Addition Of Trifluoroacetic Acid Research Articles

Synthesis and Multistimuli-responsive Behavior of Octaethylporphyrin–Dihexylbithiophene–Dimethylaniline Triads Connected with Diacetylene Linkages

Abstract Octaethylporphyrin–dihexylbithiophene–dimethylaniline triads were synthesized. The triads showed color change from green to yellow upon addition of trifluoroacetic acid. VT-NMR and UV–vis measurements revealed acid- and heat-driven multistimuli-response behaviors. In comparison with pyridine-connected analogs, a porphyrin–bithiophene diad moiety connected with diacetylene linkage expands the difference in basicity between dimethylaniline and pyridine.

Isomerization Mechanism in Hydrazone-Based Rotary Switches: Lateral Shift, Rotation, or Tautomerization?

Two intramolecularly hydrogen-bonded arylhydrazone (aryl = phenyl or naphthyl) molecular switches have been synthesized, and their full and reversible switching between the E and Z configurations have been demonstrated. These chemically controlled configurational rotary switches exist primarily as the E isomer at equilibrium and can be switched to the protonated Z configuration (Z-H(+)) by the addition of trifluoroacetic acid. The protonation of the pyridine moiety in the switch induces a rotation around the hydrazone C=N double bond, leading to isomerization. Treating Z-H(+) with base (K(2)CO(3)) yields a mixture of E and "metastable" Z isomers. The latter thermally equilibrates to reinstate the initial isomer ratio. The rate of the Z → E isomerization process showed small changes as a function of solvent polarity, indicating that the isomerization might be going through the inversion mechanism (nonpolar transition state). However, the plot of the logarithm of the rate constant k vs the Dimroth parameter (E(T)) gave a linear fit, demonstrating the involvement of a polar transition state (rotation mechanism). These two seemingly contradicting kinetic data were not enough to determine whether the isomerization mechanism goes through the rotation or inversion pathways. The highly negative entropy values obtained for both the forward (E → Z-H(+)) and backward (Z → E) processes strongly suggest that the isomerization involves a polarized transition state that is highly organized (possibly involving a high degree of solvent organization), and hence it proceeds via a rotation mechanism as opposed to inversion. Computations of the Z ↔ E isomerization using density functional theory (DFT) at the M06/cc-pVTZ level and natural bond orbital (NBO) wave function analyses have shown that the favorable isomerization mechanism in these hydrogen-bonded systems is hydrazone-azo tautomerization followed by rotation around a C-N single bond, as opposed to the more common rotation mechanism around the C=N double bond.

Species classification and quality assessment of Chaihu (Radix Bupleuri) based on high-performance liquid chromatographic fingerprint and combined chemometrics methods

A high-performance liquid chromatographic (HPLC) method was established to analyze 36 Chaihu (Radix Bupleuri) samples collected from three species (Bupleurum chinense DC., B. scorzonerifolium Willd. and B. smithii Wolff.). Addition of trifluoroacetic acid into the mobile phase resulted in fingerprint chromatograms with stable baselines. There were thirty-two characteristic peaks in the standard fingerprint of B. chinense DC. Different recognition pattern methods, including similarity analysis (SA), hierarchical cluster analysis (HCA), principal component analysis (PCA) and partial least squares-discrimination analysis (PLS-DA) were utilized to analyze the 36 samples based on the contents of chemical constituents. Consistent results from SA, HCA and PCA analysis illustrated the rationalisation for why B. smithii Wolff. was not quoted in the Chinese Pharmacopoeia and classified samples were in agreement with their species. PLS-DA loading plots showed the chemical markers which had the most influences on the separation among different species. However, SA, HCA and PCA could not differentiate between wild and cultivated B. chinense DC. as well as between samples from different provinces. HPLC fingerprint in combination with chemometric techniques provided a very flexible and reliable method for homogeneity evaluation and quality assessment of traditional Chinese medicine.

