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

CEITEC Central European Institute of Technology Masaryk University Kamenice 5 A4 62500 Brno Czech Republic

Center for Free Electron Laser Science DESY Notkestraße 85 22607 Hamburg Germany

Department of Chemistry University of Hamburg Grindelallee 117 20146 Hamburg Germany

Department of Condensed Matter Physics Faculty of Science Masaryk University Kotlářská 2 61137 Brno Czech Republic

INSERM U 1134 DSIMB 75739 Paris France

Institut National de la Transfusion Sanguine 75739 Paris France

Laboratoire d'Excellence GR Ex 75739 Paris France

National Center for Biomolecular Research Faculty of Science Masaryk University Kamenice 5 A4 62500 Brno Czech Republic

Université Paris Diderot Sorbonne Paris Cité UMRS 1134 75739 Paris France

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Lone pair-π interactions in biological systems: occurrence, function, and physical origin