Electrochemical oxidation of 2,6-diamino-8-purinol

Electrochemistry, spectroelectrochemistry and gas chromatography/mass spectrometry were used to determine the mechanism of oxidation of 2,6-diamino-8-purinol in phosphate buffer solutions. Experimental evidence indicates that the overall process is a 2 e⁻, 2 H⁺ reaction in which 2,6,-diamino-8-purinol is oxidized to form a diimine. The diimine is not stable and decomposes in a series of hydrolysis reactions to the final products 5-hydroxyghdantoin-5-carboxamide and 1-carbohydroxy2-,4,6,8-tetraaza-3,7-dioxo-4-ene-bicyclo[3,3,0]octane (BCA). Results are presented which show that the 2 e⁻ oxidation of 2,6-diamino-8-purinol proceeds in two 1 e⁻ steps and the evidence for the formation of a coupling product is discussed. A mechanism is proposed to explain the observed results.     

Polymer-modified glassy carbon electrode for the electrochemical detection of quinine in human urine and pharmaceutical formulations

Glassy carbon electrode was modified by electropolymerization of 4-amino-3-hydroxynaphthalene sulfonic acid. Cyclic voltammetric study of quinine showed higher current response at the modified electrode compared to the bare and activated glassy carbon electrodes in pH 7.0 phosphate buffer solution. Under optimized conditions, a calibration curve was obtained by square wave voltammetry at the modified electrode. The linear relationship between the peak current and the concentration of quinine in the range of 1.0?×?10^?7 to 1.0?×?10^?5 M was I _pa (in microamperes)?=?6.26C (in micromolars)?+?0.2997 (R ²?=?0.999). The detection limit calculated (S/N?=?3) was 1.42?×?10^?8 M, which is much lower than similar reports. The method was successfully applied for the determination of quinine in spiked human urine, and pharmaceutical formulations and recovery values >90 % were obtained.     

Extraction—spectrophotometric determination of boron with 2,6-dihydroxybenzoic acid and 4-(4-diethyl-aminophenylazo)-n-methylpyridinium iodide,with application to steels

The sensitive and selective method reported is based on the reaction of boric acid with 2,6-dihydroxybenzoic acid in acidic aqueous solution to form a 1:2 complex anion which can be extracted into chloroform as an ion-pair with 4-(4-diethylaminophenylazo)-N-methylpyridinium ion. The ion-pair formed by the excess of reagents and co-extracted into chloroform, is removed by washing the organic phase with phosphate buffer solution (pH 7.0) and 0.025 M sulfuric acid solution. The absorbance of azo dye in chloroform is measured at 570 nm. Calibration graphs are linear in the range 0–1.5 × 10^-5 M of boron. The molar absorptivity is 6.6 × 10⁴ l mol^-1 cm^-1 and the absorbance of the reagent blank is 0.030. The method was applied to the determination of boron at the 0.001% level in steels.     

Flow Injection Analysis of Maleic Hydrazide Using an Electrochemical Sensor Based on Palladium‐Dispersed Carbon Paste Electrode

A new analytical methodology for the electrochemical detection of the herbicide maleic hydrazide (3,6‐dihydroxypyridazine) by flow injection analysis is presented. This method is supported by the novel application of a palladium‐dispersed carbon paste electrode as an amperometric sensor for this herbicide. Maleic hydrazide shows anodic electrochemical activity on carbon‐based electrodes (glassy carbon or carbon paste electrodes) in all the pH range. This electrochemical activity is enhanced using metal‐dispersed carbon paste electrodes, especially at Pd‐dispersed CPE which displays good oxidation signals at 690 mV (0.050 M phosphate buffer pH 7.0), 140 mV lower than at unmodified electrodes. Under the optimized conditions, the electroanalytical performance of Pd‐dispersed CPE in flow injection analysis was excellent, with good reproducibility (RSD 3.3%) and a wide linear range (1.9×10^?7 to 1.0×10^?4 mol L^?1). A detection limit of 1.4×10^?8 mol L^?1 (0.14 ng maleic hydrazide) was obtained for a sample loop of 100 μL at a fixed potential of 700 mV in 0.050 M phosphate buffer solution at pH 7.0 and a flow rate of 2.0 mL min^?1. The proposed method was applied for the maleic hydrazide detection in natural drinking water samples.     

