Protonation, of bound

Fig. 3. The Chatt cycle with its intermediates. The cycle is divided in three stages (a) protonation of bound N2, (b) cleavage of the N-N bond, releasing one molecule of NH3, and (c) reduction and protonation of nitrido complexes generating the second molecule of NH3.

The mechanism and sequence of events that control delivery of protons and electrons to the FeMo cofactor during substrate reduction is not well understood in its particulars.8 It is believed that conformational change in MoFe-protein is necessary for electron transfer from the P-cluster to the M center (FeMoco) and that ATP hydrolysis and P release occurring on the Fe-protein drive the process. Hypothetically, P-clusters provide a reservoir of reducing equivalents that are transferred to substrate bound at FeMoco. Electrons are transferred one at a time from Fe-protein but the P-cluster and M center have electron buffering capacity, allowing successive two-electron transfers to, and protonations of, bound substrates.8 Neither component protein will reduce any substrate in the absence of its catalytic partner. Also, apoprotein (with any or all metal-sulfur clusters removed) will not reduce dinitrogen. 

Possible intermediates of the form M=N—NH3 have been observed and characterized they are stable when M = W but react to form NH3 when M = Mo. In a cyclic sequence of reactions that can be driven chemically or electrochemically, protonation of bound N2 occurs in a stepwise manner, with electrons flowing from the metal atom as protons are picked up from solution. 

The simulation of the dynamics of protonation of bound Bromo Cresol Green, using Equations (37) and (38), failed to reproduce the observed dynamics as long as we assumed that the measured pK is a direct function of the forward and backward rates of proton transfer. This inadequacy led to the conclusion that for this case Koh kJk and provision must be made for a mechanism where the postprotonation reaction modulates the observed dynamic and equilibrium. 

One proposed mechanism for CuNiR, which parallels catalysis by the well-characterized heme NiRs, involves the formation of a Cu -NO+ intermediate formed by protonation of bound nitrite followed by the abstraction of an oxygen atom to form OH . Although the Cu -NO intermediate has not been observed directly, the formation of N2O, a minor product of nitrite reduction by AfNiR, has been attributed to the reaction of a Cu nitrosyl intermediate with NO to form N2O. Consistent with this, large amounts of N20 were formed during turnover in the presence of N-labeled nitrite and N-labeled NO. 

The most important consequence of bound smoothing is the transfer of infonnation from those atoms for which NMR data are available to those that cannot be observed directly in NMR experiments. Within the original experimental bounds, the minimal distance intervals are identified for which all triangle inequalities can be satisfied. A distance chosen outside these intervals would violate at least one triangle inequality. Eor example, an NOE between protons pi and pj and the covalent bond between pj and carbon Cj imposes upper and lower bounds on the distance between pi and Cy, although this distance is not observable experimentally nor is it part of E hem-... 

J. I. Friedman and H. W. Kendall (Massachusetts Institute of Technology) and R. E. Taylor (Stanford) pioneering investigations concerning deep elastic scattering of electrons on protons and bound neutrons, of essential importance for the development of the quark model in particle physics. 

The retro-Claisen reaction occurs by initial nucleophilic addition of a cysteine -SH group on the enzyme to the keto group of the /3-ketoacyl CoA to yield an alkoxide ion intermediate. Cleavage of the C2-C3 bond then follows, with expulsion of an acetyl CoA enolate ion. Protonation of the enolate ion gives acetyl CoA, and the enzyme-bound acyl group undergoes nucleophilic acyl substitution by reaction with a molecule of coenzyme A. The chain-shortened acyl CoA that results then enters another round of tire /3-oxidation pathway for further degradation. 

Similarly protonation of /ac-IrMe3(PMe2Ph)3 with HBF4 gives IrMe2(BF4)(PMe2Ph)3 (XXVI), which has weakly bound BF4 (Ir—F 2.389 A) in solution the BF4 is easily displaced by neutral donors to give (XXVII) [168],... 

In reduced Fe2S2 there is a localization of valences between Fe(III) and Fe(II). The for both ions is shorter than that of the Fe(II) monomer (Table I), whereas the linewidths of the signals of the Fe(III) and Fe(II) domains depend on coefficients obtainable from the solution of Eq. (4). As a result, the signals of the H/3 protons of the cysteines bound to the Fe(III) are shifted beyond 100 ppm downfield with relatively large linewidths, while those of the cysteines bound to the Fe(II) domain are closer to the diamagnetic region and 5-10 times narrower (50-53) (Fig. 2B). There are cases in which there is delocalization of the valences (54, 55) but no NMR investigation is available. 

A Ni-bound H H species in the Ni-C form has been considered to be unlikely based on the very small hyperfine splitting observed due to exchangeable 78). It has been argued, however, that the observed small values could arise from an equatorially bound Ni hydride (79). It has also been postulated that the photolyzed hydrogen species contained in the Ni-C state is the proton of a thiol group bound to the Ni ion 80). 

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