Aqueous Buffer pH Research Articles

PH-dependent spectral behavior of substituted quinolones: potential fluorophore for metallofluorescent indicators

The 7-methoxy and 7-hydroxy derivatives of 2,3-dimethyl-4-quinolone have been characterized with respect to absorbance, steady-state fluorescence excitation and emission, and acid-base behavior in perchloric acid ( H 0 −5.68—pH 2.0) and aqueous buffers (pH 2.2–14.2); susceptibility to alkaline hydrolysis has been examined absorptiometrically. 7-Hydroxy-2,3-dimethyl-4-quinolone is resistant to attack by hydroxide ion. Aqueous solutions of it exhibit moderate to high fluorescence emission, which is dependent on solution acidity. It is recommended for incorporation into metallofluorescent indicators of the Calcein Blue type.

N3-methyl-mafosfamide as a chemically stable, alternative prodrug of mafosfamide.

The presence of an alkyl substituent at N3 in the oxazaphosphorine ring stabilizes N-substituted 4-(alkylthio)cyclophosphamides from spontaneous decomposition. Based on this finding, N3-methyl-mafosfamide was synthesized and examined as a chemically stable, biooxidative prodrug of mafosfamide. This prodrug was stable in aqueous buffer (pH 7.4, 37 degrees C) and underwent N-demethylation in a time dependent manner when incubated with rat hepatic microsomes. N3-Methyl-mafosfamide was 10-fold more cytotoxic in vitro than cyclophosphamide against mouse embryo Balb/c 3T3 cells (LC50 = 3.6 microM). Preliminary in vivo antitumor evaluation against L1210 leukemia in mice showed that this prodrug was active [Increase of life span (ILS) > 29%].

Dynamic Light Scattering Study of Gelatin−Surfactant Interactions

Binding of anionic (sodium dodecyl sulfate, SDS), cationic (cetyl trimethyl ammonium bromide, CTAB), and nonionic (TX-100) surfactants to gelatin chains in aqueous buffer (pH = 7.0) medium has been studied by dynamic light-scattering technique performed at T = 30 °C. In the surfactant concentration range varying from 0 to 100 mM, SDS exhibited electrostatic binding to the charged groups of the polypeptide chain resulting in considerable reduction in the hydrodynamic radius (Rh) of gelatin up to the critical association concentration (CAC), and at higher concentrations both the SDS micelles and gelatin−SDS complexes were found to be coexisting in equilibrium. In the case of CTAB, almost the opposite was observed: the gelatin chains showed small increase in size up to the CAC. Beyond this, the gelatin−CTAB complexes were observed to grow significantly, and these were found to be in equilibrium with CTAB micelles. TX-100 exhibited little hydrophobic binding to gelatin, and no observable change in gelatin si...

A comparative study of the passivation and localized corrosion of α-brass and β-brass in borate buffer solutions containing sodium chloride: III. The effect of temperature

The passivation and localized corrosion of α-brass and β-brass in an aqueous borate-boric acid buffer (pH 9) containing different concentrations of NaCl ( c NaCl ) in the temperature range 5 °C ≤ T ≤ 45 °C were studied comparatively by potential sweeping at 2 × 10 −2Vs −1 and 2 × 10 −4Vs −1 and de-alloying measurements. The passivation of α-brass and β-brass was related to the electroformation of a complex ZnO · xH 2O/Cu 2O layer, its thickness and compactness presumably increasing with temperature. For both α-brass and β-brass immersed in an aqueous NaCl-containing buffer, pitting corrosion was observed as the value of the breakdown potential ( E b ) was exceeded. At constant temperature, the value of E b shifted negatively as either c NaCl or the zinc content in the alloy was increased. For β-brass, the value of E b decreased slightly with increasing temperature in the range 5 °C≤ T ≤45 °C. In the range 5 °C≤ T ≤ 25 °C, for α-brass the value of E b was close to that reported for copper, whereas for T > 25 °C it approached those values measured for β-brass. De-alloying measurements in aqueous 0.5 M NaCl indicate that zinc surface enrichment of α-brass was responsible for the decrease in pitting corrosion resistance at T > 45 °C.

Chemical degradation kinetics of recombinant hirudin (HV1) in aqueous solution: effect of pH.

