Two dimensional diffusion of small molecules on protein surfaces: an EPR study of the restricted translational diffusion of protein-bound spin labels.

Heisenberg spin exchange rates and dipole-dipole spin lattice relaxation rates for deuterated 14N- and 15N-spin labels bound selectively to the histidine His15 and to the lysines Lys13, 96, 97 of the lysozyme molecule have been determined with the aid of electron spin resonance spectroscopy. The results can be interpreted in terms of a two dimensional translational diffusion of the nitroxide tips of the spin labels along the protein surface within restricted surface areas. The spin labels are regarded as models for long amino acid side chains and as probes for the dynamics of protein and water in the vicinity of the protein surface. The translational diffusion coefficient DII is reduced by a factor of between six and thirty compared to the value for T = 295 K is given by (1.3 +/- 0.6) x 10 -10m2s-1 greater than or equal to DII greater than or equal to (2.4 +/- 0.3) x 10-11M2s-1.