Bisulfite Anion Research Articles

Fluorescent Red GK as a fluorescent probe for selective detection of bisulfite anions

A coumarin-based dye, Fluorescent Red GK, was developed for the detection of bisulfite anions. The probe has exhibited high selectivity and sensitivity against other anions at room temperature. Upon addition of HSO3−, the strong fluorescence of Fluorescent Red GK was severely quenched and its color changed significantly from orange to colorless under illumination with a UV lamp. The color of the solution was also changed from pink to colorless. As a result, it can be used as a specific colorimetric and fluorescent probe for bisulfite anions. In addition, the detection limit was as low as 1.76μM. The results showed that the Fluorescent Red GK could be used in the design of the fluorescent probe for detecting HSO3− in aqueous solution. Furthermore, the probe has been used for determination of bisulfite in sugar samples with satisfactory results.

Addition of bisulfite ions to methyl vinyl ketone and methacrolein relevant to atmospheric processes

Methacrolein and methyl vinyl ketone are highly reactive carbonyls that play a pivotal role in the formation of secondary organic aerosols in the Earth’s atmosphere. Both carbonyls are the major products of isoprene oxidation. We show that among the atmospheric sinks of methacrolein and methyl vinyl ketone, the aqueous-phase addition of bisulfite anions to their molecules can be relevant under polluted conditions with the increased presence of sulfur dioxide. We demonstrate that aqueous-phase reactions of methyl vinyl ketone and methacrolein with bisulfite anions lead to the formation of primary and secondary organic hydroxysulfonates which currently are not included in the atmospheric chemistry modelling, but can be relevant in mechanisms explaining the formation and growth of the secondary organic aerosols from atmospheric carbonyls. The rate constants for all aqueous-phase reactions involved were determined. The primary addition of bisulfite anions to methacrolein was found significantly faster than that to methyl vinyl ketone, with rate constants kMAC1f = 8 and kMVK1f = 0.18 mol –1 dm s at room temperatures, respectively. The rate constant for the bisulfite addition to methacrolein was ten times faster than reported in the literature. The kinetic and mass spectrometric analyses revealed that in both cases, the dominating product was the C4 alpha-hydroxysulfonate (a primary adduct), while the C4 alpha-hydroxy disulfonate (a secondary adduct or diadduct) was produced only in trace quantities. The primary addition of bisufite anions to methacrolein and methyl vinyl ketone should be considered in atmospheric studies relevant to areas with enhanced presence of sulfur dioxide providing sufficiently high concentrations of bisulfite ions in atmospheric waters.

SO2 Hydrolysis: Ab initio MD Study of the Formation of Bisulfite Ion

Ab initio molecular dynamics simulations have been performed to study the dissolution of SO2 in water. It has been obtained that the hydrated SO2 is surrounded by the water molecules without any S-H hydrogen bond, restraining the sulfonate anion formation but allowing the bisulfite isomer formation. The metadynamics method has been employed to explore the free energy surface of the SO2 + H2O reaction. The simulations revealed that the hydrated SO2 forms bisulfite anion and hydronium cation after overcoming a ca. 17 kcal/mol free energy barrier. Direct, one-step H2SO3 formation could not been observed, in sharp contrast with earlier cluster calculations. These findings indicate a step-wise H2SO3 formation in water. The presence of the sulfur lone pair represents an important constraint on the mechanism: the nucleophilic H2O attack can occur only from certain angles as shown by the reactive trajectories.

Formation of the bisulfite anion (HSO3–, m/z 81) upon collision‐induced dissociation of anions derived from organic sulfonic acids

In the negative-ion collision-induced dissociation mass spectra of most organic sulfonates, the base peak is observed at m/z 80 for the sulfur trioxide radical anion (SO(3) (-·) ). In contrast, the product-ion spectra of a few sulfonates, such as cysteic acid, aminomethanesulfonate, and 2-phenylethanesulfonate, show the base peak at m/z 81 for the bisulfite anion (HSO(3) (-) ). An investigation with an extensive variety of sulfonates revealed that the presence of a hydrogen atom at the β-position relative to the sulfur atom is a prerequisite for the formation of the bisulfite anion. The formation of HSO(3) (-) is highly favored when the atom at the β-position is nitrogen, or the leaving neutral species is a highly conjugated molecule such as styrene or acrylic acid. Deuterium-exchange experiments with aminomethanesulfonate demonstrated that the hydrogen for HSO(3) (-) formation is transferred from the β-position. The presence of a peak at m/z 80 in the spectrum of 2-sulfoacetic acid, in contrast to a peak at m/z 81 in that of 3-sulfopropanoic acid, corroborated the proposed hydrogen transfer mechanism. For diacidic compounds, such as 4-sulfobutanoic acid and cysteic acid, the m/z 81 ion can be formed by an alternative mechanism, in which the negative charge of the carboxylate moiety attacks the α-carbon relative to the sulfur atom. Experiments conducted with deuterium-exchanged and deuterium-labeled analogs of sulfocarboxylic acids demonstrated that the formation of the bisulfite anion resulted either from a hydrogen transfer from the β-carbon, or from a direct attack by the carboxylate moiety on the α-carbon.

