Aqueous atmospheric chemistry: formation of 2,4-dinitrophenol upon nitration of 2-nitrophenol and 4-nitrophenol in solution.

Field studies have shown that the powerful phytotoxic agent 2,4-dinitrophenol is very likely to form in the atmospheric aqueous phase upon nitration of 2-nitrophenol or 4-nitrophenol. However, until now, the nitration pathway and the relative importance of the two mononitrophenols as sources of 2,4-dinitrophenol were not known. The present study shows that 2,4-dinitrophenol formation from mononitrophenols can take place upon photolysis and photooxidation of nitrite/nitrous acid (NO2-/HONO) and that nitrogen dioxide plays a key role in the process. A possible pathway might be the reaction between light-excited mononitrophenols (both 2- and 4-isomers) and nitrogen dioxide, in the presence of oxygen. As an alternative, nitration might involve *NO3 + *NO2. Possible sources of nitrogen dioxide in the atmospheric aqueous phase are dissolution from the gas phase and oxidation of NO2-. In the latter case, however, it is necessary that NO2- oxidation is faster than the oxidation of mononitrophenols. This would happen, for instance, in the presence of hematite under irradiation. Radiation absorption and scattering by hematite would also inhibit the direct photolysis of nitrophenols. The formation rate and the yield of 2,4-dinitrophenol are slightly higher when starting from 2-nitrophenol than those from 4-nitrophenol, but they are compensated by the higher concentration of 4-nitrophenol in the atmospheric aqueous phase.

Post-synthetic and site-specific modification of endocyclic nitrogen atoms of purines in DNA and its potential for biological and structural studies.

Site-specific modification of the N1-position of purine was explored at the nucleoside and oligomer levels. 2'-deoxyinosine was converted into an N1-2,4-dinitrophenyl derivative 2 that was readily transformed to the desired N1-substituted 2'-deoxyinosine analogues. This approach was used to develop a post-synthetic method for the modification of the endocyclic N1-position of purine at the oligomer level. The phosphoramidite monomer of N1-(2,4-dinitrophenyl)-2'-deoxyinosine 9 was prepared from 2'-deoxyinosine in four steps and incorporated into oligomers using an automated DNA synthesizer. The modified base, N1-(2,4-dinitrophenyl)-hypoxanthine, in synthesized oligomers, upon treatment with respective agents, was converted into corresponding N1-substituted hypoxanthines, including N1-15N-hypoxanthine, N1-methylhypoxanthine and N1-(2-aminoethyl)-hypoxanthine. These modified oligomers can be easily separated and high purity oligomers obtained. Melting curve studies show the oligomer containing N1-methylhypoxanthine or N1-(2-aminoethyl)-hypoxanthine has a reduced thermostability with no particular pairing preference to either cytosine or thymine. The developed method could be adapted for the preparation of oligomers containing mutagenic N1-beta-hydroxyalkyl-hypoxanthines and the availability of the rare base-modified oligomers should offer novel tools for biological and structural studies.

The chemical synthesis of 2-deoxy-2-fluorodisaccharide probes of the hen egg white lysozyme mechanism.

2,4-Dinitrophenyl 2-acetamido-2-deoxy-beta-d-glucopyranosyl-(1-->4)-2-deoxy-2-fluoro-beta-d-glucopyranoside (GN2FG-DNP) and 2-acetamido-2-deoxy-beta-d-glucopyranosyl-(1-->4)-2-deoxy-2-fluoro-beta-d-glucopyranosyl fluoride (GN2FG-F) were prepared using a divergent synthetic approach involving 10 steps. The key steps involved the preparation of 1-O-acetyl-3,6-di-O-benzyl-2-deoxy-2-fluoro-alpha/beta-d-glucopyranose using Selectfluor(trade mark) in the presence of acetic acid and the subsequent glycosylation of this acceptor to generate the core 2-fluorodisaccharide. After further elaboration, the target molecules were obtained and tested as probes of the mechanism of hen egg white lysozyme (HEWL). Compound GN2FG-DNP is not a substrate for the enzyme while compound GN2FG-F is cleaved slowly with an apparent K(m) greater than 5mM and a second-order rate constant of k(cat)/K(m)=9.6s(-1)M(-1). Comparison of this value to that estimated for the hydrolysis of beta-chitobiosyl fluoride by HEWL (1200s(-1)M(-1)) [Ballardie, F. W.; Capon, B.; Cuthbert, M. W.; Dearie, W. M. Bioorg. Chem.1977, 6, 483-509] revealed a 126-fold rate decrease upon substitution of a fluorine group for the 2-acetamido group of beta-chitobiosyl fluoride. This decrease resulted in the steady-state accumulation of an intermediate as visualized by mass spectrometry and the ultimate crystallographic determination of its structure [Vocadlo, D. J.; Davies, G. J.; Laine, R.; Withers, S. G. Nature2001, 412, 835-838].