On the reaction of molecular oxygen with thiyl radicals: a re-examination

Absolute rate constants for the addition of oxygen to thiyl radicals, i.e. RS. + O2----RSOO., have been determined by applying a new competition method based on RS. formation via one-electron reduction of the corresponding disulphides, and the competition between RS. reacting with O2 and an electron donor such as ascorbate. Bimolecular rate constants have been obtained for the thiyl radicals derived from cysteine (6.1 X 10(7) mol-1 dm3 s-1), penicillamine (2.5 X 10(7) mol-1 dm3 s-1), homocysteine (8.0 X 10(7) mol-1 dm3 s-1), cysteamine (2.8 X 10(7) mol-1 dm3 s-1), 3-thiopropionic acid (2.2 X 10(8) mol-1 dm3 s-1) and glutathione (3.0 X 10(7) mol-1 dm3 s-1), respectively. The values obtained for the O2 addition to the thiyl radicals from glutathione and cysteine are considerable lower (by about two orders of magnitude) than those previously published. This indicates that the RS. + O2 reaction may be of complex nature and is generally a process which is not solely controlled by the diffusion of the reactants.     

Intermolecular charge transfer involving tryptophan, tyrosine and three electron-bonded intermediates derived from methionine

Oxidation processes of radiation-generated three-electron-bonded intermediates derived from methionine Met2[S+...S] and Met[S...X] (X=Cl,Br) were investigated through reaction with tryptophan and tyrosine, using the optical pulse radiolysis method. Bimolecular rate constants have been measured for the reactions Met2[S+...S] with TrpH(k=3.8 x 10(8) dm3 mol-1 s-1 and k = 4.9 X 10(8) dm3 mol-1 s-1 at at ph 1 and 4.3, respectively) and Met2[S+...S] with tyrosine, k=3.8 x 10(7) dm3 mol-1 s-1. Rate constants for intermolecular transformation of Met[S...Br] and Met[S...Cl] into TrpH+. or Trp. were also estimated. They varied from 4.7 X 10(8) dm3 mol-1 s-1 (bromide species) to 1.0 X 10(9)dm3 mol-1 s-1 (chloride species). It has also been established azide radicals N-6, N.3 in contrast to dihalide radicals (X-2) do not form transients of Met[S...X] (X = N3)-type. However, oxidation of N-3 by Met2[S+...S] occurs with a bimolecular rate constant of 2.8 X 10(8) dm3 mol-1 s-1. These results are discussed in the light of some equilibria which have been proposed earlier for methionine-halide systems.     

The SO4(.-)-induced chain reaction of 1,3-dimethyluracil with peroxodisulphate

The sulphate radical SO4(.-) reacts with 1,3-dimethyluracil (1,3-DMU) (k = 5 X 10(9) dm3 mol-1 s-1) thereby forming with greater than or equal to 90 per cent yield the 1,3-DMU C(5)-OH adduct radical 4 as evidenced by its absorption spectrum and its reactivity toward tetranitromethane. Pulse-conductometric experiments have shown that a 1,3-DMU-SO4(.-) aduct 3 as well as the 1,3-DMU radical cation 1, if formed, must be very short-lived (t1/2 less than or equal to 1 microsecond). The 1,3-DMU C(5)-OH adduct 4 reacts slowly with peroxodisulphate (k = 2.1 X 10(5) dm3 mol-1 s-1). It is suggested that the observed new species is the 1,3-DMU-5-OH-6-SO4(.-) radical 7. At low dose rates a chain reaction is observed. The product of this chain reaction is the cis-5,6-dihydro-5,6-dihydroxy-1,3-dimethyluracil 2. At a dose rate of 2.8 X 10(-3) Gys-1 a G value of approximately 200 was observed ([1,3-DMU] = 5 X 10(-3) mol dm-3; [S2O8(2-)] = 10(-2) mol dm-3; [t-butanol] = 10(-2) mol dm-3). The peculiarities of this chain reaction (strong effect of [1,3-DMU], smaller effect of [S2O(2-)8]) is explained by 7 being an important chain carrier. It is proposed that 7 reacts with 1,3-DMU by electron transfer, albeit more slowly (k approximately 1.2 X 10(4) dm3 mol-1 s-1) than does SO4(.-). The resulting sulphate 6 is considered to hydrolyse into 2 and sulphuric acid which is formed in amounts equivalent to those of 2. Computer simulations provide support for the proposed mechanism. The results of some SCF calculations on the electron distribution in the radical cations derived from uracil and 1-methyluracil are also presented.     