To gain information on the chemical stability pattern and the kinetics of the degradation of recombinant hirudin variant HV1 (rHir), a thrombin-specific inhibitor protein of 65 amino acids, in aqueous solution as a function of pH. Stability of rHir was monitored at 50 degrees C in the framework of a classical pH-stability study in aqueous buffers pH 1-9.5. Two capillary electrophoresis (CE) protocols were used: one for the kinetics of succinimide formation at Asp53-Gly54 (C-terminal tail) and Asp33-Gly34 (loop section), the other for the kinetics of rHir degradation. To check for potential effects of conformational changes by thermal denaturation, circular dichroism (CD) measurements were performed between 25 and 80 degrees C. Throughout the pH range studied no effect of thermal denaturation on rHir confirmation at 50 degrees C was observed. rHir was most stable at a neutral pH whereas, at slightly acidic pH, an intermediate stability plateau was found. Both, strongly acidic and alkaline conditions led to fast rHir degradation. Depending on the pH of degradation, rHir was found to degrade in various combinations of multiple parallel and sequential degradation patterns. Special focus was on succinimide formation at Asp53-Gly54 (C-terminal tail) and Asp33-Gly34 (loop) and on the potential of isoAsp formation in position 53 and 33. Chemical rHir stability in the intermediate pH range depends strongly on succinimide formation. At slightly acidic conditions succinimides represent the major degradation product (up to 40%). Around neutral pH succinimides react further, presumably by isoAsp formation, and concentrations remain low. Relative preference of succinimide formation in the C-terminal tail domain versus the loop domain is explained by higher backbone flexibility in the tail.

Surface grafting of poly(ethylene glycol) onto poly(acrylamide-co-vinyl amine) cross-linked films under mild conditions

Poly(ethylene glycol) (PEG) was grafted onto poly(acrylamide-co-vinyl amine) (poly(AM-co-VA)) film using tresylated PEG (TPEG) at 37°C in aqueous buffers (pH 7.4) with a view to surface-modifying microencapsulated mammalian cells. Poly(AM-co-VA) film was synthesized by Hofmann degradation of a cross-linked poly(acrylamide) film. Conversion to vinyl amine on the surface of the film was approximately 50%, but bulk conversion was not observed; surface specificity was thought to be the result of cleavage of aminated polymer chains at the surface due to chain scission. Reaction between primary amine and TPEG gave a graft yield of 2 mol% (based on XPS) with respect to available surface amine groups, equivalent to 54 mol% ethylene oxide based on monomer units. Physical adsorption of non-activated polymer was done under identical conditions as a control and the difference in oxygen content was significant compared to TPEG. The type of buffer agent and buffer concentration did not influence graft yields. This graft reaction, which was completed in as little as 2 h was considered to be mild enough to be used for a surface modification of microcapsules containing cells without affecting their viability. Such a surface modification technique may prove to be a useful means of enhancing the biocompatibility of microcapsules (or any tissue engineering construct) even after cell encapsulation or seeding.

Nucleophilic Addition to Complexes of (Ph2P)2CCH2as a Route to Functionalized, Redox-Active Ruthenium(II)−Diphosphine Complexes

