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| - A Combined Computational and Experimental Study of the Hydrogen-Bonded Dimers ofXanthine and Hypoxanthine
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| - In addition to uracil, the noncanonical nucleobases xanthine and hypoxanthine are important lesions that areformed from the canonical bases when a cell is under oxidative stress. It is known that they lead to pointmutations; however, more detailed information about their ability to form hydrogen-bonded complexes is notavailable. In the present paper such information is obtained by a combined experimental and theoreticalapproach. Accurate association constants of xanthosine and inosine dimers are determined by concentrationdependent 1H NMR experiments, and a structural characterization of individual complexes formed in solutionis performed through measurements under slow exchange conditions at very low temperatures. An interpretationof the experimental data concerning complex geometries becomes possible through a comparison of measuredand computed NMR chemical shifts. Further qualitative insights into the hydrogen bonding abilities of xanthineand hypoxanthine are obtained by a theoretical characterization of all possible pairing modes of xanthine andhypoxanthine dimers and by a comparison with simplified model systems. The influence of a polar mediumon the bonding properties is also estimated and the importance of the various effects is discussed. Our analysisshows to what extent secondary electronic and electrostatic effects influence the hydrogen bonding propertiesof xanthine and hypoxanthine in the gas phase and in polar solvents.
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