Evidence for catalytic dismutation of superoxide by cobalt(II) derivatives of bovine superoxide dismutase in aqueous solution as studied by pulse radiolysis.

By using the technique of pulse radiolysis to generate O2-., it is demonstrated that Co(II) derivatives of bovine superoxide dismutase in which the copper alone and both the copper and zinc of the enzyme have been substituted by Co(II), resulting in (Co,Zn)- and (Co,Co)-proteins, are capable of catalytically dismutating O2-. with 'turnover' rate constants of 4.8 X 10(6) dm3.s-1.mol-1 and 3.1 X 10(6) dm3.s-1.mol-1 respectively. The activities of the proteins are independent of the pH (7.4-9.4) and are about three orders of magnitude less than that of the native (Cu,Zn)-protein. The rate constants for the initial interaction of O2-. with the Co-proteins were determined to be (1.5-1.6) X 10(9) dm3.s-1.mol-1; however, in the presence of phosphate, partial inhibition is apparent [k approximately (1.9-2.3) X 10(8) dm3.s-1.mol-1]. To account for the experimental observations, two reaction schemes are presented, involving initially either complex-formation or redox reactions between O2-. and Co(II). This is the first demonstration that substitution of a metal into the vacant copper site of (Cu,Zn)-protein results in proteins that retain superoxide dismutase activity.     

Reactions of linoleic acid peroxyl radicals with phenolic antioxidants: a pulse radiolysis study

Linoleic acid peroxyl radicals (LOO.) can be viewed as model intermediates occurring during lipid peroxidation processes. Formation and reactions of these species were investigated in aqueous alkaline solution using the technique of pulse radiolysis combined with kinetic spectroscopy. Irradiation of linoleic acid in N2O/O2-saturated solutions leads to a mixture of peroxyl radical isomers, whereas reaction of 13-hydroperoxylinoleic acid (13-LOOH) with azide radicals in N2O-saturated solution produces 13-LOO. radicals specifically. These peroxyl radicals cannot be observed directly, but their reactions with the two flavonols, kaempferol and quercetin, acting as radical-scavenging antioxidants, produced strongly absorbing aroxyl radicals (ArO.). The same aroxyl radicals were generated by .OH and N3. with rate constants exceeding 10(9) dm3 mol-1 s-1. Applying a reaction scheme that includes competing generation and decay reactions of both LOO. and ArO. radicals, we derived individual rate constants for LOO. reactions with the phenols (greater than 10(7) dm3 mol-1 s-1), with the aroxyl radicals to form covalent adducts (greater than 10(8) dm3 mol-1 s-1), as well as for their bimilecular decay (3.0 X 10(8) dm3 mol-1 s-1). These results demonstrate the high reactivity of both fatty acid peroxyl radicals and the flavone antioxidants in aqueous solution.     

The reverse of the 'repair' reaction of thiols: H-abstraction at carbon by thiyl radicals

