The gas-phase unimolecular reactions of C 2 D 5 CHFCl molecules with 94 kcal mol −1 of vibrational energy have been studied by the chemical-activation experimental technique and by electronic-structure computations. Products from the reaction of C 2 D 5 CHFCl molecules, formed by the recombination of C 2 D 5 and CHFCl radicals in a room temperature bath gas, were measured by gas chromatography− mass spectrometry. The 2,1-DCl (81%) and 1,1-HCl (17%) elimination reactions are the principal processes, but 2,1-DF and 1,1-HF elimination reactions also are observed. Comparison of experimental rate constants to calculated statistical rate constants provides threshold energies. The potential surfaces associated with C 2 D 5 (F)C: + HCl and C 2 D 5 (Cl)C: + HF reactions are of special interest because hydrogen-bonded adducts with HCl and HF with dissociation energies of 6.4 and 9.3 kcal mol −1 , respectively, are predicted by calculations. The relationship between the geometries and threshold energies of transition states for 1,1-HCl elimination and carbene:HCl adducts is complex, and previous studies of related molecules, such as CD 3 CHFCl, CD 2 ClCHFCl, C 2 D 5 CHCl 2 , and halogenated methanes are included in the computational analysis. Extensive calculations for CH 3 CHFCl as a model for 1,1-HCl reactions illustrate properties of the exit-channel potential energy surface. Since the 1,1-HCl transition state is submerged relative to dissociation of the adduct, inner and outer transition states should be considered for analysis of rate constants describing 1,1-HCl elimination and addition reactions of carbenes to HCl.

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Chemical activation experiments and computational methods have been used to study the unimolecular reactions of C2H5CH2Br and C2D5CHFBr with 90 and 93 kcal mol–1 of vibrational energy, respectively. The four-centered elimination reactions of HBr and DBr are the dominant reactions; however, 2,1-DF, 1,1-HBr, and 1,1-HF reactions are also observed from C2D5CHFBr. The main focus was to search for the role of the C2D5(F)­C:HBr adduct in the 1,1-HBr elimination for comparison with carbene adducts in 1,1-HX­(Y) elimination from RCHXY (X,Y = Cl and F) molecules. Models of transition states and molecules from electronic structure calculations were used in statistical calculations of the rate constants to assign threshold energies for each reaction based on the experimental rate constants. The threshold energy for 2,1-HBr elimination from 1-bromopropane is 50 kcal mol–1, which is in basic agreement with thermal activation experiments. Comparison of the 2,1-DBr and 2,1-HBr rate constants permits discussion of the kinetic isotope effects and the effect of F atom substitution on the threshold energy for 2,1-HBr elimination. Although CD3CDCDF from 1,1-HBr elimination of C2D5CHFBr followed by D atom migration is an experimentally observed product, dissociation of the C2D5(F)­C:HBr adduct may be the rate-limiting step rather than crossing the barrier associated with the transition state for 1,1-HBr elimination. The calculated dissociation energies of C2H5(X)­C:HF adducts are 9.9, 9.3, and 9.0 kcal mol–1 for X = F, Cl, and Br, and the values for C2H5(F)­C:HX are 9.9, 6.4, and ∼4.9 kcal mol–1.

F1Fo ATP synthase is a ubiquitous molecular motor that utilizes a rotary mechanism to synthesize adenosine triphosphate (ATP), the fundamental energy currency of life. The membrane-embedded Fo motor converts the electrochemical gradient of protons into rotation, which is then used to drive the conformational changes in the soluble F1 motor that catalyze ATP synthesis. In E. coli, the Fo motor is composed of a c10 ring (rotor) alongside subunit a (stator), which together provide two aqueous half channels that facilitate proton translocation. Previous work has suggested that Arg50 and Thr51 on the cytoplasmic side of each subunit c are involved in the proton translocation process, and positive charge is conserved in this region of subunit c. To investigate the role of these residues and the chemical requirements for activity at these positions, we generated eleven substitution mutants and assayed their in vitro ATP synthesis, H+ pumping, and passive H+ permeability activities, as well as the ability of mutants to carry out oxidative phosphorylation in vivo. While polar and hydrophobic mutations were generally tolerated in either position, introduction of negative charge caused a substantial defect. We discuss the possible effects of altered electrostatics on the interaction between the rotor and stator, water structure in the aqueous channel, and interaction of the rotor with phospholipids.

F1Fo ATP synthase is the ubiquitous biomolecular machine that catalyzes the final step of oxidative phosphorylation and is therefore the primary producer of ATP across all domains of life. The membrane‐embedded Fo motor converts the electrochemical gradient of protons into rotation, which is then used to drive the conformational changes in the soluble F1 motor that catalyze ATP synthesis. In E. coli, the Fo motor is composed of a c10 ring (rotor) alongside subunit a (stator), which provides two aqueous half channels that facilitate proton translocation. The mechanism by which proton translocation is converted into torque on the c‐ring is not fully defined. Previous work has suggested that conserved residues aAsp92, aGlu196, and cArg50 in the proton exit pathway are important for proton transport and possibly for torque generation. To clarify the roles of these residues, we generated 22 mutants and assayed their growth on succinate and in vitro ATP synthesis, H+ pumping, and permeability activities. Mutations of aGlu196 had only a mild effect on proton pumping, while moderately inhibiting ATP synthesis. These results indicate that aGlu196 is likely not interacting with cArg50 but interestingly do suggest that it may have a greater role in ATP synthesis than proton pumping. In contrast, mutations of aAsp92 were not well tolerated, and mutations that reverse the charge of cArg50 caused a substantial defect. These results along with the structural proximity of these residues suggest that they may interact electrostatically across the rotor stator interface.