Absorption Band III Kinetics Probe the Picosecond Heme Iron Motion Triggered by Nitric Oxide Binding to Hemoglobin and Myoglobin

To study the ultrafast movement of the heme iron induced by nitric oxide (NO) binding to hemoglobin (Hb) and myoglobin (Mb), we probed the picosecond spectral evolution of absorption band III (similar to 760 nm) and vibrational modes (iron-histidine stretching, v(4) and v(7) in-plane modes) in time-resolved resonance Raman spectra. The time constants of band III intensity kinetics induced by NO rebinding (25 ps for hemoglobin and 40 ps for myoglobin) are larger than in Soret bands and Q bands. Band III intensity kinetics is retarded with respect to NO rebinding to Hb and to Mb. Similarly, the v((Fe-His)) stretching intensity kinetics are retarded with respect to the v(4) and v(7) heme modes and to Soret absorption. In contrast, band III spectral shift kinetics do not coincide with band III intensity kinetics but follows Soret kinetics. We concluded that, namely, the band III intensity depends on the heme iron out-of-plane position, as theoretically predicted (Stavrov, S. S. Biopolymers 2004, 74, 37-40).