DNA-protein π-interactions in nature: abundance, structure, composition and strength of contacts between aromatic amino acids and DNA nucleobases or deoxyribose sugar

Four hundred twenty-eight high-resolution DNA-protein complexes were chosen for a bioinformatics study. Although 164 crystal structures (38% of those searched) contained no interactions, 574 discrete pi-contacts between the aromatic amino acids and the DNA nucleobases or deoxyribose were identified using strict criteria, including visual inspection. The abundance and structure of the interactions were determined by unequivocally classifying the contacts as either pi-pi stacking, pi-pi T-shaped or sugar-pi contacts. Three hundred forty-four nucleobaseamino acid pi-pi contacts (60% of all interactions identified) were identified in 175 of the crystal structures searched. Unprecedented in the literature, 230 DNA-protein sugar-pi contacts (40% of all interactions identified) were identified in 137 crystal structures, which involve C-H center dot center dot center dot pi and/ or lone-pair center dot center dot center dot pi interactions, contain any amino acid and can be classified according to sugar atoms involved. Both pi-pi and sugar-pi interactions display a range of relative monomer orientations and therefore interaction energies (up to -50 (-70) kJ mol(-1) for neutral (charged) interactions as determined using quantum chemical calculations). In general, DNA-protein pi-interactions are more prevalent than perhaps currently accepted and the role of such interactions in many biological processes may yet to be uncovered.