A novel coumarin-based fluorescent probe for selective detection of bissulfite anions in water and sugar samples

A simple aldehyde-functionalized coumarin (7-diethylaminocoumarin-3-aldehyde) was utilized for the selective detection of bisulfite anions in water. The probe has exhibited a selective and sensitive response to the bisulfite anion against other anions, including Cys, through the nucleophilic addition of the bisulfite to probe. The fluorescence of the probe changes upon the addition of bisulfite anions, such that millimolar concentrations of bisulfite are detectable. Moreover, the sensor can also be applied to detect the level of bisulfite in sugar samples.

A rhodamine-based fluorescent probe selective for bisulfite anion in aqueous ethanol media

A rhodamine-based fluorescent probe with an aldehyde functionality as a binding site was developed for the selective detection of bisulfite anion in aqueous ethanol media. The new fluorescent probe showed an excellent selectivity for bisulfite anion over other anions. The bisulfite-induced ring opening of spirolactam of the rhodamine moiety resulted in the dual chromo- and fluorogenic observation.

Novel Coumarin‐based Fluorescent Probe for Selective Detection of Bisulfite Anion in Water

AbstractCoumarins and its analogues have been widely used as chromophore in design of fluorescent probe, while less coumarin‐based fluorescent probe was reported for detection of anion in water. In this article, coumarin‐based fluorescent probes with salicylaldehyde functionality as recognition unit have been developed for selective detection of bisulfite anions in water. Four novel fluorescent probes were synthesized from 4‐haloresorcinol in three steps. The chemoprobe exhibited selective response to bisulfite over other anions. Moreover, the detection mechanism was studied. Upon bisulfite added, the fluorescent intensity of the probes was enhanced highly due to the nucleophilic addition reaction of formyl group with bisulfite anion.

Conformational Variants of Duplex DNA Correlated with Cytosine-rich Chromosomal Fragile Sites

We found that several major chromosomal fragile sites in human lymphomas, including the bcl-2 major breakpoint region, bcl-1 major translocation cluster, and c-Myc exon 1-intron 1 boundary, contain distinctive sequences of consecutive cytosines exhibiting a high degree of reactivity with the structure-specific chemical probe bisulfite. To assess the inherent structural variability of duplex DNA in these regions and to determine the range of structures reactive to bisulfite, we have performed bisulfite probing on genomic DNA in vitro and in situ; on duplex DNA in supercoiled and linearized plasmids; and on oligonucleotide DNA/DNA and DNA/2'-O-methyl RNA duplexes. Bisulfite is significantly more reactive at the frayed ends of DNA duplexes, which is expected given that bisulfite is an established probe of single-stranded DNA. We observed that bisulfite also distinguishes between more subtle sequence/structural differences in duplex DNA. Supercoiled plasmids are more reactive than linear DNA; and sequences containing consecutive cytosines, namely GGGCCC, are more reactive than those with alternating guanine and cytosine, namely GCGCGC. Circular dichroism and x-ray crystallography show that the GGGCCC sequence forms an intermediate B/A structure. Molecular dynamics simulations also predict an intermediate B/A structure for this sequence, and probe calculations suggest greater bisulfite accessibility of cytosine bases in the intermediate B/A structure over canonical B- or A-form DNA. Electrostatic calculations reveal that consecutive cytosine bases create electropositive patches in the major groove, predicting enhanced localization of the bisulfite anion at homo-C tracts over alternating G/C sequences. These characteristics of homo-C tracts in duplex DNA may be associated with DNA-protein interactions in vivo that predispose certain genomic regions to chromosomal fragility.