Direct electrochemistry and electrochemical biosensing of glucose oxidase based on CdSe@CdS quantum dots and MWNT-modified electrode

The direct electron transfer of glucose oxidase (GOx) was achieved based on the immobilization of CdSe@CdS quantum dots on glassy carbon electrode by multi-wall carbon nanotubes (MWNTs)-chitosan (Chit) film. The immobilized GOx displayed a pair of well-defined and reversible redox peaks with a formal potential (E ^θ’) of ?0.459 V (versus Ag/AgCl) in 0.1 M pH 7.0 phosphate buffer solution. The apparent heterogeneous electron transfer rate constants (k _s) of GOx confined in MWNTs-Chit/CdSe@CdS membrane were evaluated as 1.56 s^?1 according to Laviron's equation. The surface concentration (Γ*) of the electroactive GOx in the MWNTs-Chit film was estimated to be (6.52?±?0.01)?×?10^?11?mol?cm^?2. Meanwhile, the catalytic ability of GOx toward the oxidation of glucose was studied. Its apparent Michaelis–Menten constant for glucose was 0.46?±?0.01 mM, showing a good affinity. The linear range for glucose determination was from 1.6?×?10^?4 to 5.6?×?10^?3?M with a relatively high sensitivity of 31.13?±?0.02 μA?mM^?1?cm^?2 and a detection limit of 2.5?×?10^?5?M (S/N=3).     

On the syntheses of 8-Heteroaryl-substituted 9-(β-D-Ribofuranosyl)-2,6-diaminopurines through Pd-catalyzed coupling in the presence of cupric oxide

Convenient methods for the preparation of 9-(β-D-ribofuranosyl) derivatives of 8-(2- and 3-thienyl)-2,6-diaminopurine and of 8-(2- and 3-furyl)-2,6-diaminopurine, which are potential antiviral agents has been worked out. The key step was a Pd(0)-catalyzed Stille coupling between 2- and 3-tributylstannylthiophene and 2- and 3-tributylstannylfuran and trimethylsilyl protected 9-(β-D-ribofuranosyl)-2,6-diamino-8-bro-mopurine. The use of N,N-dimethylformamide as solvent at 110° and dichloro(diphenylphosphine-propane)palladium(II) [PdC1₂(dppp)] with cupric oxide as co-reagent was essential in order to obtain a fast reaction and high yields.     

Mediated electron transfer of cellobiose dehydrogenase and glucose oxidase at osmium polymer-modified nanoporous gold electrodes

Nanoporous and planar gold electrodes were utilised as supports for the redox enzymes Aspergillus niger glucose oxidase (GOx) and Corynascus thermophilus cellobiose dehydrogenase (CtCDH). Electrodes modified with hydrogels containing enzyme, Os-redox polymers and the cross-linking agent poly(ethylene glycol)diglycidyl ether were used as biosensors for the determination of glucose and lactose. Limits of detection of 6.0 (±0.4), 16.0 (±0.1) and 2.0 (±0.1) μM were obtained for CtCDH-modified lactose and glucose biosensors and GOx-modified glucose biosensors, respectively, at nanoporous gold electrodes. Biofuel cells composed of GOx- and CtCDH-modified gold electrodes were utilised as anodes, together with Myrothecium verrucaria bilirubin oxidase (MvBOD) or Melanocarpus albomyces laccase as cathodes, in biofuel cells. A maximum power density of 41 μW/cm² was obtained for a CtCDH/MvBOD biofuel cell in 5 mM lactose and O₂-saturated buffer (pH 7.4, 0.1 M phosphate, 150 mM NaCl).     

Direct electrochemistry and bioelectrocatalysis of a class II non-symbiotic plant haemoglobin immobilised on screen-printed carbon electrodes

