Spin Labels n-SASL (n = 5, 12, 16) Dynamics in Fluid Phase of DPPC Model Membranes: a Molecular Dynamics Study to Understand the EPR spectra features and their relation to membrane properties.

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

San Miguel de Tucumán

Congreso; IIILAFeBS, IX IberoAmerican congress of Biophysics, XLV SAB Annual Meeting; 2016

Sociedad Argentina de Biofísica

The interpretation of the Electronic Paramagnetic Resonance (EPR) spectra of spin labels used in biomembrane investigations is still based in heuristic models and assumptions about location, environment and orientational dynamics of the probe. This fact limits the scope of inferences provided by the experimental technique. We simulated, by Molecular Dynamics, hydrated DPPC (Dipalmitoylphosphatidylcholine) bilayers of 480 lipids dopped with 5, 12 and 16-SASL (Spin Label Stearic Acid) in fluid phase (LC) to analyze the location and orientational dynamics of the doxyl ring containing the unpaired electron, source of the magnetic properties associated with the EPR spectra. We used molar concentration of the spin labels similar to those employed in experimental conditions (<1%) to avoid for exchange narrowing of the spectra. This condition also assures that the properties of the host membrane were not modified by the molecules of the probes. Four independent long simulations (>450ns) were performed to overcome the problem of the poor statistical sampling due to the low concentration of the spin label. The results for the three spin label species show that their polar head locate approximately at the same depth inside de membrane. The interactions that determine this behavior were analyzed. The depth interval sampled by each spin label specie were determined. The Rotational Autocorrelation Functions of the normal to the doxyl ring were calculated and the rotational dynamics were identified as anomalous (subdiffusive) with correlation times τ R = 4, 0.54 and 0.059 ns for 5, 12 and 16-SASL respectively. The one order of magnitude change in the rotational correlation times τ R among different spin label species was associated with the conformational freedomof the portion of the carrier fatty acid between the polar head and the site of the doxyl ring (5, 12 and 16). The results were in good agreement with the EPR spectra of these spin labels in oriented samples of egg DPPC.