On the nature of mutual inactivation between [Cp*Rh(bpy)(H2O)]2+ and enzymes – analysis and potential remedies

Pentamethylcyclopentadienyl rhodium bipyridine ([Cp*Rh(bpy)(H2O)]2+) is a versatile catalyst to promote biocatalytic redox reactions. However, its major drawback lies in the mutual inactivation of [Cp*Rh(bpy)(H2O)]2+ and the biocatalyst. This interaction was investigated using the alcohol dehydrogenase from Thermus sp. ATN1 (TADH) as model enzyme. TADH binds 4 equiv. of [Cp*Rh(bpy)(H2O)]2+ without detectable decrease in catalytic activity and stability. Higher molar ratios lead to time-, temperature-, and concentration-dependent inactivation of the enzyme suggesting [Cp*Rh(bpy)(H2O)]2+ to function as an ‘unfolding catalyst’. This detrimental activity can be circumvented using strongly coordinating buffers (e.g. (NH4)2SO4) while preserving its activity as NAD(P)H regeneration catalyst under electrochemical reaction conditions.