The two heme-copper terminal oxidases of Thermus thermophilus have been shown to catalyze the two-electron reduction of nitric oxide (NO) to nitrous oxide (N2O) [Giuffre, A.; Stubauer, G.; Sarti, P.; Brunori, M.; Zumft, W. G.; Buse, G.; Soulimane, T. Proc. Natl. Acad. Sci. U.S.A. 1999, 96,14718-14723]. While it is well-established that NO binds to the reduced heme a(3) to form a low-spin heme {FeNO}(7) species, the role CUB plays in the binding of the second NO remains unclear. Here we present low-temperature FTIR photolysis experiments carried out on the NO complex formed by addition of NO to fully reduced cytochrome ba(3). Low-temperature UV-vis, EPR, and FIR spectroscopies confirm the binding of NO to the heme a(3) and the efficiency of the photolysis at 30 K. The nu(NO) modes from the light-induced FTIR difference spectra are isolated from other perturbed vibrations using (NO)-N-15 and (NO)-N-15-O-18. The nu(N-O)a(3) is observed at 1622 cm(-1), and upon photolysis, it is replaced by a new nu(N-O) at 1589 cm(-1) assigned to a CUB-nitrosyl complex. This N-O stretching frequency is more than 100 cm(-1) lower than those reported for Cu-NO models with three N-ligands and for CUB-NO in bovine aa(3). Because the UV-vis and FIR data do not support a bridging configuration between CUB and heme a(3) for the photolyzed NO, we assign the exceptionally low nu(NO) to an O-bound (n(1)-O) or a side-on (n(2)-NO) Cu-B-nitrosyl complex. From this study, we propose that, after binding of a first NO molecule to the heme a(3) of fully reduced Tt ba(3), the formation of an N-bound {CuNO}(11) is prevented, and the addition of a second NO produces an O-bond Cu-B-hyponitrite species bridging CUB and Fea(3). In contrast, bovine cytochrome c oxidase is believed to form an N-bound CUB-NO species; the [{FeNO}(7){CuNO}(11)] complex is suggested here to be an inhibitory complex.