Clostridium difficile (CD) is the leading cause of bacterial hospital-associated infection in North America. How CD colonizes the human host, including its response to the innate immune system and other stresses, is poorly understood. This work considers CD's defenses against two stresses found in the host - the antimicrobial peptide LL-37 and reactive oxygen species (ROS). LL-37 had bactericidal activity against CD. CD strains varied in their sensitivity to the peptide, and epidemic-associated strains were more resistant to LL-37 than others. CD became more resistant to LL-37 following exposure to sub-lethal concentrations of the peptide, suggesting the presence of inducible resistance mechanisms. A quantitative proteomics analysis revealed definite alterations in CD protein expression caused by LL-37. Specific changes included increased expression of DltB, a protein previously reported to confer resistance against other antimicrobial peptides. Notably, disruption of individual LL-37-induced genes did not sensitize CD to the peptide. This suggests functional redundancy, and that LL-37 may cause global changes in protein expression, not limited to antimicrobial peptide resistance determinants. One of the proteins most strongly induced by LL-37 was a predicted superoxide reductase (SOR). As CD is considered a strict anaerobe, expression of a predicted antioxidant protein was an interesting finding. Heterologous expression of CD SOR in a superoxide dismutase-deficient E. coli strain confirmed its action as a superoxide scavenger. Insertional inactivation of SOR rendered CD more sensitive to oxygen and ROS-generating compounds, indicating that SOR contributes to antioxidant defense in CD. SOR mutants were impaired in their ability to cause disease in hamsters, indicating a role for this protein in infection.

The electronic structures and reaction mechanisms of transition-metal complexes can be calculated accurately by density functional theory (DFT) in cooperation with the continuum solvation model. The palladium catalyzed Heck reaction, iron-model complexes for cytochrome P450 and superoxide reductase (SOR), and tetrairon hexathiolate hydrogenase model were investigated. The DFT calculations on the catalytic Heck reaction (between phenyl-bromide and ethylene to form the styrene product), catalyzed by palladium diphosphine indicate a four-step mechanism: oxidative addition of C6H5Br, migratory insertion of C6H5 to C2H4, b-hydride transfer/olefin elimination of styrene product, and catalyst regeneration by removal of HBr. For the oxidative addition, the rate-determining step, the reaction through monophosphinopalladium complex is more favorable than that through either the diphosphinopalladium or ethylene-bound monophosphinopalladium. In further study, for a steric phosphine, PtBu3, the oxidative-addition barrier is lower on monopalladium monophosphine than dipalladium diphosphine whereas for a small phosphine, PMe3, the oxidative addition proceeds more easily via dipalladium diphosphine. Of the phosphine-free palladium complexes examined: free-Pd, PdBr-, and Pd(h2-C2H4), the olefin-coordinated intermediate has the lowest barrier for the oxidativeaddition. P450 and SOR have the same first-coordination-sphere, Fe[N4S], at their active sites but proceed through different reaction paths. The different ground spin states of the intermediate FeIII(OOH)(SCH3)(L) model {L = porphyrin for P450 and four imidazoles for SOR} produce geometric and electronic structures that assist i) the protonation on distal oxygen for P450, which leads to O-O bond cleavage and formation of (FeIV=O)(SCH3)(L) H2O, and ii) the protonation on proximal oxygen for SOR, which leads to (FeIII-HOOH)(SCH3)(L) formation before the Fe-O bond cleavage and H2O2 production. The hydrogen bonding from explicit waters also stabilizes FeIII-HOOH over FeIV=O H2O products in SOR. The electrochemical hydrogen production by Fe4[MeC(CH2S)3]2(CO)8 (1) with 2,6-dimethylpyridinium (LutH ) were studied by the DFT calculations of proton-transfer free energies relative to LutH and reduction potentials (vs. Fc/Fc ) of possible intermediates. In hydrogen production by 1, the second, more highly reductive, applied potential (-1.58 V) has the advantage over the first applied potential (-1.22 V) in that the more highly reduced intermediates can more easily add protons to produce H2.

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