Resonance Raman spectroscopy has been employed to investigate the reduced cyano complexes of cytochrome aa(3) from bovine heart and Rhodobacter sphaeroides and of cytochrome bo(3) from E coli. In the aa(3)-type oxidases, the frequency of the Fe-CN stretching mode is located at 468 cm(-1), and the bending Fe-C-N vibration, at 500 cm(-1). The fully reduced cytochrome bo(3)-CN complex gives rise to a stretching vibration at 468 cm(-1), a bending vibration at 491 cm(-1), and a stretching C-N vibration at 2037 cm-1. The observed differences between aa(3) and bo(3) oxidases in the frequencies of the Fe-C-N group suggest a quantitative difference in the structure of the His-heme a(3)(2+)/Cu-B(1+) and His-heme p(3)(2+)/Cu-B(1+) binuclear pockets upon CN- binding.