Dipolar interactions with neighboring metal ions can cause enhanced spin-lattice relaxation of free radicals. We have applied the theory of dipolar relaxation enhancement and shown that the dependence of the enhanced relaxation on the protein structure surrounding the free radical can be used to obtain distances from the free radical to the protein surface. To test the theoretical predictions, we have examined the effect of added Dy3+ complexes on the microwave power saturation of free radicals in two protein complexes of known structure: myoglobin nitroxide and the reaction center from Rhodobacter sphaeroides. Three cases have been considered: (1) metal ions bound to a specific site, (2) metal ions bound randomly over the protein surface, and (3) metal ions distributed randomly in solution. Only case 3, which assumes no specific binding, gave good agreement between the distances obtained by using the two model systems. The effect of added Dy3+ complexes on the microwave power saturation of signal IIslow from photosystem II (PSII) was used to determine the location of the stable tyrosine radical giving rise to signal IIslow. Assuming that the surface of a membrane-bound protein can be approximated as planar, we have obtained distances from the tyrosine radical to the membrane surface in thylakoids, in PSII membranes, and in Tris-washed PSII membranes. The distances we have determined are in good agreement with those predicted on the basis of a structural homology between the D1 and D2 subunits of PSII and the structurally characterized L and M subunits of the reaction center from purple non-sulfur bacteria. We have also examined the temperature dependence of the microwave power at half-saturation (P1/2) of signal IIslow from 4 to 200 K in dark-adapted PSII membranes. Above 70 K, the P1/2 increases as T2.5, which is consistent with a Raman relaxation mechanism. But between 10 and 70 K, the P1/2 is nearly independent of temperature. Such temperature independence of the P1/2 is highly unusual.
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