Thiyl radicals (RS) formed by the reaction of radiolytically generated OH radicals with thiols, e.g. 1,4-dithiothreitol (DTT), react with cis- and trans-2,5-dimethyltetrahydrofuran by abstracting an H atom in the alpha-position to the ether function (k approximately equal to 5 X 10(3) dm3 mol-1 s-1). The so-formed planar ether radical is 'repaired' by the thiol (k = 6 X 10(8) dm3 mol-1 s-1) thereby regenerating a cis- or trans-2,5-dimethyltetrahydrofuran molecule. In this reaction a thiyl radical is reproduced. Thus trans-2,5-Me2THF from cis-2,5-Me2THF and vice versa are formed in a chain reaction: at a dose rate of 2.8 X 10(-3) Gys-1 and a trans-2,5-Me2THF concentration of 1 X 10(-2) mol dm-3 using DTT as the thiol, G(cis-2,5-Me2THF) = 160 has been found. The chain reaction is very sensitive to impurities and also to disulphides such as those radiolytically formed. 2,5-Me2THF can be regarded as a model for the sugar moiety of DNA where the C(4')-radical is known to lead to DNA strand breakage. The possible role of cellular thiols in the repair of the C(4') DNA radical, and also the conceivable role of thiyl radicals inducing DNA strand breakage, are discussed.     

Reaction of azide radicals with amino acids and proteins.

The azide radical N3 reacts selectively with amino acids, in neutral solution preferentially with tryptophan (k (N3 + TrpH) = 4.1 X 10(9) dm3 mol(-1s-1) and in alkaline solution also with cysteine and tyrosine (k(N3 + CyS-) = 2.7 X 10(9) dm3 mol-1s-1) and k(N3 + TyrO-) equals 03.6 X 10(9) dm3 mol-1s-1). Oxidation of the enzyme yADH by N3 involves primary attacks, mainly at tryptophan residues, and subsequent slow secondary reactions. N3-induced inactivation of yADH is likely to occur upon oxidation of tryptophan residues in the substrate binding pocket (58-TrpH and 93-TrpH) since the substrate ethanol although unreactive with N3, protects yADH and since elADH, which does not contain tryptophan in the substrate pocket, is comparatively resistant against N3-attack. N3 exhibits low reactivity with nucleic acid derivatives and it is inert towards aliphatic compounds such as methanol and sodium dodecylsulphate.     

Pulse radiolytic study of the interaction of SO4.- with deoxynucleosides. Possible implications for direct energy deposition

Using the technique of pulse radiolysis it has been demonstrated that the interaction of SO4.- with deoxynucleosides (k approximately less than 2 X 10(8)-2.3 X 10(9) dm3 mol-1 s-1) in aqueous solution at pH 7.0 results in the formation of the corresponding one-electron oxidized radicals which either deprotonate or hydrate to yield OH adducts. Based upon the ease of oxidation of the deoxynucleosides, dG, dA, dC, dT, by SO4.-, the apparent redox potentials are in the order dG much greater than dA approximately equal to dC greater than dT. With the exception of deoxyuridine, the deoxynucleoside radicals produced on interaction with SO4.- have been shown to have oxidizing properties based upon the interactions with tetranitromethane and the nitroxyls, TMPN and NPPN. The deoxynucleoside radicals (dG, dA and dC) do not interact with oxygen (k less than 10(6) dm3 mol-1 s-1) in contrast to the interaction observed with the thymidine radical (k = 2.5 X 10(7) dm3 mol-1 s-1). The implications of these findings are presented in terms of the properties of the discussed radicals as relating to those of potential DNA base radicals (positive centres) produced by direct energy deposition within DNA. The use of SO4.- to mimic, to some extent, the effects of direct energy deposition in DNA may assist in our understanding of the resulting molecular processes relevant to radiobiological studies.     

Lifetime of peroxyl radicals of poly(U), poly(A) and single-and double-stranded DNA and the rate of their reaction with thiols

Peroxyl radicals of poly(U), poly(A), and single- and double-stranded DNA have been produced by photolysing H2O2 in oxygenated aqueous solution in presence of the substrates. The peroxyl radicals are formed by the reaction of OH radicals with the polynucleotides followed by addition of oxygen. The lifetime of the peroxyl radicals and the rate constant of their reactions with the thiols cysteamine, glutathione and dithiothreithol have been measured by time-resolved e.s.r. spectroscopy. The unusually long lifetimes range from 0.2 to 3.3 s. The activation energy for the decay for all four substrates is 10.3 +/- 1 kcal/mol (43 kJ mol-1). The reaction rate constants with the thiols range from k = 0.8 X 10(4) to 1.3 X 10(5) dm3 mol-1 s-1. The reactions of the thiols with the peroxyl radical of poly(U) are known to prevent strand break formation. This shows that the peroxyl radicals of poly(U) observed by e.s.r. are intermediates in the pathway leading to strand break formation.     

