Modeling the Bacterial Photosynthetic Reaction Center. 4. The Structural, Electrochemical, and Hydrogen-Bonding Properties of 22 Mutants of <i>Rhodobacter sphaeroides</i>

Site-directed mutagenesis has been employed by a number of groups to produce mutants of bacterial
photosynthetic reaction centers, with the aim of tuning their operation by modifying hydrogen-bond patterns
in the close vicinity of the “special pair” of bacteriochlorophylls P ≡ PLPM. Direct X-ray structural measurements
of the consequences of mutation are rare. Attention has mostly focused on effects on properties such as carbonyl
stretching frequencies and midpoint potentials to infer indirectly the induced structural modifications. In this
work, the structures of 22 mutants of Rhodobacter sphaeroides have been calculated using a mixed quantum-mechanical molecular-mechanical method by modifying the known structure of the wild type. We determine
(i) the orientation of the 2a-acetyl groups in the wild type, FY(M197), and FH(M197) series mutants of the
neutral and oxidized reaction center, (ii) the structure of the FY(M197) mutant and possible water penetration
near the special pair, (iii) that significant protein chain distortions are required to assemble some M160 series
mutants (LS(M160), LN(M160), LQ(M160), and LH(M160) are considered), (iv) that there is competition for
hydrogen-bonding between the 9-keto and 10a-ester groups for the introduced histidine in LH(L131) mutants,
(v) that the observed midpoint potential of P for HL(M202) heterodimer mutants, including one involving also
LH(M160), can be correlated with the change of electrostatic potential experienced at PL, (vi) that hydrogen-bond cleavage may sometimes be induced by oxidation of the special pair, (vii) that the OH group of tyrosine
M210 points away from PM, and (viii) that competitive hydrogen-bonding effects determine the change in
properties of NL(L166) and NH(L166) mutants. A new technique is introduced for the determination of ionization
energies at the Koopmans level from QM/MM calculations, and protein-induced Stark effects on vibrational
frequencies are considered.