Disentangling the dynamics, location and conformations of n-SASL (n = 5, 12, 16) spin labels in DPPC model membranes in LC phase and their relation to EPR signal by Molecular Dynamics Simulations

GASTALDI, SALOMÉ; GARAY, A. SERGIO; RODRIGUES, DANIEL E.

Santiago del Estero

Congreso; Reunión Anual SAB 2015, Sociedad Argentina de Biofísica; 2015

Sociedad Argentina de Biofísica

p { margin-bottom: 2.12mm; }Spin-labeledstearic acid (n-SASL) species are commonly used for electronparamagnetic resonance (EPR) studies of cell membranes to investigatephase transitions, fluidity and also as a fluorescense quencher.Nevertheless the dynamics and locations of this label in membranesare usually assumed based in some heuristic models. The orientationdynamics of the doxyl radical of the n-SASL is related to the EPRsignal and the location to the environment of the membrane it isprobing. We performedMolecular Dynamics simulation of fully hydrated DPPC membranes of 512lipid in LC-phase with the n-SASL (n=5,12,16) label inserted inconcentrations similar to those used under typical experimentalconditions (0.4%mol<1%). Long simulations have been run (450ns) towarrant an appropriate sampling of the equilibrium space of thelabeled molecule, its environment and to allow precise calculationsof the selfcorrelation rotational functions up to 1ns. Thetrajectories of four independent labeled molecules have beensimulated to be sure that the probability distributions of depthprofiles of different moieties of the SASL within the membrane havereached an appropriate sampling.We have calculatedthe equilibrium depth distributions of the CO2, n-C, and C18 moietiesof the n-SASL labels. We found that for all n the CO2 depth profileis relatively narrow and located between the phosphate and carbonylgroups of the lipids of the membrane. At this position it is able tointeract with water and the choline groups. The depth distribution ofthe doxyl group are broad for n=12-16, but particularly for theformer.The rotationaldynamics of the doxyl ring normal can be characterized as asub-diffusive regime with a power that is similar for all n.Nevertheless the time scale of such dynamics change two orders ofmagnitude in going from n=5 to n=16. We explore the relation of thesechanges with the allowed conformational space of the backbone ofn-SASL.