Repair of amino acid radicals by a vitamin E analogue

Free radicals derived from one-electron oxidation of the amino acids tryptophan, tyrosine, methionine and histidine have been found to be rapidly (k = 10(7) -10(9) dm3 mol-1 s-1) and efficiently repaired by Trolox C, a vitamin E analogue. The reactions form a relatively stable phenoxyl radical of Trolox C (lambda max = 440 nm; epsilon = 5.4 X 10(3) mol dm-3 cm-1). The radical cation of tryptophan is more rapidly repaired than the neutral tryptophan radical. Repair of tryptophanyl radicals in the enzyme lysozyme has also been observed. The results suggest that a function of alpha-tocopherol in membranes may be the repair of radicals of integral membrane proteins.     

Chemical properties of water-soluble porphyrins. 4. The reaction of a 'picket-fence-like' iron (III) complex with the superoxide oxygen couple

Solution properties of the iron-(III) 'picket-fence-like' porphyrin, Fe(III)-alpha,alpha,alpha, beta-tetra-ortho (N-methyl-isonicotinamidophenyl) porphyrin, (Fe(III)PFP) were investigated. These were acid/base properties of the aquo complex with pKa of 3.9 and its aggregation (formation of dimer with K = 1 X 10(-10) dm3 mol-1), complex formation with cyanide ions and 1-methyl imidazole (1-MeIm), spectral properties of the three iron complexes in their ferric and ferrous form and the one-electron reduction potential of these complexes. Knowing these properties, the reaction of the ferric complexes, aquo, dicyano and bis (1-MeIm), with the superoxide radical and other reducing radicals were studied using the pulse radiolysis technique. The second-order reaction rate constant of O2- with the iron (III) aquo complex which governs the catalytic efficiency of the metalloporphyrin upon the disproportionation of the superoxide radical was 7.6 X 10(7) dm3 mol-1 s-1, two orders of magnitude faster when compared to the reaction of each of the other complexes. The reduction by other radicals with all iron (III) complexes had similar second-order rate constants (10(9) to 10(10) dm3 mol-1 s-1). The reduction reaction in all cases produced Fe(II)PEP and no intermediate was found. The oxidation reaction of Fe(II)PEP by O2- was one order of magnitude faster when compared to the reduction of Fe(III)PFP by the same radical. Since the reactivity of O2- toward the three iron (III) porphyrin complexes follows their reduction potentials, it is suggesting the formation of a peroxo Fe(II) porphyrin as an intermediate. The reactions of the Fe(II)PFP complexes with dioxygen were also studied. The aquo complex was found to be first order in O2 and second order in Fe(II)PFP, suggesting the formation of a peroxo Fe(II) porphyrin as an intermediate. The intermediate formation was corroborated by evidence of the rapid CO binding reaction to the aquo complex of Fe(II)PFP. The two other complexes reacted very slowly with O2 as well as with CO.     

Effects of oxygen and sulphydryl-containing compounds on irradiated transforming DNA. III. Reaction rates

The absolute rate for the repair reaction of radiation-induced, oxygen-dependent lesions in bacterial transforming DNA with the sulphydryl (SH)-containing compound dithiothreitol (DTT) has been determined using a fast response method, the gas explosion technique, to be 1.6 X 10(6) mol-1 s-1. Glutathione reacts ten times slower than DTT with the irradiated transforming DNA. It can also be calculated that transforming DNA radicals react with O2 in a damage-fixing reaction with a rate of about 3 X 10(8) dm3 mol-1 s-1. These rates are compared with values in the literature for reaction rates of SH, compounds and O2 with irradiated DNA constituents and with bacterial cells.     

