Water Molecules in DNA Recognition II: A Molecular Dynamics View of the Structure and Hydration of the trp Operator

The structure and hydration of the DNA duplex d-(AGCGTACTAGTACGCT)2 corresponding to the trp operator fragment used in the crystal structure of the half site complex (PDB entry 1TRR) was studied by a 1.4 ns molecular dynamics simulation in water. The simulation, starting from a B-DNA conformation, used a non-bonded cutoff of 1.4 nm with a reaction field correction and resulted in a stable trajectory. The average DNA conformation obtained was closer to the ones found in the crystal structures of the complexes (PDB entries 1TRO and 1TRR) than to the crystal structure of unbound trp operator (Nucleic Acid Database entry BDJ061). The DNA hydration was characterized in terms of hydrogen bond percentages and corresponding residence times. The residence times of water molecules within 0.35 nm of the DNA non-exchangeable protons were calculated for comparison with NMR measurements of intermolecular water-DNA NOEs and nuclear magnetic relaxation dispersion measurements. No significant difference was found between major and minor groove hydration. The DNA donors and acceptors were hydrogen bonded to water molecules for 77(+/-19)% of the time on average. The average residence time of the hydrogen bonded water molecules was 11(+/-11) ps with a maximum of 223 ps. When all water molecules within NOE distance (0.35 nm) of non-exchangeable protons were considered, the average residence times increased to an average of 100(+/-4) ps and a maximum of 608 ps. These results agree with the experimental NMR results of Sunnerhagen et al. which did not show any evidence for water molecules bound with more than 1 ns residence time on the DNA surface. The exchange of hydration water from the DNA occurred in the major groove primarily through direct exchange with the bulk solvent, while access to and from the minor groove frequently proceeded via pathways involving ribose O3' and O4' and phosphate O2P oxygen atoms. The most common water diffusion pathways in the minor groove were perpendicular to the groove direction. In general, water molecules visited only a limited number of sites in the DNA grooves before exiting. The hydrogen bonding sites, where hydrogen bonds could be formed with donor and acceptor groups of the DNA, were filled with water molecules with an average B-factor value of 0.58 mn2. No special values were observed at any of the sites, where water molecules were observed both in the trp repressor/operator co-crystals and in the crystal structure of unbound DNA.