Excitation transport and trapping in a synthetic chlorophyllide substituted hemoglobin: orientation of the chlorophyll S1 transition dipole.

Excitation transport in synthetic zinc chlorophyllide substituted hemoglobin has been observed by pico -second time-resolved fluorescence depolarization experiments. In this hybrid molecular system, two zinc chlorophyllide molecules are substituted into the beta-chains of hemoglobin, while deoxy hemes remain in the alpha-chains. The rate of excitation transfer between the two chlorophyllides is analyzed in terms of the distance and orientation dependences predicted by the F orster dipole-dipole theory. In this analysis, the beta-beta interchromophore geometry is assumed to be that of the deoxyhemoglobin crystal structure. When combined with steady-state fluorescence depolarization data of the complementary hybrid containing zinc chlorophyllide in the alpha-chains, these experiments provide the necessary information to determine the orientation of the S1 transition dipole moment in the zinc chlorophyllide molecule. We also find that the fluorescence lifetime of the zinc chlorophyllide is 1.42 ns when the heme is in the deoxy state but 3.75 ns when the heme is ligated to carbon monoxide. This is explained by irreversible excitation transfer from the S1 state of the zinc chlorophyllide to the lower energy excited states present in deoxy heme.