Infrared spectra of carbonyl hemoglobins: characterization of dynamic heme pocket conformers.

The infrared spectra for carbon monoxide complexed to hemoglobins were examined in the C-O stretch region. Deconvolution of the spectra requires four bands and supports the presence of four distinct conformers at the ligand binding site. Most typical hemoglobins exhibit only one predominant conformer for each subunit represented by a band at 1951 cm-1 in contrast to myoglobins, which typically exist in two major conformations. Several hemoglobins with an enlarged heme pocket are shown to shift the C-O frequency into the higher frequency conformer regions. Many factors, including pH, temperature, solvents, and divalent metals, are also shown to be capable of expanding the heme pocket. Only very specific structural changes that can reduce the size of the heme pocket will result in the lower frequency conformers. The weighted averages of the multiple CO vibrational frequencies are linearly related to the single 13CO NMR chemical shift values and to the exponential of fast CO on-rates. Conformer interconversion occurs at a rate greater than 10(4) s-1. The infrared C-O stretch spectra provide qualitative and quantitative information on the structural dynamics, stability, and ligand binding properties of hemoglobins.