Five oxalyl derivatives of beta-D-glucopyranosylamine were synthesized as potential inhibitors of glycogen phosphorylase (GP). The compounds 1-4 were competitive inhibitors of rabbit muscle GPb (with respect to alpha-D-glucose-l-phosphate) with K-i values of 0.2-1.4 mM, while compound 5 was not effective up to a concentration of 10 mM. In order to elucidate the structural basis of their inhibition, we analysed the structures of compounds 1-4 in complex with GPb at 1.93-1.96 angstrom resolution. The complex structures reveal that the inhibitors can be accommodated at the catalytic site at approximately the same position as alpha-D-glucose and stabilize the T-state conformation of the 280s loop by making several favourable contacts to Asp283 and Asn284 of this loop. Comparison with the lead compound N-acetyl-beta-D-glucopyranosylamine (6) shows that the hydrogen bonding interaction of the amide nitrogen with the main-chain carbonyl oxygen of His377 is not present in these complexes. The differences observed in the Ki values of the four analogues can be interpreted in terms of subtle conformational changes of protein residues and shifts of water molecules in the vicinity of the catalytic site, variations in van der Waals interactions, conformational entropy and desolvation effects. (c) 2006 Elsevier Ltd. All rights reserved.

Five oxalyl derivatives of $\beta$-d-glucopyranosylamine were synthesized as potential inhibitors of glycogen phosphorylase (GP). The compounds 1-4 were competitive inhibitors of rabbit muscle GPb (with respect to $\alpha$-d-glucose-1-phosphate) with Ki values of 0.2-1.4 mM, while compound 5 was not effective up to a concentration of 10 mM. In order to elucidate the structural basis of their inhibition, we analysed the structures of compounds 1-4 in complex with GPb at 1.93-1.96 {\AA} resolution. The complex structures reveal that the inhibitors can be accommodated at the catalytic site at approximately the same position as $\alpha$-d-glucose and stabilize the T-state conformation of the 280s loop by making several favourable contacts to Asp283 and Asn284 of this loop. Comparison with the lead compound N-acetyl-$\beta$-d-glucopyranosylamine (6) shows that the hydrogen bonding interaction of the amide nitrogen with the main-chain carbonyl oxygen of His377 is not present in these complexes. The differences observed in the Ki values of the four analogues can be interpreted in terms of subtle conformational changes of protein residues and shifts of water molecules in the vicinity of the catalytic site, variations in van der Waals interactions, conformational entropy and desolvation effects. © 2006 Elsevier Ltd. All rights reserved.

The photochemistry of N-hydroxypyridine-2(1H)-thione (NHPT), inserted as a photolabile modifier at the 6-position of 2′-deoxyguanosine or guanosine, has been evaluated. In particular, 6-[(1-oxidopyridin-2-yl)sulfanyl]- (1a) and 6-[(pyridin-2-yl)sulfanyl]-2′,6-dideoxyguanosine (2a), novel photolabile derivatives of the natural nucleosides, were synthesized and characterized. The observed photolysis products of 1a in organic solvents could only be rationalized by assuming a rapid equilibrium with the corresponding 6-[(2-thioxopyridin-1(2H)-yl)oxy] analogue 3a (Scheme 5). Transient spectroscopy of 1a indicated a strong triplet-excited state suitable for triplet → triplet energy transfer or singlet-oxygen generation. The NHPT function was stable enough for (slightly modified) automated solid-phase oligonucleotide synthesis. The utility of the above compounds is discussed, as well as their potential use in photosensitization of reactive oxygen species in DNA. © 2006 Verlag Helvetica Chimica Acta AG.

Despite a few decades of intense study, a full description of tautomers of one-electron-oxidized guanine remains to be achieved. Here we show that two of these tautomers are produced by the protonation of an 8-haloguanine electron adduct. The rate constants for the reactions of hydrated electrons (e(aq)(-)) with a variety of 8-substituted guanine derivatives have been measured by a pulse radiolysis technique and correlated with both inductive and resonance components of the substituents. The fate of electron adducts was investigated by radiolytic methods coupled with product studies and addressed computationally by means of time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations. The reaction of e(aq)(-) with 8-haloguanosine or 8-halo-2'-deoxyguanosine produces the first observable transient species that decay unimolecularly (k = 1 x 10(5) s(-1) at 22 degrees C) to give the one-electron oxidized guanosine or 2'-deoxyguanosine. Theory suggests that the electron adducts of 8-bromoguanine derivatives protonated at C8 form a pi-complex, with the Br atom situated above the molecular plane, that is prompt to eject Br-. The two short- lived intermediates, which show a substantial difference in their absorption spectra, are recognized to be the two purine tautomers (i.e., iminic 7 and aminic 3 forms). The spin density distributions of the two tautomers are quite different at the O6 and N10 positions, whereas they are very similar at the N3, C5, and C8 positions. The resonance structures of the two tautomers are discussed in some detail. B1B95/6-31+G** calculations show also that the tautomerization from the iminic (7) to the aminic (3) arrangement is a water-assisted process.

Despite a few decades of intense study, a full description of tautomers of one-electron-oxidized guanine remains to be achieved. Here we show that two of these tautomers are produced by the protonation of an 8-haloguanine electron adduct. The rate constants for the reactions of hydrated electrons (e aq-) with a variety of 8-substituted guanine derivatives have been measured by a pulse radiolysis technique and correlated with both inductive and resonance components of the substituents. The fate of electron adducts was investigated by radiolytic methods coupled with product studies and addressed computationally by means of time-dependent DFT (TD-B3LYP/6- 311G**//B1B95/6-31+G**) calculations. The reaction of eaq- with 8-haloguanosine or 8-halo-2′- deoxyguanosine produces the first observable transient species that decay unimolecularly (k = 1 × 105 s-1 at 22 °C) to give the one-electron oxidized guanosine or 2′-deoxyguanosine. Theory suggests that the electron adducts of 8-bromoguanine derivatives protonated at C8 form a $π$-complex, with the Br atom situated above the molecular plane, that is prompt to eject Br. The two short-lived intermediates, which show a substantial difference in their absorption spectra, are recognized to be the two purine tautomers (i.e., iminic 7 and aminic 3 forms). The spin density distributions of the two tautomers are quite different at the O6 and N10 positions, whereas they are very similar at the N3, C5, and C8 positions. The resonance structures of the two tautomers are discussed in some detail. B1B95/6-31+G** calculations show also that the tautomerization from the iminic (7) to the aminic (3) arrangement is a water-assisted process. © 2006 American Chemical Society.

Oxidatively produced DNA damage is a physiological process that has been associated with many human conditions. Of the four DNA bases, guanine is the most easily oxidized and exhibits a large number of oxidation products. The focus of this Microreview is on the structure of the proposed intermediates and final products of guanine oxidation, as determined by spectroscopic methods and independent synthetic studies. The major and minor primary and secondary oxidative DNA lesions that arise from the oxidation of guanine under a variety of conditions, as well as associated structures, are described and all the relevant information regarding the isolation, characterization, and independent synthesis or generation of these lesions in synthetic oligonucleotides is presented. Finally, information on the biological studies that have been advanced, based on the basic molecular information obtained by the aforementioned studies, is provided. © Wiley-VCH Verlag GmbH {&} Co. KGaA, 2006.