Intramolecular decay pathways

A useful tool to be gained from this theory is the predictive power based on the large-molecule limiting case requirement that, for an intramolecular decay pathway to be available to an "isolated" molecule, the condition pfV >> 1 must be met. When pf is small (usually for small molecules or small energy gaps between initial and final states), it is most likely that no final states are in resonance with the initial state and no mixing occurs an external perturbation is required to produce the transition and the process is observed to be collision-induced. Very small values of Pf, therefore, would indicate the possibility of small-molecule behavior. 

Prodi, A., C.J. Kleverlaan, M.T. Indelli, F. Scnadola, E. Alessio, and E. lengo (2001). Photophysics of pyridylporphyrin Ru(II) adducts Heavy-atom effects and intramolecular decay pathways, Inorg. Chem. 40, 3498-3504. 

Excited state lifetime tq is another important parameter to be controlled, especially if the photoactive complex is intended for bimolecular photochemical electron transfer. MLCT excited states of most polypyridine complexes decay both radia-tively and non-radiatively, with the respective rate constants and k r. The inherent excited state lifetime is defined as tq = l/( r + nr)- The non-radiative decay pathway in most cases prevails k r K. Hence To = 1 /km- Non-radiative decay of MLCT excited states can be treated as intramolecular electron transfer in the Marcus inverted region ... 

This has been already shown by Zimmerman and Paskovich [21] in the chemistry of bis(2,4,6-trimethylphenyl)carbene, which results in the formation of carbene dimer, suggesting that decay pathways of the hindered carbene are suppressed both inter- and intramolecularly and, hence, the concentration builds up in solution to the point where dimerization occurs ( Scheme 6). 

Since one of the decay pathways of l-51d is an intramolecular H abstraction from methyl groups at position 2, it is probable to extend the lifetime by using KDIEs. Thus we prepared bis[l-(2,6-dimethylnaphthyl)]diazomethane bearing fully deuterated 2-methyl groups and studied the kinetics of the carbene therefrom. 

The intramolecular electron transfer kg, subsequent to the rapid reduction, must occur because the Ru(III)-Fe(II) pairing is the stable one. It is easily monitored using absorbance changes which occur with reduction at the Fe(III) heme center. Both laser-produced Ru(bpy)3 and radicals such as CO (from pulse radiolysis (Prob. 15)) are very effective one-electron reductants for this task (Sec. 3.5).In another approach," the Fe in a heme protein is replaced by Zn. The resultant Zn porphyrin (ZnP) can be electronically excited to a triplet state, ZnP which is relatively long-lived (x = 15 ms) and is a good reducing agent E° = —0.62 V). Its decay via the usual pathways (compare (1.32)) is accelerated by electron transfer to another metal (natural or artificial) site in the protein e. g.. 

The preceding experiments prove that there is an intermediate on the reaction pathway in each case, the measured rate constants for the formation and decay of the intermediate are at least as high as the value of kcat for the hydrolysis of the ester in the steady state. They do not, however, prove what the intermediate is. The evidence for covalent modification of Ser-195 of the enzyme stems from the early experiments on the irreversible inhibition of the enzyme by organo-phosphates such as diisopropyl fluorophosphate the inhibited protein was subjected to partial hydrolysis, and the peptide containing the phosphate ester was isolated and shown to be esterified on Ser-195.1516 The ultimate characterization of acylenzymes has come from x-ray diffraction studies of nonspecific acylenzymes at low pH, where they are stable (e.g., indolylacryloyl-chymotrypsin),17 and of specific acylenzymes at subzero temperatures and at low pH.18 When stable solutions of acylenzymes are restored to conditions under which they are unstable, they are found to react at the required rate. These experiments thus prove that the acylenzyme does occur on the reaction pathway. They do not rule out, however, the possibility that there are further intermediates. For example, they do not rule out an initial acylation on His-57 followed by rapid intramolecular transfer. Evidence concerning this and any other hypothetical intermediates must come from additional kinetic experiments and examination of the crystal structure of the enzyme. 

Figure 20 Intramolecular electron-transfer pathways in hCp with the coupling decay value, p, for each. (Reprinted with permission from Ref 60. 2000 American Chemical Society)...

Ultimately, the Met(S.. N) radicals decayed via two different pH-dependent reaction pathways, (i) conversion into sulfur-sulfur, intramolecular, three-electron-bonded radical cations, Met(S.. S), and (ii) a proposed hydrolytic cleavage of the protonated form of the intramolecular, three-electron-bonded radicals Met(S.. N)/Met (S.. NH) I, followed by electron transfer and decarboxylation. Surprisingly, a-(alkylthio)alkyl radicals also enter the latter mechanism in a pH-dependent manner. 

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