Dinaphthotetraaza[14]annulene copper(II) complexes in the electrocatalytic reduction of carbon dioxide and bisulfite anion

A modified synthetic route for the complexes [Cu(II)5,7,12,14-tetramethyldinaphtho [b,i][1,4,8,11]tetraaza[14]annulene], [Cu(II)tmdnTAA], and [Cu(II) 5,7,12,14-tetramethyl-6,13-dichloro-dinaphtho[b,i][1,4,8,11]tetraaza[14]annulene], [Cu(II)dCltmdnTAA], is presented in this work. The electrochemical characterization of both complexes and their precursors, [bis(2,4-pentanedionato)copper(II)], [Cu(II)(acac)2] and [bis(3-chloro-2,4-pentanedionato)copper(II)], [Cu(II)(3-Cl-acac)2], respectively, under nitrogen and carbon dioxide is also presented. The voltammetric response of [Cu(II)(acac)2] and [Cu(II)(3-Cl-acac)2] are different compared to [Cu(II)tmdnTAA] and [Cu(II)dCltmdnTAA] under nitrogen. Precursors show the reduction of Cu(I) to Cu(0) and the tetraazadinaphtho[14]annulene complexes do not. The chlorine substituted complex has a lower reduction potential than the unsubstituted homologue under nitrogen atmosphere. However, the contrary response is obtained in the presence of carbon dioxide: the unsubstituted complex is more catalytic in terms of potential because the current discharge appears 270 mV shifted to the anodic region. These facts can be explained in terms of electronic and steric effects. The modified electrode obtained by oxidative electropolymerization of [Cu(II)tmdnTAA] over glassy carbon electrode presented a suitable amperometric response for the sulfite reduction in acidic medium (pH = 2.7). A linear correlation was observed for the catalytic current and sulfite concentration between 0.6–6.0 mM range.

A chromoreactand for the selective detection of HSO3- based on the reversible bisulfite addition reaction in polymer membranes.

A new chromoreactand for the reversible optical detection of bisulfite anion is presented with a sensitivity in the range from 0.1 to 10 mM while no absorbance changes are observed for anions such as sulfate, chloride, phosphate or hydroxide.

Kinetics and polymerization characteristics for some polyvinyl acetate emulsions prepared by different redox pair initiation systems

The emulsion polymerization of vinyl acetate was carried out using different sodium bisulfite adducts (containing the same weight equivalent of bisulfite anion) as reducing agents with potassium persulfate as an oxidising agent at low temperature. The polymerization evaluation was followed by taking samples at regular intervals. The average volume diameters, the areas and the number of polymer particles per unit volume of water ( Nt) for some prepared emulsion latices were calculated with the aid of transmission electron microscopy and image analyzer techniques. Also, it was found that the rate of emulsion polymerization decreases in the following order: acetone sodium bisulfite (AcSBS) > methyl propyl ketone sodium bisulfite (MpKSBS) > octyl aldehyde sodium bisulfite (OASBS) > acetaldehyde sodium bisulfite (ASBS) > benzaldehyde sodium bisulfite (BSBS) > salicylaldehyde sodium bisulfite (SSBS) > cyclohexanone sodium bisulfite (ChSBS), while the particle size of the prepared emulsions increases in the following order: (OASBS)<(SSBS)<(BSBS)<(ChSBS)<(MpSBS)<(AcSBS)<(ASBS). Finally, the D z, D v/ D s, number of particles per cm 3 and D w/ D n were calculated for polymer latices obtained.

Theoretical studies on possible sulfur oxides with +2 oxidation states in aqueous solution

