Denisov cycle

Another explanation of the UV protection mechanism of HALS involves the hydroxylamine ethers (XXIX) formed in Reaction 1.96. There is indirect evidence that (XXIX) can react very quickly with peroxy radicals, thereby regenerating nitroxyl radicals (Reaction 1.97). Reaction 1.96 and Reaction 1.97, which constitute the Denisov cycle, result in an overall slowdown of the usual chain oxidation Reaction 1.80 and Reaction 1.81. 

At the very beginning it was found that HALS do not absorb UV light, and so they cannot be classified as UV-absorbers or quenchers of excited states. The first mechanism proposed to explain HALS stabilization activity is known as the Denisov cycle. This mechanism attributes the key role in stabilization to HALS transformation products (nitroxyl radicals and hydroxylamino ethers), and it fits the generally accepted pathway of hydrocarbon degradation through alkyl- and alkylperoxy-radicals (Figure 3) ... 

Keywords stabilization, mechanism, hydrocarbon degradation, nitroxyl radical, Denisov cycle. 

The transformed hydroxylamines and dialkylhydroxylamine ethers are assumed to play a key role in the mechanism of stabilization of HAS, through a regenerative inhibition cycle known as the Denisov cycle [Eqs. (90a)-(90c)] [50]. 

The intermediary N-O-R formed by the reaction with the peroxy radical ROO is returned to the reactive nitroxyl radical in a regenerating process or Denisov cycle [43]. 

Scheme 18.16 Mechanism of action of HA(L)S according to the Denisov cycle.

In some cases, two-electron transfer to a metal ion leads to the formation of a reducing form. This gives a possibility to construct catalytic cycles. Oxidation of ethylene with Pd on catalysis of Cu is a striking example (Denisov 1978), that is,... 

Figure 19 Proposed general catalytic cycle of cytochrome P-450s (reactions (1)-(3) are abortive). Adapted from I. G. Denisov T. M. Makris S. G. Sligar I. Schlichting, Chem. Rev. 2005, 105, 2253-2277.

Mak PJ, Denisov IG, Victoria D, Makris TM, Deng T, Sligar SG, Kincaid JR (2007) Resonance Raman detection of the hydroperoxo intermediate in the cytochrome P450 enzymatic cycle. J Am Chem Soc 129 6382-6383... 

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