ELECTRON SPIN-LATTICE RELAXATION OF THE [CU(1.5)CENTER-DOT-CENTER-DOT-CENTER-DOT-CU(1.5)] DINUCLEAR COPPER CENTER IN NITROUS-OXIDE REDUCTASE

Relaxation times have been obtained with time-domain EPR for the dinuc lear mixed valence [CuA(1.5)... Cu-A(1.5)] S = 1/2 center in nitrous o xide reductase, N(2)OR, from Pseudomonas stutzeri, in the TN5 mutant d efective in copper chromophore biosynthesis, in a synthetic mixed vale nce complex, and in type 1 and 2 copper complexes. Data confirmed that the intrinsic electron spin-lattice relaxation time, T-1, for N(2)OR in the temperature range of 6-25 K is unusually short for copper cente rs. At best, a twofold increase of T-1 from g(perpendicular to) to g(p arallel to) was measured. Optimized fits of the saturation-recovery da ta were obtained using both double-exponential and stretched-exponenti al functions. The temperature dependence of the spin-lattice relaxatio n rate of mutant N(2)OR is about T-5.0 with the stretched-exponential model or T-3.3 and T-3.9 for the model using the sum of two exponentia ls. These T(1)s are intrinsic to the mixed valence [Cu-A(1.5)... Cu-A( 1.5)] center, and no interaction of the second copper center in wild-t ype N(2)OR with the [Cu-A(1.5)... Cu-A(1.5)] center has been observed. The T-1 of the mixed valence center of N(2)OR is not only shorter tha n for monomeric square planar Cu(II) complexes, but also shorter than for a synthetic mixed valence complex, Cu-2{N[CH2CH2NHCH2CH2NHCH2CH2]N -3}. The short T-1 is attributed to the vibrational modes of type 1 co pper and/or the metal-metal interaction in [Cu-A(1.5)... Cu-A(1.5)].