Fourier transform infrared spectroscopy (FTIR) of liganded and photoproduct states of oxymyoglobin (MbO(2)) and carbonmonoxymyoglobin (MbCO) at cryogenic temperatures reveals a number of modes sensitive to ligation state in the 1050-1150 cm(-1) region. This region contains contributions from the heme moiety, histidylimidazole side chains, and the dioxygen stretching frequency of MbO(2), all of which are influenced by photolysis and subsequent thermal recombination. For MbCO and MbO(2), we observe modes at 1080 cm(-1) (designated alpha) and 1096 cm(-1) (designated beta) that shift to lower frequencies upon photolysis and a third mode (designated gamma; in MbCO, 1107 cm(-1); in MbO(2), 1105 cm(-1)) that shifts to a higher frequency upon photolysis. As ligands rebind, these modes revert to their original positions. In addition, we observe an 1115 cm(-1) mode (designated epsilon) that is unique to compounds in which the iron atom is out of the heme plane. FTIR spectra of apomyoglobin (ape Mb) and heme model compounds : iron(III) octaethylporphyrin (FeOEP), iron(III) chloroprotoporphyrin IX (FePP IX), and (4MeIm)(2)FeOEP have aided in the assignment of the alpha- and beta-modes to heme peripheral vinyl groups and the gamma-mode to a C-H bending mode of the proximal (His-F8) histidylimidazole. We also present photolysis experiments that support the theory(1) that v(O double bond O) couples with a distal and/or proximal histidine mode. The dioxygen stretching frequency appears at 1105 cm(-1) overlapping a proximal histidylimidazole C-H bending mode at the same frequency. The frequency shifts we observe In the alpha-, beta-, and gamma-modes upon photolysis of the liganded state reflect the changing structural environment of the heme and distal and proximal heme pockets. Thus, those bands are sensitive probes of structural perturbations in hemeproteins. We observe two different vinyl deformation bands (alpha and beta), one for each peripheral vinyl group in the heme moiety, arising from unique protein environments. Synchronous, 4 cm(-1) red shifts of these bands upon photolysis indicate that these modes are sensitive to the electronic changes that are known to occur in the porphyrin pi system upon photolysis. The blue shift of the gamma-mode may be attributed to changes in the interaction between the heme and proximal histidine upon photolysis, which occurs as the histidine tilts and the iron-histidine bond length shortens.