The ligand (Ph2P)2CCH2 (dppen) reacts with [RuCl2(PPh3)3] in CH2Cl2 to give trans-[RuCl2(dppen)2], 1. Complex 1 has reversible Ru(II)/Ru(III) electrochemistry, and on treatment with NOBF4, 1 affords [RuCl2(dppen)2]BF4, 1+. Refluxing solutions of 1 in chlorobenzene affords cis-[RuCl2(dppen)2], 2. With excess RNH2 in chlorobenzene or toluene, 1 reacts to give functionalized diphosphine complexes of general formula trans-[RuCl2({Ph2P}2CHCH2NHR)2] (R: PhCH2, 3a; [CH2]3NH2, 3b; n-octyl, 3c; R-α-CH(Me)Ph, 3d; [CH2]3Si(OEt)3, 3e) characterized principally by a small upfield shift in their 31P{1H} NMR spectra compared to that of 1 and by their distinctive 1H NMR spectra. Complex 3b·4MeOH crystallizes in the space group P1̄ with a = 11.448(6) Å, b = 13.10(1) Å, c = 11.178(7) Å, α = 93.16(6)°, β = 99.51(5)°, γ = 92.19(5)°, V = 1648(2) Å3, and Dcalcd = 1.250 g cm-3 for Z = 1. Complex 3e reacts with the surface hydroxyl groups of indium-doped tin oxide (ITO) electrodes, to give monolayers of anchored, redox-active Ru(II)−diphosphine complexes, characterized by cyclic voltammetry. The modified electrodes are stable to repetitive cycling over the Ru(II)/Ru(III) redox wave in acetonitrile−tetraethylammonium tetrafluoroborate and in aqueous buffer (pH 8). With anodized Pt electrodes, however, 3e reacts to form multilayers. Reaction of 1 with secondary amines is more sluggish than reaction with primary amines, and only adducts with pyrrolidine (3f) and morpholine (3g; impure) were isolated. With LiC⋮C(CH2)3CH3, 1 reacts to give trans-[Ru(C⋮CR)2({Ph2P}2CHCH2C⋮CR)2] (R = n-butyl, 5); acetylide nucleophiles displace the chloride ligands as well as adding to the dppen double bonds of 1. Other carbanions fail to react with 1. The cis complex 2 reacts with a limited range of primary amines, to afford cis-[RuCl2({Ph2P}2CHCH2NHR)2] (R: n-hexyl, 4a; [CH2]3Si[OEt]3, 4b). However, 4b was too insoluble for electrode derivatization. The “half-sandwich” complex [CpRuCl(dppen)] (6; Cp = η5-C5H5) also reacts with RNH2 to give [CpRuCl({PPh2}2CHCH2NHR)] (R: −[CH2]3NH2, 7a; −CH2Ph, 7b).

An Improved Synthesis of O-Benzoyl Protected Hydroxamates.

Several primary amines (R-NH(2)) were converted to their corresponding O-benzoyl protected hydroxamates under biphasic conditions. The intermediate (benzoyloxy)amines (R-NHOCOPh) were generated using benzoyl peroxide dissolved in CH(2)Cl(2) and an aqueous carbonate buffer (pH 10.5) at room temperature. Subsequent acylation with R'COCl gave the protected hydroxamates (R'CONROCOPh) in good overall yields (56-89%).

Surface Enhanced Roman Spectroscopy of Iron Oxide Thin Films: Comparison with the Passive Film on Iron

Thin films (33 nm to 1.4 μm) of iron oxides were deposited onto roughened silver substrates by laser ablation of and targets. Surface enhanced Raman (SER) spectra from the 33 to 37 nm thick films, both in air and in aqueous borate buffer (pH 8.4), were identical to SER spectra of the passive film formed on iron in the same solution; that is, all were characterized by a strong broad peak centered at approximately 530 to . The 33 to 37 nm thick films were potentiodynamically reduced in borate buffer. Electrochemical reduction of the ablated iron oxide films initiated at a more cathodic potential than that found for the passive film on iron. Thicker 178 nm films formed by laser ablation of exhibited spectra partially resembling the iron passive film and partially resembling the normal Raman spectrum of bulk . The 153 nm and 1.4 μm thick films formed from and 1.3 μm thick films formed from gave very weak Raman spectra that were characteristic of the bulk oxides, and did not resemble the iron passive film. The observation that the SER spectra from passive films on iron, as measured in this and previous studies, are identical to the SER spectra from thin films laser ablated from targets of and (in both cases, resulting in iron oxides most probably hydrated and of small particle size) permits the more accurate assignment of spectral features in future as well as past SERS studies.

Detection of menadione sodium bisulfite (vitamin K 3) by reversed-phase high performance liquid chromatography with series dual-electrode amperometric detector

A reversed-phase high-performance liquid chromatography with series dual glassy carbon electrodes for the amperometric detection of water-soluble menadione is described. The complex post-column derivatization reaction and the high background currents were avoided. The menadione sodium bisulfite was reduced at −0.3 V vs. SCE at the upstream (generator) electrode and oxidized at +0.2 V vs. SCE at the downstream (collector) electrode. The mobile phase was 0.2 mol l −1 HAcNaAc aqueous buffer (pH 5.50) and 40% (v/v) methanol. The linear response was in the range of 35 ng to 15 μg, with a detection limit of 15 ng ( S N = 3 ). The correlation coefficient was 0.9997 ( n = 6). The electrochemical detection with series dual electrodes has a higher selectivity for menadione (vitamin K 3) compound than with UV detection.

