Low-Temperature Femtosecond-Resolution Transient Absorption Spectroscopy of Large-Scale Symmetry Mutants of Bacterial Reaction Centers

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Abstract
Reaction centers isolated from three large-scale symmetry mutants, sym0, sym2−1, and sym5−2 described in the previous article of this issue [Taguchi, A. K. W., Eastman, J. E., Gallo, D. M., Jr., Sheagley, E., Xiao, W., & Woodbury, N. W. (1996) Biochemistry 35, 3175−3186] have been investigated by low-temperature ground state and femtosecond-resolution transient absorption spectroscopy. All three of these large-scale symmetry mutants undergo electron transfer at 20 K. The mutants sym0 and sym5−2 have yields and dominant rates of charge separation comparable to wild type. However, the sym2−1 mutant shows a roughly 35% quantum yield at this temperature, and the major kinetic component of the initial electron transfer is slower than wild type by nearly a factor of 100. The sym0 mutant showed substantial changes in the monomer bacteriochlorophyll ground state and transient spectra, and both sym0 and sym2−1 showed changes in the bacteriopheophytin ground state and transient spectra. In particular, sym2−1 shows a small absorbance decrease in the region of the QX band of the B side bacteriopheophytin which could be attributed to 10%−20% electron transfer along the B pathway.

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