In this study, direct electron transfer (ET) has been achieved between an immobilised non-symbiotic plant haemoglobin class II from Beta vulgaris (nsBvHb2) and three different screen-printed carbon electrodes based on graphite (SPCE), multi-walled carbon nanotubes (MWCNT-SPCE), and single-walled carbon nanotubes (SWCNT-SPCE) without the aid of any electron mediator. The nsBvHb2 modified electrodes were studied with cyclic voltammetry (CV) and also when placed in a wall-jet flow through cell for their electrocatalytic properties for reduction of H₂O₂. The immobilised nsBvHb2 displayed a couple of stable and well-defined redox peaks with a formal potential (E°′) of ?33.5 mV (vs. Ag|AgCl|3 M KCl) at pH 7.4. The ET rate constant of nsBvHb2, k _s, was also determined at the surface of the three types of electrodes in phosphate buffer solution pH 7.4, and was found to be 0.50 s^?1 on SPCE, 2.78 s^?1 on MWCNT-SPCE and 4.06 s^?1 on SWCNT-SPCE, respectively. The average surface coverage of electrochemically active nsBvHb2 immobilised on the SPCEs, MWCNT-SPCEs and SWCNT-SPCEs obtained was 2.85?×?10^?10 mol cm^?2, 4.13?×?10^?10 mol cm^?2 and 5.20?×?10^?10 mol cm^?2. During the experiments the immobilised nsBvHb2 was stable and kept its electrochemical and catalytic activities. The nsBvHb2 modified electrodes also displayed an excellent response to the reduction of hydrogen peroxide (H₂O₂) with a linear detection range from 1 μM to 1000 μM on the surface of SPCEs, from 0.5 μM to 1000 μM on MWCNT-SPCEs, and from 0.1 μM to 1000 μM on SWCNT-SPCEs. The lower limit of detection was 0.8 μM, 0.4 μM and 0.1 μM at 3σ at the SPCEs, the MWCNT-SPCEs, and the SWCNT-SPCEs, respectively, and the apparent Michaelis–Menten constant, $ {\hbox{K}}_{\rm{M}}^{\rm{app}} $ , for the H₂O₂ sensors was estimated to be 0.32 mM , 0.29 mM and 0.27 mM, respectively.     

Screen-printed electrochemical sensors for label-free potentiometric and impedimetric detection of human serum albumin

Herein, two electrochemical methods based on potentiometric and impedimetric transductions were presented for albumin targeting, employing screen-printed platforms (SPEs) to make easy and cost-effective sensors with good detection merits. The SPEs incorporated ion-to-electron multi-walled carbon nanotubes (MWCNTs) transducer. Sensors were constructed using either tridodecyl methyl-ammonium chloride (TDMACl) (sensor I) or aliquate 336S (sensor II) in plasticized polymeric matrices of carboxylated poly (vinyl chloride) (PVC-COOH). Analytical performances of the sensors were evaluated using the above-mentioned electrochemical techniques. For potentiometric assay, constructed sensors responded to albumin with −81.7 ± 1.7 (r² = 0.9986) and −146.2 ± 2.3 mV/decade (r² = 0.9991) slopes over the linearity range 1.5 μM–1.5 mM with 0.8 and 1.0 μM detection limits for respective TDMAC- and aliquate-based sensors. Interference study showed apparent selectivity for both sensors. Impedimetric assays were performed at pH = 7.5 in 10 mM PBS buffer solution with a 0.02 M [Fe(CN)₆]^−3/−4 redox-active electrolyte. Sensors achieved detection limits of 4.3 × 10⁻⁸ and 1.8 × 10⁻⁷ M over the linear ranges of 5.2×10⁻⁸–1.0×10⁻⁴ M and 1.4×10⁻⁶–1.4×10⁻³ M, with 0.09 ± 0.004 and 0.168 ± 0.009 log Ω/decade slopes for sensors based on TDMAC and aliquate, respectively. These sensors are characterized with simple construction, high sensitivity and selectivity, fast response time, single-use, and cost-effectiveness. The methods were successfully applied to albumin assessment in different biological fluids.     

Stability-Indicating LC Method for the Determination of Lacidipine in Tablets. Application to Degradation Kinetics and Content Uniformity Testing

A simple, stability-indicating, reversed-phase liquid chromatographic method was developed for the determination of lacidipine in the presence of its degradation products. The analysis was carried out using a 150 mm × 4.6 mm i.d., 5 μm particle size Nucleodur MN-C18 column. Mobile phase containing a mixture of acetonitrile and 0.02 M phosphate buffer (70:30) at pH = 5.0 was pumped at a flow rate of 1 mL min^?1 with UV-detection at 254 nm. The method showed good linearity in the range of 0.06–15 μg mL^?1 with a limit of detection (S/N = 3) of 0.016 μg mL^?1 (3.5 × 10^?8 M). The suggested method was successfully applied for the analysis of lacidipine in bulk and in commercial tablets with average recoveries of 100.19 ± 0.81% and 100.05 ± 0.69%, respectively. The results were favorably compared to those obtained by a reference method. The suggested method was utilized to investigate the kinetics of alkaline, acidic, peroxide and photo-induced degradation of the drug. The apparent first-order rate constant, half-life times and activation energies of the degradation process were calculated. The pH profile curve was derived. The proposed method was successfully applied to the content uniformity testing of tablets.     