Degradable poly(ester amide)s based on l-tartaric acid

A series of poly(ester amide)s with ester/amide group ratios ranging from 1 99 to 1 4 were obtained using 1,6-hexanediamine, 1,6-hexanediol and 2,3-di- O-methoxy- l-tartaric and succinic acids as building blocks. The ester linkages were introduced in pairs using as comonomer the diacid resulting from the esterification of 1,6-hexanediol with 2 mol of succinic anhydride. Polycondensastion reactions were carried out in solution at room temperature with the diamine activated as the N,N′-bis(trimethylsilyl) derivative and the two diacids as bis(pentachlorophenyl) esters. The prepared poly(ester amide)s have number average molecular weights in the range 10 000–40 000, display optical activity and are soluble in chloroform. These copolymers were found to be highly crystalline with melting points above 200°C and mechanical moduli comparable to those reported for the parent polyamide poly(hexamethylene-di- O-methyl- l-tartaramide). They were degraded by aqueous buffer of pH 7.4 at a rate that increased with the content of the copolymer in succinic acid units. 1H n.m.r. evidenced that no reactions other than those entailing the hydrolysis of the main chain ester bonds appear to take place at polymer degradation.

Polymer-Surfactant Interactions: The Gelatin-SDS Example

Binding of the anionic surfactant, sodium dodecyl sulfate, SDS to gelatin chains in aqueous buffer (pH = 7.0) medium has been studied by dynamic light scattering technique at 30°C. In the surfactant concentration range varying from 0 to 100 mM, SDS exhibited electrostatic binding to the charged groups of the polypeptide chain resulting in considerable reduction in the hydrodynamic radius (R h ) of gelatin upto the critical aggregation concentration (CAC); at higher concentrations both the SDS micelles and gelatin-SDS complexes were found to coexist in equilibrium. The micellar shapes were found to be near-spherical for SDS. Results have been interpreted through the necklace-bead model of polymer-surfactant interactions.

Fluorescence Studies of Lipid Association-Induced Conformational Adaptations of an Exchangeable Amphipathic Apolipoprotein

The conformational adaptability ofManduca sextaapolipophorin III (apoLp-III) has been evaluated by monitoring the spectroscopic properties of its sole tyrosine residue, Tyr145, present in the fifth helical segment of the protein.M. sextaapoLp-III adopts a globular five-helix bundle structure in solution and has been postulated to undergo an opening at putative hinge domains upon interaction with lipid surfaces. Previous results have shown that the intrinsic fluorescence of Tyr145is highly quenched in the closed, water-soluble conformation but is dramatically enhanced upon lipid association. We have carried out a spectroscopic characterization of Tyr145and its microenvironment, to enable its use as a structural probe of lipid-induced conformational changes of apoLp-III. The pKaof Tyr145in lipid-free apoLp-III was found to be 10.5, as determined from uv-spectrophotometry, indicating that, in the ground state, the tyrosyl phenolic group is not ionized under physiological conditions. Compared to free tyrosine in aqueous buffer (pH 7.0), a red shift (77 nm) in the λmaxof absorbance of Tyr145was observed, suggesting that an H-bonding interaction is responsible for the quenched state of tyrosine fluorescence. In an effort to explain the observed quenching phenomenon, the quantum yield and lifetimes of Tyr145fluorescence emission were investigated as a function of pH and lipid binding. The quantum yield of Tyr145in lipid-free apoLp-III was enhanced fivefold upon decreasing the pH, with a half-maximal point around pH 5.5. Time-resolved fluorescence decay analysis showed that Tyr145exhibits nonexponential emission decay with two components having lifetimes of 3.3 ns (76%) and 0.89 ns (24%) in the lipid-free state. The lifetime and amplitude of Tyr145remain essentially unaltered upon lipid association or decreasing the pH. This is consistent with the hypothesis that, in the lipid-free helix bundle conformation, a quenching residue exists within H-bonding distance of the phenolic side chain of Tyr145which, at physiological pH, is responsible for the observed fluorescence quenching. Opening of the helix bundle repositions this acceptor base, possibly a carboxylate or an imidazole side chain, making it unavailable for quenching. Using differential polarized phase and modulation fluorometry, it was seen that the segmental motion of Tyr145is also altered considerably upon lipid interaction. These spectroscopic and motional properties of Tyr145distinguish this unique residue as a useful probe to monitor structural flexibility of apoLp-III.