Preparation and optical and electrochemical properties of octaethylphthalocyanines fused with several tetrathiafulvalene units

Abstract3,6‐Diethylphthalonitrile (3) with a tetrathiafulvalene (TTF) unit at 4,5‐positions was prepared from 4,5‐xylylenedithio‐3,6‐diethylphthalonitrile (1a) via elimination of the xylylene group, connection of a carbonyl group to benzenedithiolate generated, and condensation of 4,5‐bis(methylthio)‐1,3‐dithiole‐2‐thione with benzo‐1,3‐dithiole‐2‐one (2‐O) produced. A 1:1 mixture of phthalonitrile (3) and 4,5‐bis(benzylthio)‐3,6‐diethylphthalonitrile (1b) was treated with lithium in n‐hexanol at 120°C to produce hexakis (benzylthio)mono(tetrathiafulvaleno)phthalocyanine (5), tetrakis(benzylthio)bis(tetrathiafulvaleno)phthalocyanine (6), and bis(benzylthio)tris(tetrathiafulvaleno)phthalocyanine (7). The structures of 5, 6, and 7 were determined by 1H NMR, FAB MS, MALDI‐TOF MS (matrix assisted laser desorption ionization time‐of‐flight mass spectrometry), and UV‐‐vis spectroscopy. Compound 6 is a mixture of trans and cis isomers (6‐trans and 6‐cis). The UV‐‐vis spectrum of 5 measured in chloroform changed by addition of trifluoroacetic acid (TFA). The Q band absorption at λmax = 755 nm (chloroform) decreased in intensity and resulted in a new absorption at λmax = 740 nm (chloroform/TFA). The electrochemical properties of 5, 6, and 7 were determined by cyclic voltammetry using Ag/AgNO3 as a reference electrode. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:605–611, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20694

Near-infrared BODIPY dyes modulated with spirofluorene moieties

New structurally constrained BODIPY dyes having electron-donating substituents were synthesized. As the key compounds for the construction of the BODIPY dyes, 1′ H-spiro-[fluorene-9,4′-indeno[1,2- b]pyrrole] ( sp- FIP) derivatives with electron-donating groups, such as OMe and NMe 2 at its 6′-position, were prepared using palladium-catalyzed intramolecular direct C–H arylation of a pyrrole moiety. The resulting BODIPY dyes showed bathochromic shift in absorption and fluorescence spectra in comparison to the unsubstituted analogs. Furthermore, pH-dependent reversible spectrum changes of the BODIPY dye were observed with the addition of trifluoroacetic acid (TFA) and subsequent addition of i-Pr 2NEt.

Controlled Intramolecular Electron Transfers in Cyanide-Bridged Molecular Squares by Chemical Modifications and External Stimuli

A series of cyanide bridged Fe-Co molecular squares, [Co(2)Fe(2)(CN)(6)(tp*)(2)(dtbbpy)(4)](PF(6))(2)·2MeOH (1), [Co(2)Fe(2)(CN)(6)(tp*)(2)(bpy)(4)](PF(6))(2)·2MeOH (2), and [Co(2)Fe(2)(CN)(6)(tp)(2)(dtbbpy)(4)](PF(6))(2)·4H(2)O (3) (tp = hydrotris(pyrazol-1-yl)borate, tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, bpy =2,2'-bipyridine, dtbbpy =4,4'-di-tert-butyl-2,2'-bipyridine), were prepared by the reactions of [Fe(CN)(3)(L)](-) (L = tp or tp*) with Co(2+) and bidentate ligands (bpy or dtbbpy) in MeOH. In the molecular squares, Fe and Co ions are alternately bridged by cyanide ions, forming macrocyclic tetranuclear cores. Variable temperature X-ray structural analyses and magnetic susceptibility measurements confirmed that 1 exhibits two-step charge-transfer induced spin transitions (CTIST) centered at T(1/2) = 275 and 310 K in the solid state. The Fe and Co ions in 1 are the low-spin (LS) Fe(III) and high-spin (HS) Co(II) ions, described here in the high-temperature (HT) phase ([Fe(III)(LS2)Co(II)(HS2)]) at 330 K, while a low-temperature (LT) phase ([Fe(II)(LS2)Co(III)(LS2)]) with LS Fe(II) and Co(III) ions was dominant below 260 K. X-ray structural analysis revealed that in the intermediate (IM) phase at 298 K 1 exhibits positional ordering of [Fe(III)(LS2)Co(II)(HS2)] and [Fe(II)(LS2)Co(III)(LS2)] species with the 2:2 ratio. In photomagnetic experiments on 1, light-induced CTIST from the LT to the HT phase was observed by excitation of Fe(II) → Co(III) intervalence charge transfer (IVCT) band at 5 K and the trapped HT phase thermally relaxed to the LT phase in a two-step fashion. On the other hand, 2 and 3 are in the HT and LT phases, respectively, throughout the entire temperature range measured, and no CTIST was observed. UV-vis-NIR absorption spectral measurements and cyclic voltammetry in solution revealed that the different electronic states in 1-3 are ascribable to the destabilization of iron and cobalt ion d-orbitals by the introduction of methyl and tert-butyl groups to the ligands tp and bpy, respectively. Temperature dependence of UV-vis-NIR spectra confirmed that 1 exhibited a one-step CTIST in butyronitrile, of which T(1/2) varied from 227 to 280 K upon the addition of trifluoroacetic acid.

