Low-temperature femtosecond-resolution transient absorption spectroscopy of large-scale symmetry mutants of bacterial reaction centers

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., and 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 Q(x) band of the B side bacteriopheophytin which could be attributed to 10%-20% electron transfer along the B pathway.