Determination of the antileukemic drug mitoguazone and seven other closely related bis(amidinohydrazones) in human blood serum by high-performance liquid chromatography.

A reversed-phase (C18) HPLC method with diode-array detection was developed for the separation and determination of methylglyoxal bis(amidinohydrazone) (mitoguazone) and seven closely related aliphatic analogs thereof, namely the bis(amidinohydrazones) of glyoxal, dimethylglyoxal, ethylmethylglyoxal, methylpropylglyoxal, butylmethylglyoxal, diethylglyoxal and dipropylglyoxal. The mobile phase consisted of a non-linear binary gradient of methanol and 0.03 M aqueous sodium acetate buffer (pH 4.3). Good separation of the eight congeners was achieved. On increasing the methanol content of the eluent, the bis(amidinohydrazones) eluted in the order of increasing number of carbon atoms in the side-chains. The method was also applied to the quantitative analysis of the compounds in aqueous solution and, combined with ultrafiltration, for the separation of the eight congeners in spiked human blood serum. A separate simplified method for the quantitative determination of each of the compounds in spiked human blood serum samples was also developed. The methods developed made for the first time possible the simultaneous HPLC analysis of more than one bis(amidinohydrazones). The results obtained indicate that the bis(amidinohydrazones) studied obviously have a distinct tendency to form ion associates with acetate ions and probably also other carboxylate ions in aqueous solution. This aspect may be of biochemical significance, especially concerning the intracellular binding of the compounds. Each one of the compounds studied invariably gave rise to one peak only, this result supporting the theory that the conventional synthesis of each of the compounds gives rise to one geometrical isomer only. This result is completely in agreement with the results of previous proton and carbon NMR spectroscopic as well as X-ray diffraction studies.

Binding Properties of Near-IR Dyes to Proteins and Separation of the Dye/Protein Complexes Using Capillary Electrophoresis with Laser-Induced Fluorescence Detection

The noncovalent binding of the near-infrared (NIR) dyes, DTTCI (cationic) and IR-125 (anionic), to several model proteins was investigated with the use of steady-state and picosecond laser fluorescence measurements. In an aqueous borate buffer (pH = 9.2), minimal fluorescence emission from these NIR dyes was observed. When a protein was added to the solution, enhancements in the fluorescence emission were found for both dyes. Time-resolved fluorescence measurements for IR-125 in the presence of the protein, β-casein, indicated a biexponential decay with lifetimes of 195 and 682 ps (χ2 = 1.94). Our data suggest that these dyes distribute themselves between the hydrophobic core of the protein and the interstitial aqueous solution. The dye molecules residing in the interior of the protein exhibit enhancements in their fluorescence due to a more favorable microenvironment. The binding and enhanced fluorescence properties allowed the use of these dyes as noncovalent stains for the low-level detection of proteins separated via capillary electrophoresis (CE). Detection limits for some model proteins separated by CE and stained with these NIR dyes were found to be superior to those obtained by using UV detection in CE.

High-sensitivity fluorescence derivatization for the determination of hydroxy compounds in aqueous solution by high-performance liquid chromatography

The utility of the fluorescent labelling reagent 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) for the determination of alcohols in aqueous media was evaluated. The labelling conditions were optimized for C 1C 8 alcohols. Under the very mild reaction conditions of 25°C for 1 min in a basic buffer, all the alcohols tested were derivatized with the reagent to yield highly stable fluorescent carbamate derivatives. The maximum excitation (290 nm) and emission (365 nm) wavelengths were the same for all the alcohols tested. The stability of the derivatives and their yield were affected by the chosen pH in the range 6–9.5 and by the steric hindrance of the alcohol function. All the derivatives obtained with AQC were completely separated by reversed-phase high-performance liquid chromatography on a 20-cm C 18 column with a gradient of aqueous sodium acetate buffer (pH 6) and acetonitrile. The detection limit ( S/ N = 3) with fluorescence detection is at the picomole level. Some preliminary applications in the fields of foods and beverages are described.

Hydrolysis of Azadirachtin in Buffered and Natural Waters

The hydrolysis of azadirachtin was studied in several aqueous buffers of pH 4.1−8.1 and in four natural waters (pH 6.2, 7.3, 8.0, and 8.1) at 20−45 °C. Depending on the pH, several unidentified con...