Tunable Electronic Properties of a Proton‐ResponsiveN,N‐Dimethylaminoazobenzene Fullerene (C60) Dyad

A basic N,N-dimethylaminoazobenzene-fullerene (C(60)) dyad molecular skeleton is modelled and synthesized. In spite of the myriad use of azobenzene as a photo- and electrochromic moiety, the idea presented herein is to adopt a conceptually different path by using it as a bridge in a donor-bridge-acceptor single-molecular skeleton, connecting the electron acceptor N-methylfulleropyrrolidine with an electron donor N,N-dimethylaniline. Addition of trifluoroacetic acid (TFA) results in a drastic colour change of the dyad from yellow to pink in dichloromethane (DCM). The structure of the protonated species are established from electronic spectroscopy and time-dependent density functional theory (TD-DFT) calculations. UV/Vis spectroscopic investigations reveal the disappearance of the 409 nm (1)(π→π*) transition with appearance of new features at 520 and 540 nm, attributed to protonated β and α nitrogens, respectively, along with a finite weight of the C(60) pyrrolidinic nitrogen. Calculations reveal intermixing of n((N=N))→π*((N=N)) and charge transfer (CT) transitions in the neutral dyad, whereas, the n((N=N))→π*((N=N)) transition in the protonated dyad is buried under the dominant (1)(π →π*) feature and is red-shifted upon Gaussian deconvolution. The experimental binding constants involved in the protonation of N,N-dimethylanilineazobenzene and the dyad imply an almost equal probability of existence of both α- and β-protonated forms. Larger binding constants for the protonated dyads imply more stable dyad complexes than for the donor counterparts. One of the most significant findings upon protonation resulted in frontier molecular orbital (FMO) switching with the dyad LUMO located on the donor part, evidenced from electrochemical investigations. The appearance of a new peak, prior to the first reduction potential of N-methylfulleropyrrolidine, clearly indicates location of the first incoming electron on the donor-centred LUMO of the dyad, corroborated by unrestricted DFT calculations performed on the monoanions of the protonated dyad. The protonation of the basic azo nitrogens thus enables a rational control over the energetics and location of the FMOs, indispensable for electron transport across molecular junctions in realizing futuristic current switching devices.

Effect of addition of trifluoroacetic acid on the photophysical properties and photoreactions of aromatic imides

The UV and IR spectra of N-methyl-1,8-naphthalimide in benzene showed a two-step consecutive complexation (hydrogen bond formation) with trifluoroacetic acid (TFA). The equilibrium constant K 1 for the first complexation in benzene was determined from the UV spectrum to be 48 M −1. The fluorescence intensities of the imide in benzene were found to be remarkably enhanced by the addition of TFA. Furthermore, photochemical cyclobutane formation of the imide with styrene in benzene was enhanced by the addition of TFA. Enhancement of the fluorescence intensity and the photoreaction of the imide by complexation with TFA was explained by a decrease of the efficiency of the intersystem crossing from 1(ππ ∗) to 3( nπ ∗), that results from an increase in the energy of the 3( nπ ∗) level due to the complexation.

