Conformational sensitivity of .beta.-93 cysteine sulfhydryl to ligation of hemoglobin observed by FT-IR spectroscopy

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Abstract
The SH vibrational absorption of cysteine F9(.beta.-93) in concentrated aqueous solutions of native liganded hemoglobin (human HbA, horse, and bovine) has been observed by use of Fourier transform infrared spectroscopy. The pattern of .beta.-93 SH absorption intensity is ligand dependent. In bovine hemoglobin derivatives the SH absorption intensity pattern is (carbonmonoxy)hemoglobin (HbCO) > oxyhemoglobin (HbO2) = cyanomethemoglobin (HbCN) .mchgt. aquomethemoglobin (metHb) and deoxyhemoglobin (deoxyHb). In horse and human hemoglobin derivatives the pattern is HbCO .gtorsim. HbO2 > HbCN > metHb. The bovine metHb .beta.-93 SH shows a much lower absorptivity than that of horse or human metHb, and thus it has a different local tertiary equilibrium conformation than does horse or human hemoglobin. X-ray diffraction studies have shown the .beta.-93 SH in carbon monoxide or oxygen bound hemoglobin to be situated within a nonpolar pocket between the F, G, and H helices. The higher than usual SH absorption frequency (2592 cm-1) that we observe implies there is no hydrogen bonding for this thiol group while situated within this nonpolar pocket. A similar .beta.-93 SH absorption has been observed in the .beta.-chain tetramer (thalassemic hemoglobin H in vivo). The .beta.-112 SH stretching band, previously observed in the .alpha.2.beta.2 tetramer, was observed for the first time in the .beta.-chain tetramer. A band at 2610 cm-1 that is not due to SH was resolved and found to be ligand dependent.