Anion‐π interactions in flavoproteins involve a substantial charge‐transfer component

Anion-pi interactions have been shown to stabilize flavoproteins and to regulate the redox potential of the flavin cofactor. They are commonly attributed to electrostatic forces. Here we show that anion-flavin interactions have a substantial charge-transfer component. Our conclusion emanates from a multiapproach theoretical analysis and is backed by previously reported observations of absorption bands, originating from charge transfer between oxidized flavin and proximate cystein thiolate groups. This partial covalency of anion-flavin contacts renders classical simulations of flavoproteins questionable.