ELECTROSTATICS AND HYDRATION AT THE HOMEODOMAIN-DNA INTERFACE - CHEMICAL PROBES OF AN INTERFACIAL WATER CAVITY

Electrostatics and hydration of a homeodomain-DNA complex are dissecte d by chemical modification. Selective neutralization of phosphate char ges by methylphosphonate substitution demonstrates the differential im portance of short- and long-range electrostatic interactions. Whereas the footprint of direct contacts is in accord with crystal structures, interference is also observed at non-contacted sites. Such sites adjo in a novel interfacial water cavity in the major groove. Non-contacted phosphodiester groups in the cavity are proposed to contribute to lon g-range ordering of an extended protein-water-DNA interface. Use of is olated Sp and methylphosphonate diastereomers demonstrates that interf erence extended interface is stereoselective and charge-independent. A ttenuation of protein binding presumably reflects groove-specific reor ganization of bound water. Surprisingly, such attenuation can exceed t hat due to neutralization of a direct phosphate-side-chain salt bridge . These results support the hypothesis that hydration of an interfacia l cavity functions as a non-covalent extension of the DNA surface. Ste reospecific interrogation of bound water by chemical synthesis provide s a general method to assess the coupling between solvation and DNA re cognition. (C) 1997 Academic Press Limited.