Electrochemical Characterization and Rapid Voltammetric Determination of Riluzole in Pharmaceuticals and Human Serum

The electrochemical oxidation of riluzole was investigated using cyclic and linear sweep voltammetry. Under optimized conditions, current and concentration showed linear dependence in Britton Robinson buffer at pH 3.00 for boron doped diamond and pH 3.00 phosphate buffers for glassy carbon electrodes. Differential pulse and square wave voltammetry were used for the determination of riluzole levels in serum samples and pharmaceutical formulations. The limit of detections were found as 5.25 × 10^?7 M and 8.26 × 10^?8 M for glassy carbon electrode and 1.78 × 10^?7 M and 8.42 × 10^?8 M for boron-doped diamond electrodes, in serum samples, using differential pulse and square wave methods, respectively.     

The Electropolymerization of 2,6‐Diaminopyridine and Its Application as Mercury Ion Selective Electrode

A composite graphite (CG) electrode modified with poly(2,6‐diaminopyridine) (PDAP) was used as solid state‐ion selective electrode for determination of mercury. The electrooxidation of monomer 2, 6 diaminopyridine (DAP) onto CG was accomplished from the 30 mM DAP in 5% H₂SO₄ and 0.5 M ZnSO₄. The electrode displayed Nernstian response with slope of 28.4±1 mV decade⁻¹ in concentration range of 1×10⁻⁶ to 1×10⁻¹ M and in solution of pH 3–5. The limit of detection for electrode was 3×10⁻⁸ M with response time of 25 s. The electrode was also suitable as an indicator electrode in the potentiometric titration of Hg²⁺ with iodide.     

Complexation of Osmium(IV) with 3-Methyl-2,6-Dimercapto-1-Thiopyrone-4 and Its Catalytic Application

The reaction of osmium(IV) with 3-methyl-2,6-dimercapto-1-thiopyrone-4 was studied by spectrometry, amperometry, and potentiometry. In a pH range of 2–4 M HCl (2 M H₂SO₄), insoluble osmium methyl thiopyrone dimercaptides formed with a metal-to-ligand ratio of 1 : 1, 1 : 2, 1 : 3, 1 : 4, and 1 : 6 in HCl solutions and of 1 : 1, 1 : 2, 1 : 3, and 1 : 5 in H₂SO₄ solutions. Conditional stability constants and molar absorption coefficients were determined for the products formed in the presence of excess reagent (n = 3, P = 0.95): logβ = 24.42 ± 0.02 and ε = (2.95 α 0.01) × 10⁴ for the 1 : 5 complex and logβ = 30.24 ± 0.10 and ε = (2.92 ± 0.01) × 10⁴ for the 1 : 6 complex. It was shown that the reagent can be used for the potentiometric titration of osmium(IV). Conditions were determined for the successive titration of components of the osmium(IV)-ruthenium(IV) mixture.     

Electrochemical Behavior of a Gold Electrode Modified with SWNTs/DDAB Films and Its Electrocatalytic Activity Towards Ascorbic Acid

A film of single-wall carbon nanotubes (SWNTs) and didodecyldimethylammonium bromide (DDAB) is prepared by casting a solution of SWNTs and DDAB onto the surface of a gold electrode. The electrochemical behavior of the film is investigated by electrochemical impedance spectroscopy and cyclic voltammetry. In a 0.10 M phosphate buffer solution of pH 7.0, the film-modified electrode gives a pair of redox peaks in cyclic voltamograms, with the anodic and cathodic peak potentials of 0.095 and 0.042 V. The peak currents change linearly with the scan rate at 30–500 mV/s. The modified electrode has an excellent electrocatalytic activity towards the oxidation of ascorbic acid (AA). The catalysis currents are proportional to the AA concentration in the range of 5.0 × 10⁻⁴ to 3.2 × 10⁻² M. The linear-regression equation is i (μA) = 1.2079 + 1.3987 × 10³ c _AA (M), with a correlation coefficient of 0.9995. The detection limit is 2.2 × 10⁻⁴ M (signal-to-noise ratio of 3). The Michaelis-Menten constant (K _m) is 1.0 × 10⁻⁴ M by the Lineweaver-Burk equation. __________ From Elektrokhimiya, Vol. 41, No. 10, 2005, pp. 1193–1199. Original English Text Copyright ? 2005 by Cheng, Jin, Zhang. The text was submitted by the authors in English.     