Palladium-Catalyzed Ortho-Arylation of O-Phenylcarbamates with Simple Arenes and Sodium Persulfate

By palladium catalysis, the C-H bond functionalization of O-phenylcarbamates with simple arenes has been achieved using sodium persulfate (Na(2)S(2)O(8)), an inexpensive, easy-to-handle, and environmentally friendly oxidant. This oxidative cross-coupling involves two aromatic C-H bonds undergoing concomitant oxidation to furnish a new biaryl C-C linkage. Excellent reaction efficiencies and regioselectivities were observed with a range of electron-rich, electron-neutral, and electron-deficient arenes; minimal homocoupling of either component was observed. When two reactive C-H bonds are present on the O-phenylcarbamate, selective diarylation can be achieved via quadruple C-H bond functionalization. This work represents a rare example of using O-carbamates as directing groups for catalytic C-H bond activation. Additionally, a palladacycle obtained from an O-phenylcarbamate was prepared and fully characterized. This trifluoroacetate-bridged bimetallic Pd complex exhibits clean conversion to the ortho-arylation product upon treatment with simple arenes. The addition of trifluoroacetic acid (TFA) was found to be critical for successful cyclopalladation of O-phenylcarbamates. We propose this oxidative arene cross-coupling occurs via two discrete C-H bond activations, namely cyclopalladation and electrophilic metalation, within a Pd(0/II) catalytic cycle.

Aromatic Fumaronitrile Core-Based Donor−Linker−Acceptor−Linker−Donor (D-π-A-π-D) Compounds: Synthesis and Photophysical Properties

A new class of aromatic fumaronitrile core-based compounds with different donors and linkers has been synthesized and well characterized. Compounds 1 and 2 have indole and 2-phenylindole groups as electron donors, respectively. Compounds 3 and 4 have a diphenylamino group as the electron donor, and compound 5 has a 3,6-di-tert-butylcarbazole group as an electron donor. These compounds absorb in the blue-to-green region and emit in the blue-to-red region depending on the electron donor, linker, and solvents. The quantum yields of fluorescence of these compounds in solution are measured and found to be moderate, but in solid states, they are high. These compounds display strong emission solvatochromism that is reflected by a large shift in their fluorescence emission maxima on changing the solvents. This change is accompanied by a successive decrease in fluorescence intensity. The fluorescence lifetimes of these compounds are measured in different solvent and found to vary from <1 to 7 ns. Optical switching of these compounds with solvents, concentration, and excitation energy have been studied. The correlation between the functional group and optical properties has been established to some extent. The ability of these compounds to function as colorimetric and luminescence pH sensors is demonstrated with color changes and luminescence switching upon the addition of trifluoroacetic acid. The potentiality of these compounds for application in optoelectronics has been optically assessed.

Synthesis, Electronic, and Emission Spectroscopy, and Electrochromic Characterization of Azulene−Fluorene Conjugated Oligomers and Polymers

Two model compounds, 1,3-bis(9,9-dihexylfluoren-2-yl)azulene (M1), and 1,3-bis[7-(9,9,9′,9′-tetrahexyl-2,2′-bifluoren-7-yl)azulene (M2), and polymers, poly[2,7-(9,9-dialkylfluorenyl)-alt-(1′,3′-azulenyl)] (P1−P4) and poly{[1,3-bis(9′,9′-dihexylfluoren-2′-yl)azulenyl]-alt-[2′′,7′′-(9′′,9′′-dialkylfluorenyl]} (P5, P6) were synthesized by reacting 1,3-dibromoazulene or 1,3-bis(7-bromo-9,9-dihexylfluoren-2-yl)azulene with a suitable 9,9-dialkylflourenyl-2-borate or 2,7-diborate via Suzuki cross-coupling reactions. The thermal and optical properties of the polymers were characterized by thermogravimetric analysis, differential scanning calorimetry, and UV−vis and fluorescence spectroscopy. M1, M2, and P1−P6 are nonfluorescent in the neutral state in different organic solvents. However, M1, M2, P5, and P6 become fluorescent upon the addition of trifluoroacetic acid in THF, with relative quantum efficiencies of 0.004−0.06. The “switching on” of the fluorescence for the azulene−fluorene copolymers and model compo...

Hydrogen Peroxide Oxygenation of Saturated and Unsaturated Hydrocarbons Catalyzed by Montmorillonite or Aluminum Oxide

