Dynamics and reactivity of HbXL99α: A cross-linked hemoglobin derivative

Resonance Raman spectroscopy, transient absorption, and fluorescence techniques have been employed to investigate the structure and dynamics of the α-cross-linked hemoglobin derivative, HbXL99α. The resonance Raman spectra of the deoxy form of Hb-XL99α are identical to those of native NbA (ν_Fe-His ≈ 222 cm^-1), which exhibit a T-state (low affinity) structure regardless of solvent conditions. The resonance Raman spectra of the transient heme photoproduct resulting from CO photolysis from HbXL99α appear to have structures intermediate between deoxy-T and ligand-bound R structures (ν_Fe-His ≈ 222 cm^-1). Time-resolved resonance Raman data of HbXL99α-CO show that complete CO recombination occurs after ≈5 ms, with only a small amount of the CO-bound species reforming within ≈200 ns (geminate recombination). Transient absorption spectra of HbXL99α-O₂ indicate that the extent of sub-nanosecond geminate recombination of O₂ is also reduced in the cross-linked derivative relative to native HbA. The decrease in tryptophan fluorescence of HbXL99α upon oxygenation further indicates that tertiary structural changes at the α₁-β₂ interface upon ligation are apparently reduced, but not eliminated in the cross-linked derivative relative to HbA.