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Reactivity and Accessibility of Cellulose, Hemicelluloses, and Lignins
Published in David N.-S. Hon, Chemical Modification of Lignocellulosic Materials, 2017
Figure 4 illustrates one of the generally accepted mechanisms for the cleavage of glycosidic linkages catalyzed by acid [109–112] or alkali [109,112–117]. Both reactions display common features. An initial rapid equilibrium-controlled process involves acidic protonation of the glycosidic oxygen or alkali ionization of the C2 OH. In the slow rate-determining step, the conjugate acid (13) decomposes in an unimolecular heterolysis to form a carbonium ion intermediate (75), whereas the C2 hydroxyl anion (77) undergoes an intramolecular displacement process to yield a 1,2-anhydride intermediate (18). Both acidic and alkaline hydrolysis reactions are significantly influenced by the type of glycosides as illustrated in Table 2 for a variety of methyl pyranosides pertinent to wood polysaccharides [117–120].
The interaction of carbon-centered radicals with copper(I) and copper(II) complexes*
Published in Journal of Coordination Chemistry, 2018
Thomas G. Ribelli, Krzysztof Matyjaszewski, Rinaldo Poli
In subsequent work, it was shown that the interaction between radicals and L/CuII compounds proceeds via two competing pathways: direct atom transfer, ISET (path a in Scheme 3), which is the basis of ATRA and ATRP, and a second pathway named oxidative substitution, via the organocopper(III) intermediate (path b in Scheme 3). The second step of path b involves heterolysis of the L/CuIII-R bond to produce carbocations, as suggested by the observation of polar solvent effects, by the analysis of structural rearrangements and by the results of H/D isotope labelling experiments [34]. The atom transfer pathway usually represents the major course of the reaction, with the alternative pathway becoming preferred only for alkyl radicals capable of forming stabilized carbenium ions. The ratio between the two pathways is somewhat influenced by the copper coordination sphere. These dichotomy was also observed later in ATRP [35, 36].
Theory of chemical bonds in metalloenzymes XXII: a concerted bond-switching mechanism for the oxygen–oxygen bond formation coupled with one electron transfer for water oxidation in the oxygen-evolving complex of photosystem II
Published in Molecular Physics, 2019
K. Yamaguchi, M. Shoji, H. Isobe, K. Miyagawa, K. Nakatani
Five steps (Si; i = 0∼4) have been elucidated for water oxidation in OEC of PSII, providing the Kok cycle [49,50] for water oxidation (see supporting Fig. S2). Two different mechanisms have been proposed at the S2 to S3 transition in the Kok cycle: (I) no insertion of a new water molecule and (II) insertion of the water molecule at the water oxidation site. Two important papers concerning with the S3 state of OEC of PSII were recently published in relation to the water oxidation in Equation (1). Young et al. (Berkeley group) recently reported the S3 (two flash irradiation) structure of OEC of PSII by the serial femtosecond crystallography (SFX) method [51–54], concluding no insertion of hydroxide anion in the S1 to S3 transition, supporting the former assumption I. Suga et al. (Okayama group) [52] elucidated the SFX difference map responsible for the insertion of water molecule in the S1 to S3 transition, supporting the latter assumption II. Key conclusions on the S3 structure of OEC of PSII are different between SFX results by Berkeley [51] and Okayama [52] groups at the present stage. The observed O(5)–O(6) (newly inserted oxygen) distance by the SFX [52] was 1.5 Å, suggesting a possibility of the O–O bond formation in the S3 state. However, the O–O bond formation in the S3 state indicated the Ca(II)Mn(III)a(4)Mn(IV)b(3)Mn(IV)c(2)Mn(III)d(1), namely (3443), valence state in contradiction to early EXAFS and XES results [3,12]. This may indicate the necessity of further refinements of the SFX S3 structure [52] by theoretical calculations in combination with EXAFS, XES and EPR results [3–31]. We have performed extensive theoretical investigation for elucidation of geometrical structures of possible intermediates in the S3 state, and for understanding possible mechanisms of water oxidation in relation to different conclusions by Berkeley [51], Okayama [52] and many other groups. In this paper, we fully examine the structure and reactivity of the CaMn4O5 cluster in OEC of PSII on the theoretical grounds, proposing a concerted bond switching (CBS) mechanism for the O–O bond formation via the smooth switching between the Mn4–O(5) and O(5)–O(6) bonds coupled with one electron transfer (OET) for reductions of the high-valent Mn ions in OEC of PSII. Implications of present CBS-OET mechanism are briefly discussed in relation to radical coupling (RC) and acid-base (AB) mechanisms for water oxidation in artificial and native photosynthesis systems where two high-valent Mn = O bonds are formed at reaction sites. As an alternative mechanism for CBS-OET, a proton-coupled electron transfer (PCET) mechanism with participation of Tyrosine radical is finally touched in relation to the O–O bond formation in OEC and the oxygen heterolysis in cytochrome c oxidase (CcO).