Montmorillonites K-10, Na0.60K0.12Ca0.02(Al1.78Fe0.12Mg0.10)oct(Si3.89Al0.11)tetO10(OH)2, and NT-25, Na0.10K0.04Ca0.08(Al1.28Fe0.45Mg0.28)oct(Si3.94Al0.06)tetO10(OH)2, and aluminum oxide, Al2O3, catalyze alkane hydroperoxidation and olefin epoxidation with hydrogen peroxide. Alkanes afford alkyl hydroperoxides as main primary products which partially decompose to produce corresponding ketones and alcohols. The oxidation of cis- and trans-isomers of 1,2-dimethylcyclohexane on K-10 proceeds stereoselectively. Benzene in the presence of montmorillonite K-10 is transformed into phenol. Styrene is oxidized to afford benzaldehyde and styrene epoxide. It is proposed on the basis of a kinetic study and measurements of the selectivity parameters that in the case of alumina the oxidation occurs with the participation of hydroxyl radicals generated from an aluminum peroxo derivative on the catalyst surface. In the cases of montmorillonites iron centers are also responsible for the H2O2 activation which gives hydroxyl radicals. Addition of trifluoroacetic acid (TFA) leads to the noticeable acceleration of all oxidation reactions and enhancement of the product yield. It has been shown that in the presence of TFA some parts of solid catalysts are transferred into the homogeneous solution. Thus, very cheap natural solid materials provide soluble species that are active in efficient homogeneous oxidations of hydrocarbons with hydrogen peroxide. In the absence of TFA the oxidation reactions in the case of the three catalysts are truly heterogeneous in nature.

Modulating Large‐Area Self‐Assembly at the Solid–Liquid Interface by pH‐Mediated Conformational Switching

Dynamic surfaces: The conformational transition of 2,6-bis(1-aryl-1,2,3-triazol-4-yl)pyridine (BTP) derivatives, triggered by a change in pH, has been observed with a sub-nm resolution by STM at the solid-liquid interface. Upon addition of trifluoroacetic acid two different BTP molecules, each forming a highly ordered physisorbed monolayer, underwent significant conformational changes from their "rosette" to their "tetragon" forms, as reflected in dramatically altered 2D self-assembly over large areas extending over hundreds of nanometers (see graphic).

A rapid and robust liquid chromatography/tandem mass spectrometry method for simultaneous analysis of anti-tuberculosis drugs—Ethambutol and pyrazinamide in human plasma

Ethambutol and pyrazinamide are two first-line anti-tuberculosis drugs. Though they are normally combined for the treatment, their highly different polarity complicates simultaneous liquid chromatography/tandem mass spectrometry (LC/MS/MS) analysis of these two drugs in human plasma with decent peak shape and retention. Here we report a rapid and robust LC/MS/MS method for the simultaneous determination of these two drugs in human plasma. Human plasma samples, together with the isotopically labeled internal standards were extracted using protein precipitation, and then separated on a Chromolith SpeedROD RP-18e column and detected with mass spectrometry. The mobile phase is 0.1% trifluoroacetic acid in water and 0.1% trifluoroacetic acid in methanol. Addition of trifluoroacetic acid in the mobile phases was found to be able to improve peak shape as well as to increase the retention of ethambutol, thus being able to analyze these two drugs at the same time with both drugs having decent peak shape and enough retention on a C 18 column. An atmospheric pressure chemical ionization interface was chosen to reduce ion suppression from sample matrix components and provide high sensitivity. The standard curve range was 10.0–5000 ng/mL for ethambutol and 50.0–25,000 ng/mL for pyrazinamide using a plasma sample volume of 50.0 μL. This method has a very short run time of 3.8 min. The method has been fully validated, and <15% relative standard deviation was obtained for both analytes.

A Novel Chip-based Electroelution System for Rapid and Efficient Recovery of Intact Proteins from Polyacylamide Gels

A Novel Chip-based Electroelution System for Rapid and Efficient Recovery of Intact Proteins from Polyacylamide Gels

Synthesis of Highly Substituted Indoles via an Intramolecular Heck Cyclization

A combined multi-component Ugi-Smiles reaction followed by a Heck cyclization for the synthesis of substituted indole derivatives in good to high yield has been developed. Thus the Ugi-Smiles reaction of o-iodophenols, aldehydes, isocyanides and allyl amines gives highly functionalized aniline derivatives which, under Pd-catalyzed cyclization followed by isomerization, yield the highly substituted indoles. Furthermore, hydroxy pyrimidines were found to give indolopyrimidines by the corresponding reactions. Both reactions were successfully carried out in one-pot, with the addition of trifluoroacetic acid (0.2-0.3 equiv to affect the hydrolysis of the remaining isocyanide) before Heck cyclization.

