Tuning of B 12 photochemistry in the CarH photoreceptor to avoid radical photoproducts.

Time-resolved infrared spectroscopy reveals the flow of electron density through coenzyme B 12 in the light-activated, bacterial transcriptional regulator, CarH. The protein stabilises a series of charge transfer states that result in a photoresponse that avoids reactive, and potentially damaging, radical photoproducts.