In the oxidation of sulfide to sulfate a number of different intermediate products are commonly observed, but products with S oxidation states of +2 seem generally to be absent. Recently Vairavamurthy and Zhou (hereafter VZ) [Vairavamurthy, M.A., Zhou, W., 1995. Characterization of a transient +2 sulfur oxidation state intermediate from the oxidation of aqueous sulfide. In: Vairavamurthy, M.A., Schoonen, M.A.A. (Eds.), Geochemical Transformations of Sedimentary Sulfur. ACS Symposium Ser., 612, ACS, Washington, DC, pp. 280–292] characterized an intermediate oxidation state sulfur oxide whose concentration increased during the early stages of sulfide oxidation and then declined. On the basis of IR and XANES spectra they formulated this as So 2 −2, with the sulfur in oxidation state +2. Steiger and Steudel [Steiger, T., Steudel, R., 1992. Sulphur compounds. Part 149. Structures, relative stabilities and vibrational spectra of several isomeric forms of sulphoxylic acid (H 2SO 2) and its anion (HSO 2 −1 ): an ab initio study. J. Molec. Struct. (THEOCHEM) 257, 313–323] had earlier studied theoretically the species (OH)SO −1, which had a strong S=O IR absorption near the energy observed by VZ for the +2 oxidation state intermediate, but which was calculated to be of higher energy (in the gas phase) than its isomer HSO 2 −1(with the H bonded to S). We have calculated the relative energies and the vibrational spectra of (OH)SO −1 and HSO 2 −1coordinated to four water molecules, establishing that the O-H form, i.e. (OH)SO −1, is more stable than the SH form in the presence of small numbers of waters (we find a similar result for the OH and SH isomers of the bisulfite anion) and that the calculated S=O stretching frequency for (OH)SO −1 in the presence of water is still very close to that observed by VZ (about 911 cm −1 calculated vs. 918 cm −1 experimental). The calculated SOH stretching frequency in (OH)SO −1 is around 690 cm −1, and thus could not have been observed in water solution by VZ. XANES energies have also been estimated from ground state orbital energies for (OH)SO −1 and a range of other sulfur oxides, establishing that the XANES of (OH)SO −1 falls into the range observed for the +2 oxidation state intermediate by VZ. The IR and XANES evidence and the calculated relative stabilities are thus consistent with the assignment of this species as (OH)SO −1. To more completely characterize this and related species we have also calculated their 17O NMR spectra, which we find to depend in a fairly complicated way on S oxidation state, coordination number and degree of protonation. For the (OH)SO −1 species in solution we predict 17O NMR shifts of 63 (compared to gas-phase H 2O) for the OH oxygen and 92 for the unprotonated O, both considerably more shielded than either (OH)SO 2 −1 or S 2O 3 −2. S oxides with more negative oxidation states, e.g. SO −2, show more shielded 17O NMR signals and lower vibrational energies. The reaction of SH − + O 2 to give (OH)SO −1 is calculated to be slightly endothermic in solution while the reaction of (OH)SO −1 to give thiosulfate is strongly exothermic.

Stable sulfur isotope fractionation by anion exchange chromatography. production of compounds enriched in 34S

The separation of the 34S isotope was carried out by isotopic exchange between sulfurous acid in solution and bisulfite anions adsorbed on a quaternary ammonium (Dowex 1X8 and Dowex 2X8, 100-200 mesh) packed in six anion exchange columns. Each resin column packed with the anion exchange resin had a height of 115 cm and 2.2 cm in diameter. The columns were connected in series during the displacement of bisulfite bands. For the experiments, a band of bisulfite was fixed to the anion resin, initially in the hydroxyl ion form, and subsequently eluted with 0.2, 0.3, 0.4, or 0.6 mol L-1 HCl solution. The hydrochloric acid solution was kept under nitrogen at 245 kPa, in order to prevent the evolution of gases and the oxidation of the bisulfite band. The experiments with the Dowex 1X8 resin showed that the best results in terms of maximum enrichment (18.66 atom %34S) and total34S accumulated mass (161.7 mg, excess) were obtained by elution of the bisulfite band for 50 m with 0.2 and 0.3 mol L-1 HCl, respectively. The depleted portion of the band was periodically replaced with natural bisulfite, at 10 m intervals during elution.

Quantitation of 5-hydroxymethylfurfural (5-HMF) and related substances in dextrose injections containing drugs and bisulfite

The spectrophotometric test for 5-hydroxymethylfurfural and related substances, which is included in multiple compendial monographs for dextrose-containing injections, was modified to eliminate interference of bisulfite anion. Experimental formulations containing heparin or dopamine hydrochloride were used in this study. The modified assay was shown to be accurate, precise and rugged. With an appropriate standard, the method can be used to measure 5-HMF in solutions with an approximate quantitation limit of 0.06 ppm.