Quenching and Recovery of Fluorescence of Azobenzenes by Acid–Base Reactions

Abstract 2-[Bis(pentafluorophenyl)boryl]-4′-(dimethylamino)azobenzene was synthesized as a hybrid of the intensely fluorescent 2-borylazobenzene and the pH-responsive 4-(dimethylamino)azobenzene. Quenching and recovery of fluorescence of this azobenzene were reversibly performed by addition of trifluoroacetic acid and triethylamine, respectively. The reason of fluorescence quenching was investigated by theoretical calculations.

Control of the Photoreactivity of Diarylethene Derivatives by Quaternarization of the Pyridylethynyl Group

Photochromic behavior of diarylethene derivatives with (4-pyridyl)ethynyl group directly attached to the 6-pi hexatriene moieties of the diarylethenes was investigated. Upon quaternarization of the pyridine moieties, the photoreactivity was strongly suppressed. On the other hand, diarylethene derivatives with nonconjugated (4-pyridyl)ethyl group exhibited the photochromic reactivity, regardless of whether pyridyl rings are quaternarized or not. In the case of the (4-pyridyl)ethynyl-substituted compounds, the photochromic reactivity was suppressed by the addition of trifluoroacetic acid and was restored by diethylamine.

Preparation and chemical properties of soluble π‐conjugated poly(aryleneethynylene) consisting of azabenzothiadiazole as the electron‐accepting unit

AbstractA soluble charge‐transfer type poly(aryleneethynylene), PAE‐AzaBzTdz, consisting of a highly electron‐accepting azabenzothiadiazole unit was prepared in 99% yield by palladium‐catalyzed polycondensation between 4,7‐dibromo‐2,1,3‐azabenzothiadiazole (Br2‐AzaBzTdz) and 1,4‐diethynyl‐2,5‐didodecyloxybenzene. PAE‐AzaBzTdz showed a number‐average molecular weight, Mn, of 6000 in gel‐permeation chromatography analysis and had good thermal stability as measured by TGA. UV–vis spectrum of PAE‐AzaBzTdz exhibited an absorption peak at 529 nm in chloroform, and the absorption peak shifted to a longer wavelength (601 nm) in film. Addition of MeOH to a CHCl3 solution of PAE‐AzaBzTdz led to aggregation of the polymer to form stable colloidal particles. Results of filtration experiments using 0.2 and 0.02 μm membranes supported aggregation of the polymer. Addition of trifluoroacetic acid (TFA) to a chloroform solution of PAE‐AzaBzTdz led to a red‐shift of the UV–vis peak from 529 to 640 nm. An X‐ray diffraction pattern of powdery PAE‐AzaBzTdz indicated that the polymer assumed a layer‐to‐layer stacked structure with an interlayer distance of 3.4 Å in the solid state. An X‐ray diffraction pattern of cast film of PAE‐AzaBzTdz revealed that the polymer molecules in the cast film were ordered on the surface of Pt plate with the dodecyl side chain oriented toward the surface of the Pt plate. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2975–2982, 2008

Inhibition of Human Dimethylarginine Dimethylaminohydrolase-1 by S-Nitroso-L-homocysteine and Hydrogen Peroxide

The plasma concentrations of two cardiovascular risk factors, total homocysteine (tHcy) and asymmetric dimethylarginine (ADMA), correlate with decreased levels of endothelium-derived nitric oxide and subsequent endothelial dysfunction. Homocysteine has been proposed to inhibit the catabolic enzyme of ADMA, dimethylarginine dimethylaminohydrolase (DDAH), but the mechanism of this inhibition has not been fully elucidated. Here, the human DDAH isoform-1 (DDAH-1) is heterologously expressed and purified. Cys(274) and His(173) are identified as active site residues and the pH rate dependence is described. Because oxidation of the active site Cys has been suggested as an inhibitory mechanism in patients with hyperhomocysteinemia, the sensitivity of DDAH-1 to inhibition by L-homocysteine, H(2)O(2), and S-nitroso-L-homocysteine is quantified. DDAH-1 is surprisingly insensitive to inactivation by the powerful oxidant, H(2)O(2) (0.088 M(-1) s(-1)), possibly because of a substrate-assisted mechanism that allows the active site cysteine to remain predominantly protonated and less reactive in the resting enzyme. In contrast, DDAH-1 is sensitive to inactivation by S-nitroso-L-homocysteine (3.79 M(-1) s(-1)). This work illustrates how a particular catalytic mechanism can result in selective redox regulation and has possible implications for hyperhomocysteinemia.