Phenylalanine forms an aromatic hydrogen bond—an interaction between the pi-electron cloud of an aromatic ring and a hydrogen-bond donor—with cytosine in the [Phe181]CAP-DNA complex.
[Parkinson, G., Gunasekera, A., Vojtechovsky, J., Zhang, X., Kunkel, T., Berman, H., and Ebright, R. (1996). Aromatic hydrogen bond in sequence-specific protein-DNA interaction. Nature Struct. Biol., 3, 837-841.]