Model studies for the direct effect of high-energy irradiation on DNA. Mechanism of strand break formation induced by laser photoionization of poly U in aqueous solution

Laser flash photolysis of polyuridylic acid (poly U) in anoxic aqueous solutions leads to biphotonic photoionization of the uracil moiety followed by the formation of single strand breaks (ssb). The rate constant for ssb formation (1.0 s-1, obtained from the slow component of conductivity increase at 23 degrees C and pH 6.8) increases with decreasing pH to 235 s-1 at pH 3.5. The activation energy (pre-exponential factor) was measured to be 66 kJ mol-1 (5 X 10(11) s-1) at pH 6.8. Addition of dithiothreitol (DTT) or glutathione (GSH) prevents ssb formation by reacting with a poly U intermediate (rate constant = 1.2 X 10(6) and 0.16 X 10(6) dm3 mol-1 s-1, respectively). Since with OH radicals as initiators very similar data have been obtained for the kinetics of ssb formation and for the reaction with DTT, we conclude that photoionization of the uracil moiety in poly U leads eventually to the same chemical pathway for ssb formation as that induced by OH radicals. Furthermore, we propose that protection by DTT and GSH occurs via H donation to the C-4' radicals of the sugar moiety of DNA and to the C-4' and the C-2' radicals of poly U.     

Gamma-radiolysis of N2O-saturated formate solutions. A chain reaction

In the radiolysis of aqueous formate-containing solutions a chain reaction (i, ii) proceeds in the presence of N2O. CO2-. + N2O + H2O----CO2 + N2 + .OH + OH- (i) .OH + HCO2-.----CO2-. + H2O (ii) The chain length depends on the dose rate and the N2O concentration but not on the formate concentration. Typically, G(CO2) approximately 140 molecules (100 eV)-1 is found, with an equivalent amount of N2, at a dose rate of 3 X 10(-3) Gy s-1. The rate constant for the rate-determining step in this chain reaction has been calculated at k(i) = 1600 dm3 mol-1 s-1. The possible relevance of this chain reaction in radiation biological studies is briefly discussed.     

Chemical properties of water-soluble porphyrins. 5. Reactions of some manganese (III) porphyrins with the superoxide and other reducing radicals

Solution properties of three manganese porphyrins, in monomeric form, were investigated. These were the 'picket-fence-like' porphyrin Mn(III)-alpha,alpha,alpha,beta- tetra-ortho(N-methylisonicotinamidophenyl)porphyrin (Mn(III)PFP) and two 'planar unhindered' porphyrins, the Mn(III)TMPyP (tetrakis (4-N-methylpyridyl)porphyrin) and Mn(III)TAP (tetra(4-N,N,N-trimethylanilinium)porphyrin). The porphyrin properties studied were: the absorption spectra in their manganic and manganous forms; acid/base properties of the aquo complexes; the effect of potential axial ligands (up to a concentration of 0.1 mol dm-3) and their one electron reduction potentials. Knowing these properties, the reaction of the Mn(III) porphyrins with the superoxide radical and other reducing radicals were studied using the pulse radiolysis technique. The second-order reaction rate constant of O2- with the Mn(III) porphyrins, which governs the catalytic efficiency of the metalloporphyrins upon the disproportionation of the superoxide radical, was 5.1 X 10(7) to 4.0 X 10(5) dm3 mol-1 s-1, depending on the pH and the nature of the metalloporphyrin. These values are at least one order of magnitude lower than found for Fe(III)TMPyP. One electron reduction of the three Mn(III) porphyrins by eaq-, CO2-, CH2OH and (CH3)2COH had similar second-order rate constants (10(9)-10(10) dm3 mol-1 s-1). That for (CH3)2(CH2)COH was about 10(5) dm3 mol-1 s-1. Reduction in all cases produced the corresponding Mn(II) porphyrin and no intermediate was found. The oxidation reaction of the Mn(II) porphyrins by O2- was approximately two orders of magnitude faster when compared to the reduction of Mn(III) porphyrins with the same radical. Since the reactivities of O2- towards the three manganese (III) compounds follow their reduction potentials, it is suggested that these reactions are governed by an outer-sphere mechanism. This suggestion is corroborated by the finding that water molecules acting as axial ligands, in these aqueous solution systems, are not replaced by another potential ligand when the latter is in the concentration range of 100 mM or less.     

