Musical mechanisms - ACS Publications

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Peter Jones University of Newcastle upon Tyne, Newcastle upon Tyne. NEI 7RU, U.K

Analogies hetween music and chemical reactions are very close. Both contain the concepts of pace and theme and a structure based on advancement in a sequence of unit processes (hars/elementary reactions). Catalytic processes introduce harmony in the coupled changes in catalyst and suhstrate(s) by which the overall reaction progresses. The turbulent catalysis of hydrogen peroxide decomposition by the haem-enzyme catalase has fascinated chemists and biochemists since the reaction was first observed by Louis-Jacques Thenard in 1818. The underlying reaction mechanism can readily he portrayed using an adaptation of musical notation. The five important substrate and product species (02.0.2-, H202,OH., HzO) are assigned to the five lines of a "substrate stave" (Fig. l a ) so that transitions between adjacent lines correspond to one-electron redox processes. Correspondingly the enzyme (an Fe(II1) species) and its derivatives are assigned to a "catalyst stave" (Fig. lb). The hasic catalase mechanism is shown in Fimre 2a. where each bar represents a mechantstic step. The r& constants for the maior catalvtic reactions are very large (10'' dm3 mol-1 ~-'/~;estissim"o) hut not diffusiod c o ~ r o l l e d (prestississimo). Note that the enzyme is almost completely successful in avoiding formation of the biologically dangerous free radicals 0.7- and OH.-althouph compound I1 gradually accumulate^ and inhibits the vig&ous reaction. If catalase is stripped of its protein and porphyrin ligand-leaving only a Fe3+ ion-catalysis still occurs if pH