title: Geometry and cooperativity effects in adenosine-carboxylic acid complexes.
abstract: NMR experiments and theoretical investigations were performed on hydrogen bonded complexes of specifically 1- and 7-15N-labeled adenine nucleosides with carboxylic acids. By employing a freonic solvent of CDClF2 and CDF3, NMR spectra were acquired at temperatures as low as 123 K, where the regime of slow hydrogen bond exchange is reached and several higher-order complexes were found to coexist in solution. Unlike acetic acid, chloroacetic acid forms Watson-Crick complexes with the proton largely displaced from oxygen to the nitrogen acceptor in an ion pairing structure. Calculated geometries and chemical shifts of the proton in the hydrogen bridge favorably agree with experimentally determined values if vibrational averaging and solvent effects are taken into account. The results indicate that binding a second acidic ligand at the adenine Hoogsteen site in a ternary complex weakens the hydrogen bond to the Watson-Crick bound carboxylic acid. However, substituting a second adenine nucleobase for a carboxylic acid in the trimolecular complex leads to cooperative binding at Watson-Crick and Hoogsteen faces of adenosine.
authors.affiliation: Institut für Organische Chemie, Universität Würzburg, Am Hubland, D-97070 Würzburg, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Schlund
authors.forename: Milena
authors.initials: M
authors.lastname: Mladenovic
authors.forename: Eline M
authors.initials: EM
authors.lastname: Basílio Janke
authors.forename: Bernd
authors.initials: B
authors.lastname: Engels
authors.forename: Klaus
authors.initials: K
authors.lastname: Weisz
mesh_terms: D000241:Adenosine / Q000737:chemistry*; D002264:Carboxylic Acids / Q000737:chemistry*; D003201:Computers; D006860:Hydrogen Bonding; D006878:Hydroxides / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D044367:Phase Transition; D013696:Temperature; D014732:Vibration
chemical_list: D002264:Carboxylic Acids; D006878:Hydroxides; C031356:hydroxide ion; D000241:Adenosine