One-Electron Oxidation and Reduction of Sulfites and Sulfinic Acids in Oxygenated Media: The Formation of Sulfonyl and Sulfuranyl Peroxyl Radicals

Bisulfite anions and sulfinic acids are shown by ESR spectroscopy to undergo reduction with radiolytically produced free electrons and oxidation by hole centers (Cl2•-) in a low-temperature aqueous LiCl glass. Electron-reduced intermediates display g anisotropy within the 2.010−2.002 range and are considered as neutral or singly deprotonated forms of sulfuranyl radicals (HO)3S•, (RO)2SOH•, and RS(OH)2•. The oxidized species produced by hole trapping are sulfite (SO3•-) and sulfonyl (RSO2•) radicals. Dialkyl sulfites scavenge free electrons only and are not oxidized by Cl2•-. Both oxidized and reduced intermediates add oxygen on annealing forming peroxyl radicals which are identified by their characteristic oxygen-17 hyperfine couplings. A large 100 G oxygen-17 coupling with the terminal oxygen atom found for oxidized sulfite−oxygen adduct -O3SOO• is in accord with its known high reactivity. Sulfuranyl peroxyls formed by the reductive pathway show lower 17O couplings (e.g., 90 G with terminal oxygens) and are expected to be far less reactive than alkyl peroxyls. Ab initio MO calculations predict stable structures for these sulfuranyl peroxyls, suggesting a trigonal-bipyramid coordination of the central sulfur atom. However, the sulfuranyl intermediates formed through one-electron reduction of dimethyl and methylphenyl sulfoxides are found to be unstable even in low-temperature glass, fragmenting into a methyl radical and sulfenic acid or into a hydroxyl anion and sulfide radical cation, respectively.

Utilisation of the bisulfite addition reaction for the separation of neutral aldehydes by capillary electrophoresis

In-column reaction of neutral aldehydes with the bisulfite anion leads to charged complexes that can be separated by electrophoresis. Initially experiments were carried out with sodium bisulfite as a buffer system. These indicated that complexation with bisulfite was effective at low concentrations, opening up the concept of using bisulfite as an additive to more conventional buffers.

Effect of some sodium bisulfite adducts of different chain lengths on the course of the emulsion polymerization of vinyl acetate

AbstractThe emulsion polymerization of vinyl acetate using some bisulfite adducts of different chain lengths with potassium persulfate as the redox initiation system, in the absence of emulsifier, was studied. The effects of such adducts, which contain the same weight equivalent of bisulfite anion, on the rate of polymerization, maximum conversion, and stability of the produced polymer lattices as well as the morphological characteristics including volume‐average diameters and the number of polymer particles per unit volume of water were investigated. The rate of polymerization was found to be dependent on the concentration of benzaldehyde, acetaldehyde, octyl aldehyde, methyl propyl ketone, and acetone sodium bisulfite adducts to the powers 0.54, 0.66, 0.95, 1.0, and 1.1, respectively. Also, it was found that increasing adduct chain lengths decreases the volume‐average diameter, increases the obtained lattice stability, and improves the morphology of the polymeric particles, where a spherical morphology and extra stability were obtained in the presence of the octyl aldehyde adduct. © 1993 John Wiley &amp; Sons, Inc.This article is a US Government work and, as such, is in the public domain in the United States of America.

Sulfur-dioxide fluxes into different cellular compartments of leaves photosynthesizing in a polluted atmosphere

A computer model is used to analyze fluxes of SO2 from polluted air into leaves and the intracellular distribution of sulfur species derived from SO2. The analysis considers only effects of acidification and of anion accumulation. (i) The SO2 flux into leaves is practically exclusively controlled by the boundary-layer resistance of leaves to gas diffusion and by stomatal opening. At constant stomatal opening, flux is proportional to the concentration of SO2 in air. (ii) The sink capacity of cellular compartments for SO2 depends on intracellular pH and the intracellular localization of reactions capable of oxidizing or reducing SO2. In the mesophyll of illuminated leaves, the chloroplasts possess the highest trapping potential for SO2. (iii) If intracellular ion transport were insignificant, and if bisulfite and sulfite could not be oxidized or reduced, leaves with opened stomata would rapidly be killed both by the accumulation of sulfites and by acidification of chloroplasts and cytosol even if SO2 levels in air did not exceed concentrations thought to be permissible. Acidification and sulfite accumulation would remain confined largely to the chloroplasts and to the cytosol under these conditions. (iv) Transport of bisulfite and protons produced by hydration of SO2 into the vacuole cannot solve the problem of cytoplasmic accumulation of bisulfite and sulfite and of cytoplasmic acidification, because SO2 generated in the acidic vacuole from the bisulfite anion would diffuse back into the cytoplasm. (v) Oxidation to sulfate which is known to occur mainly in the chloroplasts can solve the problem of cytoplasmic sulfite and bisulfite accumulation, but aggravates the problem of chloroplastic and cytosolic acidification. (vi) A temporary solution to the problem of acidification requires the transfer of H(+) and sulfate into the vacuole. This transport needs to be energized. The storage capacity of the vacuole for protons and sulfate defines the extent to which SO2 can be detoxified by oxidation and removal of the resulting protons and sulfate anions from the cytoplasm. Calculations show that even at atmospheric levels of SO2 thought to be tolerable, known vacuolar buffer capacities are insufficient to cope with proton production during oxidation of SO2 to sulfate within a vegetation period. (vii) A permanent solution to the problem of acidification is the removal of protons. Protons are consumed during the reduction of sulfate to sulfide. Proteins and peptides contain sulfur at the level of sulfide. During photosynthesis in the presence of the permissible concentration of 0.05μl·l(-1) SO2, sulfur may be deposited in plants at a ratio not far from 1/500 in relation to carbon. The content of reduced sulfur to carbon is similar to that ratio only in fast-growing, protein-rich plants. Such plants may experience little difficulty in detoxifying SO2. In contrast, many trees may contain reduced sulfur at a ratio as low as 1/10 000 in relation to carbon. Excess sulfur deposited in such trees during photosynthesis in polluted air gives rise to sulfate and protons. If detoxification of SO2 by reduction is inadequate, and if the storage capacity of the vacuoles for protons and sulfate is exhausted, damage is unavoidable. Calculations indicate that trees with a low ratio of reduced S to C cannot tolerate long-term exposure to concentrations of SO2 as low as 0.02 or 0.03 μl·l(-1) which so far have been considered to be non-toxic to sensitive plant species.

