Evidence for sub-picosecond heme doming in hemoglobin and myoglobin: a time-resolved resonance Raman comparison of carbonmonoxy and deoxy species.

Separation of the photophysical aspects of the sub-picosecond (sub-ps) time-resolved resonance Raman signal from contributions due to conformation has been achieved by comparing deoxyhemoglobin (Hb) in the T state with (carbonmonoxy)hemoglobin (HbCO), deoxy-beta 4 (beta 4 CO) (All R state), and monomers deoxymyoglobin and (carbonmonoxy)myoglobin (MbCO) [beta 4 consists of a tetramer of four beta-subunits and shows no cooperativity]. In all photolyzed species, Hb*(CO), Mb*(CO), and beta 4*(CO), the iron-histidine out-of-plane mode (vFe-His), indicative of heme doming, achieves 90% of its full intensity in 1 ps. The frequency of this mode (223-228 cm-1) is shifted significantly relative to equilibrium deoxy-Hb (210-216 cm-1) in the T state, but not with respect to either equilibrium deoxy-Mb or deoxy-beta 4. A correlation between the +12 cm-1 bandshift of vFe-His and the -2 cm-1 shift of the electron density marker band (v4 at 1370 cm-1) relative to T-state deoxy-Hb is shown to hold on all time scales, including the sub-picosecond time scale. Photolyzed Hb*(CO) consists of R-state or weakly interacting tetramers on the picosecond time scale and is shown to have properties similar to those of photolyzed Mb*(CO) and beta 4*(CO) on the picosecond time scale. These results establish that heme doming occurs as an ultrafast reaction to ligand dissociation and that heme doming is the primary event in the sequence of conformational changes leading to the cooperative R-->T transition.