UV resonance Raman spectra of ligand binding intermediates of sol-gel encapsulated hemoglobin

We report for the first time specific conformational changes for a homogeneous population of ligand-bound adult deoxy human hemoglobin A (HbA) generated by introducing CO into a sample of deoxy-HbA with the effector, inositol hexaphosphate, encapsulated in a porous sol-gel. The preparation of ligand-bound deoxy-HbA results from the speed of ligand diffusion relative to globin conformational dynamics within the sol-gel (1). The ultraviolet resonance Raman (UVRR) difference spectra obtained reveal that E helix motion is initiated upon ligand binding, as signaled by the appearance of an α14β15 Trp W3 band difference at 1559 cm^-1. The subsequent appearance of Tyr (Y8a and Y9a) and W3 (1549 cm^-1) UVRR difference bands suggest conformational shifts for the penultimate Tyrα140 on the F helix, the 'switch' region Tyrα42, and the 'hinge' region Trpβ37. The UVRR results expose a sequence of conformational steps leading up to the ligation-induced T to R quaternary structure transition as opposed to a single, concerted switch. More generally, this report demonstrates that sol-gel encapsulation of proteins can be used to study a sequence of specific conformational events triggered by substrate binding because the traditional limitation of substrate diffusion times is overcome.