Spectroscopic and Redox Properties of sym1 and (M)F195H: Rhodobacter capsulatus Reaction Center Symmetry Mutants Which Affect the Initial Electron Donor

The redox properties, absorption, electroabsorption, CD, EPR, and P⁺Q_A ^- recombination kinetics have been measured for the special pairs of two mutants of Rhodobacter capsulatus reaction centers involving amino acid changes in the vicinity of the special pair, P. Both mutants symmetrize amino acid residues so that portions of the M-sequence are replaced with L-sequence: syml symmetrizes all residues between M187 and M203, whereas (M)F195H is a single amino acid subset of the syml mutation. (M)-F195H introduces a His residue in a position where it is likely to form a hydrogen bond to the acetyl group of the M-side bacteriochlorophyll of P. For both mutants compared with wild-type, (i) the redox potential is at least 100 meV greater, (ii) the P⁺Q_A ^- recombination rate is about twice as fast at room temperature, and (iii) the large electroabsorption feature for the Q_y band of P is shifted relative to the absorption spectrum. The comparison of the properties observed for the sym 1 and (M) F19 5 H reaction center mutants and the differences between these mutants and wild-type suggest that residue M195 is an important determinant of the properties of the special pair.