Hydrogen-transfer reactions, water

Ethylene dehydrogenation was poisoned by oxygen, and direct hydrogen transfer reactions between water and oxygen and between methanol and oxygen were observed. 

To understand the fundamental photochemical processes in biologically relevant molecular systems, prototype molecules like phenol or indole - the chromophores of the amino acids tyrosine respective trypthophan - embedded in clusters of ammonia or water molecules are an important object of research. Numerous studies have been performed concerning the dynamics of photoinduced processes in phenol-ammonia or phenol-water clusters (see e. g. [1,2]). As a main result a hydrogen transfer reaction has been clearly indicated in phenol(NH3)n clusters [2], whereas for phenol(H20)n complexes no signature for such a reaction has been found. According to a general theoretical model [3] a similar behavior is expected for the indole molecule surrounded by ammonia or water clusters. As the primary step an internal conversion from the initially excited nn state to a dark 7ta state is predicted which may be followed by the H-transfer process on the 7ia potential energy surface. 

In this section, we discuss the photoinduced hydrogen transfer from phenol to water and ammonia in phenol-water and phenol-ammonia clusters, respectively, as a representative model of excited-state chromophore-to-solvent hydrogen transfer reactions. 

The 17r<7 states also dominate the photoinduced processes in hydrogen-bonded chromophore-solvent clusters. The photoinduced hydrogen transfer reaction is experimentally and computationally well documented in clusters of phenol and indole with ammonia [14,16,32], There is no clear evidence for the existence of an excited-state proton transfer process in these systems [14], The same conclusion applies to bi functional chromophores solvated in finite clusters, such as 7HQ-ammonia and 7HQ-water clusters [15]. In future work, the photochemistry of larger and biologically relevant chromophores (such as tyrosine, tryptophan, or the DNA bases) should be investigated in a finite solvent environment. 

Several of the most common hydrogen donors, such as formic acid and formates, ascorbic acid, EDTA or 2-propanol are well or at least sufficiently soluble in water. In addition, H20 itself can serve as a source of hydrogen. Frequently, hydrogenation of unsaturated substrates is achieved by using C0/H20 mixtures such reactions are discussed in 3.8. As written in eq. (3.11) the hydrogen transfer reaction is often reversible, an obvious example being the reduction of ketones using 2-propanol as donor. 

Fig. 12. Hydrogen transfer reactions catalyzed by Lactobacillus lekhmannii ribonucleotide reductase via deoxyadenosylcobalamin. (Co) indicates the cobalt-corrin complex. Hydrogen isotopes ( H) are exchanged between water and coenzyme, or vice versa, in presence of enzyme and a dithiol (i.e., reduced enzyme, E(SH)2) and an allosteric effector (not drawn) by reactions 1-3. Reaction 4 indicates degradation of the radical pair (center) to 5 -deoxyadenosine and cob(II)alamin. Reduction of a ribonucleotide substrate is described in step 5 the nucleotide radical (proposed in >) is a hypothetic intermediate...

The radical part of the reaction mechanism was developed similar to other radical mechanisms in supercritical water [61, 62], which are only modifications of free-radical mechanisms at low pressures. The reaction classes considered are initiation reactions, P-scissions, hydrogen-transfer reactions, radical isomeriza-tions, radical additions, radical dehydratizations, radical substitutions, and radical-termination reactions. 

---