Mercury ion selective poly(aniline) solid contact electrode based on 2-mercaptobenzimidazol ionophore

A mercury(II) ion selective poly(aniline) solid contact electrode based on 2-mercaptobenzimidazol (2MBI) ionophore as a sulfur containing sensing material was successfully developed. The electrode exhibits a good linear response of 29.1 mV/decade (at 20 ± 0.2°C, r ² = 0.997) within the concentration range of 1 × 10^?2?1 × 10^?7 M Hg(II). The composition of this electrode was: ionophore 0.100, polyvinylchloride (PVC) 0.330, dibutylphthalate (DBP) 0.470, potassiumtetrakis(4-chlorophenyl)borate (KTpCIPB) 0.090, and oleic acid (OA) 0.010. A poly(aniline) solid contact electrode based on 2MBI with DBP and OA plasticizers exhibited the best response characteristics of the results obtained for similarly coated wire type electrodes and solid contact electrodes based on only one DBP plasticizer. The electrode shows good selectivity for mercury(II) ions in comparison with alkali, alkaline earth, transition and heavy metal ions. This electrode is suitable for use with aqueous solutions of pH 3.3?C8.0 and the standard deviation in the measured EMF difference was ±0.5 mV in a mercury nitrate sample solution of 1.0 × 10^?2 M and ±1.1 mV in a mercury nitrate sample solution of 1.0 × 10^?3 M. The stabilization time was less than 15 min and the response time was less than 33 s. The electrode was applied as a sensor for the determination of Hg(II) content in a sea water sample and some amalgam alloys. The results show good correlation with data obtained by atomic absorption spectrometry.     

Synthesis and crystal structure of a 1D coordination polymer constructed by cobalt with 4-hydroxypyridine-2,6-dicarboxylic acid

A new one-dimensional chain complex [Co₃(Hpda)₂(H₂O)₈] _n (I), where HpdaH₃ = 4-hydroxypyridine-2,6-dicarboxylic acid, has been synthesized and characterized structurally. It crystallizes in the triclinic space group $P\bar 1$ with a = 7.3718(11), b = 8.4881(13), c = 9.4513(14) Å; α = 110.3300(10)°, β = 100.7470(10)°, γ = 95.338(2)°, V = 536.77(14) ų, C₁₄H₂₀Co₃N₂O₁₈, Mr = 681.11, Z = 1, ρ_c = 2.107 g/cm³, μ = 2.390 mm^?1, F(000) = 343, R = 0.0182 and wR = 0.0486 for 1893 observed reflections with I > 2σ(I). Complex I possesses a one-dimensional (1D) chain structure composed of continuous trinuclear Co(II) clusters, which further forms a three-dimensional (3D) supramolecular architecture via hydrogen bonds.     

o-phthalic acid as a standard substance for calibrations of hydrogen ion concentrations with a glass electrode

o-Phthalic acid is proposed as a standard substance for buffer solutions of known hydrogen ion concentration (I ? 0.2 M KCl, p[H⁺] = 3.0–5.4, 25°C). Its crystallinity, purity and slightly wide buffer range afford advantages over acetic acid. Empirical relationships between measured pH (pH_m) and calculated [H⁺] were derived for sequences of buffer solutions at several ionic strengths: pH_m - Mp[H⁺] + C. These calibration lines were parallel and of unit slope as required by theory. A table of p[H⁺] values for o-phthalic acid buffer solutions at I = 0.1 M (KCl) is presented and the method of calculation of p[H⁺] values for a buffer series generated by additions of potassium hydroxide is outlined.     