Chitosan-Drug Conjugate Microspheres: Preparation and Drug Release Properties of Microspheres Composed of the Conjugate of 2′- or 3′-(4-Carboxy-butyryl)-5- Fluorouridine with Chitosan

AbstractThe conjugate microspheres (Chi-glu-FUR-m) were prepared by the dry-in-oil method using chitosan-5-fuorouridine conjugate. Chi-glu-FUR-m were characterized by drug content, particle shape and size, swelling property, and drug release. Their characteristics were compared with those of the simple microspheres (Chi/ FUR-m), which were prepared under similar conditions using a mixture of chitosan and 5-fluorouridine. Both microspheres prepared showed a high retention of the drug after preparation and similar particle size and shape. Swelling ratios after incubation in aqueous buflers of pH 7.4 for 6 hr were similar for both microspheres. Chi-glu-FUR-m swelled quickly in aqueous buffers of pH 7.4 and the disintegration was observed to occur gradually from 24 hr afrer the incubation. Chi-glu-FUR-m showed a gradual drug release (50% release time = 61 hr), while Chi/FUR-m released the drug very rapidly, Such characteristics of Chi-glu-FURm as swelling, slow disintegration, and gradual drug release propose...

Permeability of a soft steroid, loteprednol etabonate, through an excised rabbit cornea.

The objective of this work was to study the in vitro permeability characteristics of a "soft" steroid, loteprednol etabonate 1 in different vehicles across excised rabbit cornea using a transcorneal permeation system. The vehicles studies were aqueous solutions of beta-cyclodextrin derivatives; 2-hydroxypropyl beta-cyclodextrin (HPCD) and heptakis (2,6-di-O-methyl) beta-cyclodextrin (DMCD); and DMCD together with 5 and 10% propylene glycol (PG) or dimethyl sulfoxide (DMSO). Two experimental suspension products of the steroid, 0.1% and 0.5%, and the suspensions of the steroid in commercial lubricant ophthalmic solutions which include Liquifilm Tears and Tears Plus were also studied for their possible use as vehicles to deliver the steroid across the cornea. The observed permeability rate of the soft steroid from DMCD was found to be 13-fold greater than from HPCD. The presence of varying percentages of PG and DMSO with DMCD decreased the flux of the steroid. Transcorneal permeability of the steroid was found to be insignificant from aqueous buffer (pH 7.4) and commercial lubricant ophthalmic solutions. The flux of the steroid in 20% DMCD (23 micrograms/hr/cm2) was found to be comparable with 0.5% commercial suspension (20.5 micrograms/hr/cm2). The barrier function of corneal epithelium for the steroid in 0.5% experimental suspension was also investigated. When the corneal epithelium was removed, the lag time of the lipophilic soft steroid decreased but the flux value did not change.

A comparison of the photoproperties of zinc phthalocyanine and zinc naphthalocyanine tetrasulfonates: model sensitizers for the photodynamic therapy of tumors

Phthalo- and naphthaocyanines are of interest as sensitizers for the photodynamic therapy of tumors because of their strong absorption in the 680 and 760 nm ranges respectively. Both zinc phthalocyanine and naphthalocyanine tetrasulfonates (ZnPcS 4 and ZnNcS 4) were aggregated and photochemically inactive in aqueous buffer of pH 7.4, while in 10 mM cetyl pyridinium chloride in buffer they were monomeric and active. Therefore all these studies were carried out using the buffered detergent. The triplet lifetimes of ZnPcS 4 and ZnNcS 4 under argon were 490 and 110 μs respectively, with oxygen bimolecular quenching constants of 4.2 × 10 9 and 2.0 × 10 8 M −1s −1 respectively. Triplet decay curves in argon, air and 100% oxygen were first order, suggesting that there was little back reaction of the triplet states with oxygen as has been observed with some naphthalocyanines. The quantum yield of singlet oxygen generation by ZnPcS 4 was 0.70 and that for ZnNcS 4 was 0.25. Both compounds sensitized the photo-oxidation of furfuryl alcohol, cysteine, histidine, methionine, tryptophan, tyrosine and guanosine; ZnPcS 4 was three times more efficient than ZnNcS 4. These reactions were 50% inhibited by about 0.5 mM azide, suggesting the involvement of singlet oxygen. Both sensitizers photobleached on illumination, with quantum yields of 1.7 × 10 −5 for ZnPcS 4 and 4.2 × 10 −3 for ZnNcS 4.