Chlorophyllide-substituted hemoglobin tetramers and hybrids: preparation, characterization, and energy transfer

Three chlorophyllide-substituted human hemoglobin (Hb) complexes have been prepared: the tetrameric complex in which zinc pyrochlorophyllide alpha (ZnPChl alpha) is substituted for all four hemes and the two complementary hybrids in which ZnPChl alpha is substituted for heme in either the alpha- or beta-chains, while heme remains in the other chains. In each of these complexes, intramolecular Chl-Chl singlet energy transfer occurs. A variety of probes demonstrate that ZnPChl alpha-deoxyheme hybrids and the ZnPChl alpha-Hb complexes consistently exhibit properties associated with the well-known T-state tertiary and quaternary structure of deoxyHb itself. Using the known crystal structure of human deoxyHb, we have analyzed the steady-state fluorescence anisotropy of these complexes within the framework of the Förster energy-transfer theory. The result is the determination of the orientation of the Qy transition dipole moment of ZnPChl alpha. Nuclear magnetic resonance data for the hybrids offer insight into specific tertiary structural changes in the heme pocket surrounding the diamagnetic ZnPChl alpha, which accompany changes in the ligation state of the heme on the opposite chain.