Joe Frazier

Tautomerism and Protonation of Guanosine

Guanosine has been demonstrated, by infrared and nuclear magnetic resonance spectroscopy, to have a keto-amino structure in neutral aqueous solution and to undergo protonation at N7 in acid solution.

Poly 2-amino-6-N-methyladenylic acid: synthesis, characterization and interaction with polyuridylic acid.

Abstract A new polynucleotide containing adenylic acid residues modified by introduction of a 2-amino group and a 6- N -methyl group has been synthesized by enzymic polymerization of the nucleoside diphosphate. Properties of the polymer and its interaction products have been investigated by a variety of physical and spectroscopic methods. The polymer forms a non-regular stacked structure in neutral solution and a regular helical structure in acid. The former is more stable and the latter less stable thermally than the corresponding forms of poly A. In interesting contrast to the model proposed for acid poly A the acid helix of the new polymer cannot have an interchain amino-phosphate hydrogen bond. Both the nucleoside 2-amino-6- N -methyladenosine and poly 2NH 2 6Me A ‡ form 1:1 helical complexes with poly U. The effects of the 2-amino and 6- N -methyl groups on T m of the complexes with poly U are approximately additive and appear to be exerted independently of each other. The monomer-polymer helix provides a valuable model system for studying electrostatic interactions in polynucleotides since it has no interstrand electrostatic repulsion.

Polyadenylate polyuridylate helices with non-Watson-Crick hydrogen bonding.

Abstract Two-stranded helical complexes of poly(A) and poly(U) and their simple derivatives may, in principle, exist in structurally isomeric forms, having different hydrogen bonding arrangements and different strand polarities. Previous experimental studies of A-U interactions, however, have revealed only a single 1:1 complex in each case, presumably of Watson-Crick structure. We have investigated the possibility of alternative hydrogen bonding arrangements by introducing a substitution in poly(A) which will prevent formation of Watson-Crick pairs with poly(U) or poly(BrU) but which allows bonding at N (7) . Interaction of poly-2-dimethylaminoadenylic acid with poly(U) and with poly(BrU) has been investigated by infrared, ultraviolet and circular dichroism spectroscopy. The three spectroscopic methods provide consistent and compelling evidence of specific base pair formation between the complementary polynucleotides. These are the first two-stranded A · U helices shown to have non-Watson-Crick pairing. The melting temperature ( T m ) is 8 °C for the complex with poly(U) and 46 °C for that with poly(BrU) in 0.1 m -Na + . These relatively low values may be due either to an intrinsically less stable bonding scheme or to a non-specific destabilizing effect of the amino-methyl groups.

Infrared study of G · C complex formation in template-dependent oligo(G) synthesis

Abstract G · C complex formation was studied by infrared spectroscopy for a system that has been shown by Inoue & Orgel (1982) to give efficient, template-dependent synthesis of oligo(G). Guanosine-5′-phosphor-2-methylimiazolide (2-MeImpG) exhibits rapid formation with poly(C) of a G · C double helix at p D ~ 8 and of a C · G · CH + triple helix at p D ~ 6.5 in the presence of Na + . Significant oligo(G) synthesis does not occur under these conditions. In the presence of synthetically effective concentrations of Mg 2+ G · C complex formation is much slower but eventually goes to completion. The rate of complex formation parallels that of chemical synthesis. Infrared spectra and melting curves confirm that oligo(G) of high molecular weight is formed in high yield. The bulk of the G · C complex at any given time during the reaction is composed of G residues that have already been polymerized and not of the monomer 2-MeImpG. Evidence indicates that synthesis proceeds primarily at growing points at the ends of the G · C helical regions and not randomly on a fully occupied template.