Chain selectivity of tyrosine contributions to hemoglobin static and time-resolved UVRR spectra in C-13 isotopic hybrids

Recombinant hemoglobin [Hb] has been labeled with C-13(6)-tyrosine in order to characterize the tyrosine bands in static and time-resolved ultraviolet resonance Raman [UVRR] spectra. The large isotope shift for the Y8a/ 8b ri ng modes permits complete resolution of the important 1580-1660 cm(-1) regi on. Underlying bands from tryptophan [W1 and W17+18] and from phenylalanine [F8a] make only small contributions to static T-R and 150 ns time-resolved difference spectra. Isotopic hybrid Hb's were constructed in order to eval uate tyrosine contributions separately for alpha and beta chains. The Y8a a nd Y8b bands shift up significantly [2.5 and 3.2 cm(-1)] in the alpha but n ot the beta chains for the T vs the R state. These shifts, along with an up shift of the Y9a band, are attributed to the T-state H-bonding ofTyr alpha 42, which is reinforced by a nearby positive charge. In the 150 ns time-res olved difference UVRR spectrum, both alpha and beta chains contribute to th e negative tyrosine bands. These are suggested to arise from H-bond weakeni ng of the penultimate residues, Tyr alpha 140 and beta 145, as a result of helix displacement in the initial [R-deoxy] intermediate along the path fro m the R to the T states.