N-OH-containing compounds play important roles in many biological, pharmacological, and industrial processes. The traditional emphasis on the metal ion chelating property of the compounds has recently been shifted to the redox chemistry of the N-OH site, which is of great interest in developing mediated oxidoreductase-based biocatalyses, such as laccase-catalyzed delignification, decontamination, and organic synthesis. In an N-OH-mediated laccase biocatalysis system, N-OH is first oxidized into N-O ·by laccase. As shown by a comparative study of 33 N-OH-containing compounds and seven fungal laccases, the oxidation is controlled by the electron-transfer from N-OH to laccase whose rate depends on the redox potential difference between laccase and N-OH. Higher redox potential tends to reduce the oxidation rate of N-OH, similar to the cases of other laccase substrates such as phenols. The redox potential of N-OH is related to the frontier molecular orbital energy, which is proportional to electron-withdrawing ability of N-phenyl substituents. Using cyclic and differential pulse voltammetry, the N-O· decay can be quantitated. The stability of N-O· varies, depending on the heat of formation of the radical. Lower redox potential, better delocalization/resonance, or more extensive steric effect tend to make N-O· more stable. However, stabilization of N-O· mitigates against its oxidation of the target molecule of the biocatalysis. Balancing the reactivity and stability of N-O· is key to the catalytic efficiency. The prospect of N-OH mediated laccase biocatalysis is discussed in terms of applying quantum calculation, rational design, and methodology development.
展开▼