Synthesis and calcium channel antagonist activities of new derivatives of dialkyl 1,4-dihydro-2,6-dimethyl-4-(5-phenylisoxazol-3-yl)pyridine-3,5-dicarboxylates

The new analogues of nifedipine, in which 2-nitrophenyl group at position 4 is replaced by phenylisoxazolyl substituent, were synthesized. The symmetrical dialkyl 1,4-dihydro-2,6-dimethyl-4-(5-phenylisoxazol-3-yl)pyridine-3,5-dicarboxylates were prepared by classical Hantzsch condensation, and the asymmetrical analogues were synthesized using a procedure reported by Dagnino that involved the condensation of alkyl acetoacetate with alkyl 3-aminocrotonate and 5-phenylisoxazole-3-carboxaldehyde. The structure of all compounds was confirmed by IR, ¹H NMR and Mass spectra. In vitro calcium channel antagonist activities were evaluated as calcium channel antagonists using the high K⁺ concentration of guinea-pig ileum longitudinal smooth muscle (GPILSM) assay. These compounds exhibited moderate calcium antagonist activity (IC₅₀ = 10^?7 to 10^{? 5} M range) relative to the reference drug nifedipine (IC₅₀ = 1.10 ± 0.40 × 10^?8 M).     

The linear pockels effect in crystals of the iodoform-sulphur complex

Crystals of the 1:3 adduct of iodoform and sulphur (CHI₃·3 S₈) have been found to exhibit a strong electro-optic effect. The values of the linear Pockels effect coefficients determined by using a modulation technique are: ∣r₁₂∣ = (4.4 ± 2.4) × 10⁻¹² m/V and ∣r₁₃ − 0.5 i 2r₃₃ ∣ = (0.29 ± 0.12) × 10⁻¹² m/V. A comparison of the electro-optic susceptibility derived from these data with that deduced from powder second-harmonic generation tests shows the electro-optic phenomenon to be mainly of electronic origin.     

Fluorescence quenching in molecular recognition by hydrogen bonding

Steady-state fluorescence and single photon timing have been used to study the effect of the presence of hydrogen bonding on the intermolecular quenching of pyrene covalently linked to a guanine-like receptor I by an aliphatic amine (N,N-dimethylpropylamine) covalently linked to cytosine derivative II. By comparing the fluorescence quenching of I by II with that of 10methylpyrene (1-MP) by triethylamine (TEA), as a model system in which no hydrogen bonding can occur, one could possibly analyze the effect of the hydrogen bonding between receptor and substrate as a quenching as it leads to a higher local concentration of donor and acceptor. While the quenching of I by II was observed with an apparent rate constant k_q of (1.78 ± 0.10) × 10⁹ M⁻¹ s⁻¹ and (8.72 ± 0.42) × 10⁸ M⁻¹ s⁻¹ in toluene and acetonitrile, respectively, no quenching could be observed in methanol. Upon excitation of 1-MP, no quenching by II could be detected in the same concentration range as used in the quenching of I. Quenching of I and of 1-MP by TEA (⩾ 10⁻² M) in toluene leads to exciplex formation with maxima centred at 540 and 514 nm, respectively. The rate constants of exciplex formation and dissociation of I with TEA were analyzed using a global compartmental analysis. The following values were obtained for the rate constants: k₀₁ = (9.70 ± 0.01) × 10⁶ s⁻¹, k₂₁ = (1.12 ± 0.003) × 10⁹ M⁻¹ s⁻¹, k₀₂ = (5.24 ± 0.01) × 10⁷ s⁻¹ and k₁₂ = (7.74 ± 0.08) × 10⁶ s⁻¹. Quenching of I by TEA in the presence of III, a hydrogen-bonding system without an alkyl amine substituent, leads to exciplex formation centred at 538 nm. The rate constant values for the exciplex formation and dissociation of I with TEA in the presence of III were: k₀₁ = (9.32 ± 0.08) × 10⁶ s⁻¹, k₂₁ = (9.32 ± 0.003) × 10⁸ M⁻¹ s⁻¹, k₀₂ = (6.16 ± 0.03) × 10⁷ s⁻¹ and k₁₂ = (21.90 ± 0.3) × 10⁶ s⁻¹. The apparent rate constants k_q for this system was (7.26 ± 0.56) × 10⁶ M⁻¹ s⁻¹. The observed decrease in the rate of exciplex formation of I with TEA in the presence of III could suggest that the guanine-like moiety in I forms hydrogen bonds with the cytosine-like moiety and this could decrease the electron affinity of I. The rate constant of exciplex dissociation increased, indicating that the exciplex is less stable in the presence of III. Because of the single exponential decay of I in the presence and absence of II and of the agreement between steady-state and transient fluorescence measurements, the information available for quantitative analysis of the association between I and II is limited.