Reactions of Muonium with simple biochemical solutes in water and micelles

Rate constants are reported for the reaction of muonium atoms in water with some 36 organic compounds, many of interest in biology. These kM values range from < l05 to 7 × 1010 M−1 s−1, according to the type of reaction involved, with the sugar–phosphate backbones of nucleic acids being at the low end and their bases at the high end. They are compared with corresponding published H-atom data (kH), where possible, and show kinetic-isotope-effects ranging over five orders of magnitude. Since all kH data were obtained at pH = 1, while kM values refer to pH ~ 7 of the natural state, the pH-dependence of kM was examined in representative cases. The changes found result from protonation of the solute rather than a changed reactivity of Mu on being converted to MuH+. On localizing the solutes in the hydrophobic phase of dilute micelles, the reactivity of Mu was again measured (kM(mic)). The resulting "enhancement" factor was considered in terms of: the reaction mechanism, its dependence on microenvironment (solvation), and the concentrating effect of mutual confinement to small sections of a biphasal system. Key words: kinetic isotope effects, muonium, biochemicals, micelles.