Near-ultraviolet magnetic circular dichroism spectroscopy of protein conformational states: Correlation of tryptophan band position and intensity with hemoglobin allostery

The near-UV magnetic circular dichroism spectroscopy of the aromatic amino acid bands of hemoglobin was investigated as a potential probe of structural changes at the α₁β₂ interface during the allosteric transition. Allosteric effectors were used to direct carp and chemically modified human hemoglobins into the R (relaxed) or T (tense) state in order to determine the heine-ligation-independent spectral characteristics of the quaternary states. The tryptophan magnetic circular dichroism (MCD) peak observed at 293 nm in the R state of N-ethylsuccinimide- (NES-) des-Arg- modified human hemoglobin (Hb) was shifted to a slightly longer wavelength in the T state, consistent with the shift expected for tryptophan acting as a proton donor in a T-state hydrogen bond. Moreover, the increase observed in the T-state MCD intensity of this band relative to the R-state intensity was consistent with the effect expected for proton donation by tryptophan on the basis of the Michl perimeter model of aromatic MCD. The peak-to-trough magnitude of the R - T MCD difference spectrum is equal to 30% of the total R-state peak intensity contributed by all six tryptophans present in the human tetramer; the relative magnitude specific to the two β37 tryptophans undergoing conformational change is estimated accordingly to be 3 times larger. The Trp-β37 spectral shift, about 200 cm^-1, is in good agreement with the shifts observed in other H-bonded proton donors and provides corroborating spectral evidence for the formation in solution of a T-state Trp β37-Asp α94 hydrogen bond observed in X-ray diffraction studies of deoxyHb crystals.