS molecules and the solvent. Weexplore the relevance of the specie of the spin label molecule in determining the shape of themicelle.