Electron paramagnetic resonance (EPR) at 15 K was used to probe the magnetic interaction between the visible copper CuA2+ and ferric cytochrome a in the carbon monoxide compound of beef heart cytochrome oxidase. At pH 8.6, the midpoint potentials (Em's) for one-electron oxidation of CuA+ and cytochrome a2+ were found to be 195 and 235 mV, respectively. Because the Em of CuA is well below that of cytochrome a under these conditions, the microwave power saturation of CuA could be measured as a function of percentage cytochrome a oxidized. Although progressive power saturation data directly provide only the product of the spin-lattice and transverse relaxation rates delta [1/(T1T2)], Castner's theory for the saturation of inhomogeneously broadened lines [Castner, T.G., Jr. (1959) Phys. Rev. 115 (6), 1506-1515], along with our own theoretical formulation of the dipolar T2, enabled us to determine the change in T1 of CuA due to dipolar relaxation by cytochrome a. The orientation of the principal g values of CuA with respect to those of cytochrome a was evaluated in partially oriented membranous multilayers. When allowance was made for uncertainties in the relative CuA-cytochrome a configuration and in the dipolar axis-magnetic field orientation, a range for the spin-spin distance r was calculated on the basis of the dipolar T1 of the gx component of CuA. This distance range was further restricted by consideration of T1 for the nonunique orientations of CuA giving rise to the gy signal. Only those values of r are possible for which the calculated T1 ratio (gx/gy) is equal to the experimentally determined ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

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