Reaction of bovine erythrocyte green hemoprotein with oxygen and hydrogen peroxide.

Bovine erythrocyte green hemoprotein was shown to undergo rapid and complete bleaching during aerobic reduction by dithionite. In order to explain this phenomenon, we have studied the reactions of the ferrihemoprotein with dithionite and H202 and the reactions of the ferrohemoprotein with HzOz and OZ. Bleaching caused by aerobic dithionite was shown not to involve either superoxide or bisulfite anions. Inhibition by the ligands CO and pyridine suggested that a free heme site was necessary for bleaching and that the ferrous hemoprotein may be involved. Catalase strongly inhibits this bleaching, implying that HzOz (produced in the aerobic dithionite solution) is involved. The ferrihemoprotein is bleached by HzOz at pH 7.2, 25”C, in a reaction which involves a complex with Hz02 (KD = 1.3 mM) that decays at a rate of 0.14 s-l. This reaction is inhibited by CN(Z& = K. = 0.083 mu). HzOz reacts more rapidly with ferrohemoprotein; within 5 s, 35 to 50% of the hemoprotein is bleached and the remainder is oxidized to ferrihemoprotein. The anaerobic reduction of ferrihemoprotein by dithionite is very fast. These findings lead to the conclusion that in the aerobic bleaching reaction dithionite both generates HzOz by reaction with O2 and maintains the heme in the more reactive ferrous state. The ferrohemoprotein reacts rapidly with O2 to form a transient Complex (& = 8.7 X Ioe5 M) which decays to the ferrihemoprotein at a rate of 501 S -’ at 5°C. This oxidation is accompanied by a loss of 13 to 20% of the original absorbance of the ferrihemoprotein. Like green hemoprotein, cytochrome P-450 is bleached by HzOz, but at a much slower rate; ferrocytochrome P-450 reacts more rapidly than does ferricytochrome P-450. The mechanism for the bleaching is envisioned to be similar to the peroxidatic cycle of Yokota and Yamazaki ((1965) Biochim Biophys. Actu 105, 301) with the important difference that the addition of reducing equivalents to Compounds I and II of the peroxidatic cycle has been replaced by the loss of HO l radicals. We postulate that HO* generated by such a mechanism attacks, and thereby bleaches, the porphyrin ring.

Carbon-13 nuclear magnetic resonance investigations into the interactions of bisulfite with pyrimidine nucleosides and nucleotides.

Carbon-13 NMR is utilized to demonstrate the attack of bisulfite anion on uridine, 5-fluorouridine, and uridine 5'-monophosphate. The attack produces a pair of diastereomeric adducts similar in structure to those seen in the uracil series. Intensity data from the equilibrium system give an estimate for the individual equilibrium constants. Thymidine and thymidine 5'-monophosphate show no evidence of nucleophilic attack by bisulfite. This evidence indicates that bisulfite addition to nucleosides and nucleotides models the enzymatic methylation of uridine by the enzyme thymidylate synthetase better than the uracil bisulfite system.