PROTEIN DYNAMICS AND EPR-SPECTROSCOPY - COMPARISON OF MOLECULAR DYNAMIC SIMULATIONS WITH EXPERIMENTS

Electron paramagnetic resonance (EPR) experiments and molecular dynami cs simulations on the residual motion of hemoglobin bound spin labels provide information about dynamical and rate processes in proteins. Ra pid (ps time range) reorientational fluctuations of the nitroxide ring of the spin labels within a conformational substate can be experiment ally characterized and distinguished from low frequency (ns time range ) transitions between the conformational substates which are observed above 200K in solved samples. A comparison of the experimentally deter mined magnitudes and frequencies of the reorientational motions and th e results of molecular dynamics simulations for several nitroxide loca lizations yields good agreement for the short time dynamics. Evidence for relatively large scale motions involving rare jumps are observed i n the molecular dynamics trajectories. A simple approach using Kramers ' theory and a two state jump model permits to combine EPR results and molecular dynamics simulations in the case of these rare events.