The unusual and varied spin dynamics of vitamin B12

In many ways the chemistry of coenzyme B12 is synonymous with radical mechanism. In its best-described role as cofactor to metabolic enzymes, B12 generates a radical pair upon substrate binding that triggers catalysis. The same radical pair is produced following photolysis, which undergoes reaction dynamics on the ultrafast timescale. In this lecture, I will provide an overview of the complex interplay between the reaction and spin dynamics of B12 radical pairs. In doing so, I hope to emphasise some important features that impact radical pair spin dynamics, including: the precise nature of the constituent radicals; how they are formed; the nature of their environment; and their reactivity in a particular context. B12 thus not only serves to illustrate how varied spin dynamics can be, but also that one should be cautious when making assumptions about how spin effects might manifest in biology. I will finish the lecture by summarising how the B12-dependent photoreceptor protein, CarH, appears to avoid radical chemistry altogether.