CH PI INTERACTION IN THE PACKING OF THE ADENINE RING IN PROTEIN STRUCTURES/

One of the weak but directional interactions in protein structures inv olves the O-H or N-H bond that is oriented along the center of a benze ne ring. Even a CH group can have enthalpically favorable interaction with an aromatic ring if the latter is made electron-rich by incorpora ting nitrogen atoms. This CH/pi interaction is brought into play in th e binding of the adenine rings, which are sandwiched between protein r esidues such that saturated carbon atoms are on top of ring nitrogen a toms at distances of approximately 3.7 Angstrom. There is a prepondera nce of residues with branched side-chains that have specific locations on the tertiary fold that is employed for binding the adenine-contain ing cofactors. In addition to the conventional hydrogen bonding, the C H/pi interaction can be important for the recognition of DNA and RNA m olecules by proteins. The main- and the side-chain atoms of the same r esidue can participate in both types of interaction, so that a protein can engage an adenine moiety by employing only a limited number of re sidues. (C) 1995 Academic Press Limited