title: Low-temperature NMR studies on the geometry of base pairs involving 5-substituted uracil derivatives.
abstract: Uridine (U) imino proton chemical shifts, measured in the slow hydrogen-bond exchange regime at low temperatures in a freonic solution, show that electron-withdrawing 5-bromo and 5-fluoro substituents on the uracil base strengthen NHN hydrogen bonds in (X)U·A base pairs formed by the free nucleosides. Whereas the halogens do not alter the preferential formation of Watson-Crick geometries, self-associates of the halouracils point to a more favorable engagement of the 2-carbonyl as proton acceptor in the cyclic hydrogen bonds, suggesting increased formation of reverse geometries and wobble base pairs with guanine when compared to the thymine base. Employing (15)N-labeled 5-bromouridine, no noticeable population of minor enol tautomers is found in the freonic mixture at 113 K.
authors.affiliation: Institute of Biochemistry, Ernst-Moritz-Arndt University Greifswald, Felix-Hausdorff-Straße 4, D-17487 Greifswald, Germany.
authors.forename: Eline M
authors.initials: EM
authors.lastname: Basílio Janke
authors.forename: Klaus
authors.initials: K
authors.lastname: Weisz
mesh_terms: D000241:Adenosine / Q000737:chemistry; D020029:Base Pairing; D001976:Bromouracil / Q000031:analogs & derivatives; D003080:Cold Temperature; D005472:Fluorouracil / Q000737:chemistry; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D009587:Nitrogen Isotopes / Q000737:chemistry; D013936:Thymidine / Q000737:chemistry; D014498:Uracil / Q000737:chemistry*; D014529:Uridine / Q000031:analogs & derivatives / Q000737:chemistry
chemical_list: D009587:Nitrogen Isotopes; D001976:Bromouracil; D014498:Uracil; D000241:Adenosine; C006824:5-bromouridine; D005472:Fluorouracil; D013936:Thymidine; D014529:Uridine