Time-resolved visible and infrared study of the Cyano complexes of Myoglobin and of Hemoglobin I from Lucina pectinata

The dynamics of the ferric CN complexes of the heme proteins Myoglobin and Hb I from the clam Lucina pectinata upon Soret band excitation is monitored using IR and broad band visible pump-probe spectroscopy. The transient response in the UV-vis spectral region does not depend on the heme pocket environment and is very similar to that known for ferrous proteins. The main feature is an instantaneous, broad, short-lived absorption signal that develops into a narrower red-shifted Soret band. Significant transient absorption is also obsd. in the 360-390 nm range. At all probe wavelengths the signal decays to zero with a longest time const. of 3.6 ps. The IR data on MbCN reveal a bleaching of the C == N stretch vibration of the heme-bound ligand, and the formation of a five-times weaker transient absorption band, 28 cm-1 lower in energy, within the time resoln. of the expt. The MbC == N stretch vibration provides a direct measure for the return of population to the ligated electronic (and vibrational) ground state with a 3-4 ps time const. In addn., the CN-stretch frequency is sensitive to the excitation of low frequency heme modes, and yields independent information about vibrational cooling, which occurs on the same timescale. [on SciFinder (R)]