The nature of conformational correlations in alpha- and beta-anomers of nucleic acid components in aqueous solution by nuclear magnetic resonance

1H NMR Conformational Study of a Variety of α-Anomers of C5-Substituted 2′-Deoxyuridines: Comparison to Their Antiherpetic β Counterparts

Although alpha-nucleosides are not found in nucleic acid, they do occur as constituents of smaller molecules in living cells, e.g., in vitamin B(12). There are now several examples of alpha-nucleosides exerting a biological activity in some instances equal to, or even exceeding, that of the corresponding beta-anomer. Examples include growth inhibitory properties against mouse leukemia cells and antitumor activity. From stereochemical point of view, alpha-anomers serve as references for studying of interaction of the base with the sugar moiety in beta-anomers and may help in better understanding of structure-activity relationships. One important problem preventing conformational analysis of alpha nucleosides is uncertainty in the determination of vicinal coupling constants from simulation of overlapping sugar proton resonances of strongly coupled spin systems. A successful resolution of near-isochronous H3' and H4' resonances made possible a full conformational analysis for a series of alpha-anomers C5-substituted 2'-deoxyuridines, including methyl, ethyl, isopropyl, fluor, vinyl, and bromovinyl, in comparison to their beta counterparts. Conformation of the sugar ring is determined from proton-proton coupling constants and described in terms of pseudorotation between two main puckering domains C2'endo (S) and C3'endo (N). A thorough analysis of chemical shifts as well as conformation of the sugar ring and C4'-C5' rotamers made possible determination of conformational preferences in equilibrium about the glycosidic bond between two regions, anti and syn. This work provides insights into the role of anomeric configuration of the base in conformational behavior of the sugar moiety, a link in the backbone of nucleic acids.

A code relating sequence to structure in nucleic acids

Nucleic acids are elucidated in configuration space. An algorithm relating sequence to stability in A and B helical secondary structures, is stated to incorporate NMR conformational and optical melting data. This made possible a classification of elementary sequences in terms of configuration forces driving between A and B states, a finding useful in prediction of structural behavior of different sequences of DNA, RNA and their hybrids.

1H NMR conformational study of antiherpetic C5-substituted 2'-deoxyuridines: insight into the nature of structure-activity relationships

1H NMR study and conformational analysis of a broad series of biologically important C5-substituted 2'-deoxyuridines, including alkyl, halogen, vinyl, hydroxymethyl, and hydroxy derivatives as well as nitro, formyl, trifluoromethyl, and dimethylamino substituents, is presented. A thorough analysis of chemical shifts in correlation with C5-substituent electronegativity as well as calculations by SCF semi-empirical method of the formal charge localized on C6 carbon is discussed in terms of charge distribution for electron attracting and electron donating groups. Conformation of the sugar ring is determined from proton-proton coupling constants and described in terms of pseudorotation between two main puckering domains C2'endo (S) and C3'endo (N). Generally, electron donating groups destabilise the N conformation, simultaneously decreasing the mean pseudorotation amplitude. Absolute assignments of the H5' and H5'' methylene protons in 1H NMR spectra permitted the unequivocal determination of molar fractions of the three classical exocyclic C4'-C5' rotamers gauche+, trans, and gauche-, and correlation of them with the sugar ring puckering domains. Conformation about the glycosidic bond is described in terms of equilibrium between two conformational regions, anti and syn. Finally, the role of the C5-substituent in the creation of cytotoxic activity is considered on the basis of a simplified model assuming that compound activity is a function of substituent polar surface, its molecular volume, and its molecule polarity defined at the relative partition of the polar atoms.

Thermodynamic cycle between DNA and RNA constituents for conformation of the sugar ring from nuclear magnetic resonance study

The effect of a structural change of ribose to deoxyribose, by replacement of 2'-OH by 2'-H, on the conformational equilibrium of the sugar ring is described in terms of one thermodynamic cycle. The method is based on the observation that conformational correlations of the sugar ring--side chain ensemble in DNA and RNA components show one general pattern, reflecting an intrinsic physical property of this ensemble. The pattern determines a choice of model systems to study. The systems consist of pairs of DNA and RNA components, nucleosides and nucleotides in aqueous solution, where all conformational factors are fully controlled. This approach allowed us to describe the thermodynamic cycle and measure its fundamental parameters, equilibrium constants and free energy differences, delta delta G, from a nuclear magnetic resonance study. The delta delta G values as determined for pairs of ribo- and deoxyribo-nucleosides in classes of syn-constrained and anti-preferred models, are comparable and lie in a narrow range, delta delta G = 1.7 +/- 0.1 [kJ/mol]. For pairs of ribo- and deoxyribo-nucleotides, the delta delta G values also lie in narrow ranges, delta delta G = 1.7 +/- 0.1 [kJ/mol] for 5'-phosphate nucleotides and delta delta G = 1.9 +/- 0.1 [kJ/mol] for 3'-phosphate nucleotides, i.e. similar to those observed for nucleosides. The measured quantity, delta delta G, is generally observed in a relatively narrow range, delta delta G = 1.75 +/- 0.15 [kJ/mol], irrespective of the class of the model system. This quantity represents a "pure" constant contribution, pe one sugar moiety, as a "driving force" for the N-->S shift in the sugar ring conformational equilibrium, when one compares RNA and DNA. This important thermodynamic quantity, delta delta G, has not hitherto been determined for nucleic acids. Ultimately the delta delta G quantity is revealed in the tendency to adopt S(C2'endo) sugar puckering domain by the majority of DNA structures, whereas RNA generally adopt an N(C3'endo) puckering domain. A possible biological significance of the delta delta G quantity may include evolutionary aspects of nucleic acids.