Oxidation of polyunsaturated fatty acids and lipids through thiyl and sulfonyl radicals: reaction kinetics, and influence of oxygen and structure of thiyl radicals.

Thiyl free radicals have been shown to react with polyunsaturated fatty acids via abstraction of bisallylic hydrogen, forming pentadienyl radicals, and via addition to the double bonds. In the absence of oxygen, the latter pathway leads to regeneration of thiyl radicals through beta-elimination or "repair" of the adduct radicals by thiols. In the presence of oxygen, fixation of thiyl-induced damage occurs through reaction of O2 with the pentadienyl radical (yielding conjugated dienyl peroxyl radicals) and also with the thiyl-to-double bond adduct radical. A quantitative reaction scheme evaluated from these data considers abstraction, addition, rearrangement, and repair reactions, and the evaluation of rate constants for the individual steps. Absolute rate constants have been measured, in particular, for reactions of thiyl free radicals from glutathione, cysteine, homocysteine, N-acetylcysteine, cysteine ethyl ester, penicillamine, captopril, mercaptoethanol, and dithiothreitol with polyunsaturated fatty acids (PUFAs) ranging from 18:2 to 22:6, and the lipids trilinolein and trilinolenin. The rate constants for hydrogen abstraction were found to be typically of the order of 10(7) mol-1 dm3 s-1 and to increase with increasing lipophilicity of the attacking thiyl radical. Thioperoxyl radicals, RSOO., were found to be rather unreactive toward PUFAs, in contrast to the isomer sulfonyl radicals, RSO2., which not only abstract hydrogen from the bisallylic methylene groups of the PUFAs (although only at relatively small yield) but also readily add to the PUFA double bonds (major pathway). Specific information was obtained on the optical properties of the thiyl radical derived from the ACE inhibitor captopril, CpS. (lambda max = 340 nm, epsilon = 460 +/- 50 mol-1 dm3 cm-1), and its conjugate disulfide radical anion (CpS:.SCp) (lambda max = 420 nm).     

Vitamin E analogue Trolox C. E.s.r. and pulse-radiolysis studies of free-radical reactions.

The reactions between Trolox C, a water-soluble vitamin E analogue, and several oxidizing free radicals including the hydroxyl radical and various peroxy radicals were examined by using the pulse-radiolysis technique. The results demonstrate that Trolox C may undergo rapid one-electron-transfer reactions as well as hydrogen-transfer processes; the resulting phenoxyl radical is shown to be relatively stable, in common with the phenoxyl radical derived from vitamin E. The reactions between the Trolox C phenoxyl radical and a variety of biologically relevant reducing compounds were examined by using both pulse radiolysis and e.s.r. The results demonstrate that the Trolox C phenoxyl radical is readily repaired by ascorbate (k = 8.3 x 10(6) dm3.mol-1.s-1) and certain thiols (k less than 10(5) dm3.mol-1.s-1) but not by urate, NADH or propyl gallate. Evidence from e.s.r. studies indicates that thiol-containing compounds may also enter into similar repair reactions with the alpha-tocopherol phenoxyl radical. Kinetic evidence is presented that suggests that Trolox C may 'repair' proteins that have been oxidized by free radicals.     

