Response of the local heme environment of (carbonmonoxy)hemoglobin to protein dehydration

The effects of protein dehydration upon the equilibrium and dynamic properties of the heme active site in human hemoglobin (HbA) have been probed by resonance Raman scattering. Spectra of equilibrium carbonmonoxy-HbA and the photolytic heme transient species generated within 10 ns of ligand photolysis have been obtained from thin films of protein in various stages of dehydration. These data provide detailed information concerning the response of the heme and its bonding interactions with both the proximal histidine and carbon monoxide as a function of protein hydration. For protein hydration levels of 0.4-1.0 g of H2O/g of protein, our results indicate that the C = O stretching mode of carbonmonoxy-HbA is dramatically affected by protein hydration levels, thus corroborating the infrared results of Brown et al. [Brown, W. E., Sutcliffe, J. W., & Pulsinelli, P. D. (1983) Biochemistry 22, 2914-2923]. However, we find that both heme skeletal modes and the Fe-C bond strength are largely insensitive to dehydration. Moreover, the proximal pocket geometry (as reflected in the behavior of the Fe-proximal histidine stretching mode) immediately following ligand photolysis was found to be very similar to that of R-state solution hemoglobin. At protein hydration levels below the theoretical monolayer limit, small changes in the resonance Raman spectra of both equilibrium HbCO and the transient heme species generated subsequent to ligand photolysis are detected. These include broadening of the Fe-C stretching mode in equilibrium HbCO and a small shift to lower frequency of the Fe-His mode in the photolytic transient species.(ABSTRACT TRUNCATED AT 250 WORDS)