Mechanism of reduction of bleomycin-Cu(II) by CO2- and oxidation of bleomycin-Cu(I) by H2O2 in the absence and presence of DNA

The reduction reaction of bleomycin-Cu(II) by CO2- has been studied by gamma and pulse radiolysis at pH7. The CO2- radical reduces bleomycin-Cu(II) at a rate of (6.7 +/- 0.7) X 10(8) dm3 mol-1 s-1. In the presence of calf thymus DNA the rate of the reduction decreased as the concentration of DNA increased, indicating that the reduction reaction proceeds through free bleomycin-Cu(II). The stoichiometry and the kinetics of the oxidation of bleomycin-Cu(I) by H2O2 in the presence and absence of DNA have been studied. Our observations suggest that the OH. radical is not produced during this reaction and the degradation of the drug occurs in the absence and presence of DNA. We assume that bleomycin-Cu(II) in the presence of a reducing agent and molecular oxygen or H2O2 does not cleave DNA since the oxidizing species, which are formed during the oxidation reaction by H2O2, attack the drug even in the presence of DNA.     

The effect of oxygen on the radiolysis of tyrosine in aqueous solutions

The effect of oxygen on the radiolysis of tyrosine in aqueous solutions was investigated by using gamma and pulsed electron irradiation. Steady-state radiolysis was reexamined and extended to include the effect of pH and determination of hydrogen peroxide. The loss of tyrosine, G(-Tyr), during irradiation and yields of 3,4-dihydroxyphenylalanine, G(DOPA), and hydrogen peroxide, G(H2O2), are determined in the pH range from 1 to 9. In the whole pH range used G(-Tyr) equals G(DOPA), and a higher G(H2O2) than expected was observed. In slightly acid and neutral media, both G(-Tyr) and G(DOPA) equal the yield of hydroxyl radicals, GOH, formed in the radiolysis of water, while the excess of hydrogen peroxide equals 1/2 GOH. Hence it was concluded that all tyrosine OH-adducts react with oxygen yielding peroxy radicals. In acid and alkaline media all measured yields decrease. This is caused by formation of tyrosine phenoxyl radicals (TyrO), which react with superoxide anion (O2-) and hydroperoxy (HO2) radicals regenerating tyrosine. By using pulse radiolysis K(TyrO + O2) less than or equal to 2 X 10(5) mol-1 dm3 s-1 and k(TyrO + O2-) = (1.7 +/- 0.2) X 10(9) mol-1 dm3 s-1 were determined. On the basis of the results, a reaction mechanism is proposed.     

The reactivity of tryptophan residues in proteins. Stopped-flow kinetics of fluorescence quenching.

The quenching of tryptophan fluorescence by N-bromosuccinamide, studied by the fluorescence stopped-flow technique, was used to compare the reactivities of tryptophan residues in protein molecules. The reaction of N-bromosuccinamide with the indole group of N-acetyltryptophanamide, a model compound for bound tryptophan, followed second-order kinetics with a rate constant of (7.8 +/- 0.8) . 10(5) dm3 . mol-1 . s-1 at 23 degrees C. The rate does not depend on ionic strength or on the pH near neutrality. The non-fluorescent intermediate formed from N-acetyltryptophanamide on the reaction with N-bromosuccinamide appears to be a bromohydrin compound. The second-order rate constant for fluorescence quenching of tryptophan in Gly-Trp-Gly by N-bromosuccinamide was very similar, (8.8 +/- 0.8) . 10(5) dm3 . mol-1 . s-1. Apocytochrome c has the conformation of a random coil with the single tryptophan largely exposed to the solvent. The rate constant for the fluorescence quenching of the tryptophan in apocytochrome c by N-bromosuccinamide was (3.7 +/- 0.3) . 10(5) dm3 . mol-1 . s-1. The fluorescence quenching by N-bromosuccinamide of the tryptophan residues incorporated in alpha-chymotrypsin at pH 7.0 showed three exponential terms from which the following rate constants were derived: 1.74 . 10(5), 0.56 . 10(5) and 0.11 . 10(5) dm3 . mol-1 . s-1. This protein is known to have eight tryptophan residues in the native state, six residues at the surface, and two buried. Three of the surface tryptophans have the indole rings protruding out of the molecule and may account for the fastest kinetic phase of the quenching process. The intermediate phase may be due to three surface tryptophans whose indole rings point inwards, and the slowest to the two interior tryptophan residues.