Stephen G. Schulman

Dependence of the fluorescence of immobilized 1-hydroxypyrene-3,6,8-trisulfonate on solution pH : extension of the range of applicability of a pH fluorosensor

Abstract 1-Hydroxy-3,6,8-pyrenetrisulfonate (HPTS), covalently immobilized on cellulose attached to a plastic strip, has been used as a fluorescent optical pH sensor for the neutral pH region. At near neutral pH, HPTS fluoresces green, i.e., from its conjugate base form, exclusively and independent of whether it was excited in its conjugate base form or acidic (phenol) form, or both. However, the fluorescence of immobilized HPTS also varies in the low pH region from about pH 3 to below pH 1, in that not only the green emission almost completely disappears, but also a second (blue) band appears which can be attributed to the fluorescence of the conjugate acid (the phenol). The proton transfer in the lowest excited singlet state which accounts for the variation of the fluorescence in the low pH region is here evaluated kinetically. The application of this sensor to measure pH over a wide range of pH values is suggested.

Chemiluminescence-based detection: principles and analytical applications in flowing streams and in immunoassays

The present paper provides the principles of chemiluminescence (CL) and its powerful applications in analytical chemistry, mainly in the area of flow injection analysis, column liquid chromatographic and capillary electrophoretic separating systems, and its potential in immunoassays. CL is light produced by a chemical reaction. The most common advantages of chemiluminescent reactions are the relatively simple instrumentation required, the very low detection limits and wide dynamic ranges, which have contributed to the interest of CL detection in flow injection analysis, high performance liquid chromatography, including miniaturized systems, and, most recently, the exploding area of capillary electrophoresis. The latter powerful microanalytical separation technique offers high numbers of theoretical plates and relatively short analysis times requiring only small sample volumes, the migrating system comprising aqueous buffer solutions. In non-isotopic immunoassays, covering a great variety of applications in human and veterinary medicine, forensic medicine, agriculture and food industry, the radioisotope is replaced by a fluorescence or chemiluminescent label. The use of CL as a detection principle permits quantitative determination of various compounds at low concentrations. Disadvantages of the CL-based technique may include lack of sufficient selectivity and sensitivity to various physicochemical factors.

Biprotonic versus intramolecular phototautomerism of salicylic acid and some of its methylated derivatives in the lowest excited singlet state

Abstract Salicylic acid demonstrates intramolecular proton transfer (phototautomerism) in its uncharged and singly charged anionic species in both chloroform and aqueous media. Methyl salicylate on the other hand, exhibits intramolecular phototautomerism in chloroform but in aqueous media phototautomerism appears to be biprotonic, involving diffusion limited protonation of the carboxyl group and dissociation of the hydroxyl group by the solvent. The biprotonic process in methyl salicylate is complete, during the lifetime of the excited state in moderately concentrated acid solutions but is incomplete in dilute acid media.

Fluorescence and phosphorescence of 5- and 8-aminoquinoline.

Abstract 5-Aminoquinoline and 8-aminoquinoline fluoresce moderately to weakly in low dielectric media but not in strongly hydrogen-bonding or acidic aqueous media. At low temperatures and in rigid media moderately intense fluorescence is observed in aqueous, media at all pH values. Quenching of the aminoquinoline fluorescences in fluid media is believed to be due primarily to hydrogen bonding in the lowest excited singlet state. The fluorescences of the aminoquinolines arise from the 1La states in the respective molecules. Calculation of approximate excited singlet state pKa* values indicates that upon excitation to the 1La state the amino group becomes a weaker base while the heterocyclic ring nitrogen becomes a stronger base. Phosphorescence is observed for the doubly protonated cations of both aminoquinolines but not for the singly protonated or neutral species. Both neutral species exhibit delayed fluorescence at 77° K.

Characteristics of the binding of europium(III) to tetracycline

Abstract Lanthanide-sensitized luminescence was described previously as a sensitive means of quantifying tetracycline in solution. The results of potentiometric, absorbance, fluorescence and lifetime measurements on the tetracycline/E 3+ system are presented here. The tetracycline/Eu 2+ complexes which exist in aqueous solution have 1:1 stoichiometry; their formation constants are a function of pH.

Excited state prototropism and solvent dependence of the fluorescence of 3-aminoquinoline

Abstract The effects of solvent polarity and hydrogen-bonding capability on the fluorescence of 3-aminoquinoline have been studied. The failure of the solvent dependence of the absorption spectrum to parallel that of the fluorescence spectrum is attributed to differences in hydrogen bonding in ground and lowest excited singlet states. The large Stokes shifts produced by addition of small amounts of hydrogen-bonding solvents to solutions of 3-aminoquinoline in non-activating solvents suggest the formation of a well defined solute-solvent complex in the excited state. 3-Amino-quinoline is a stronger acid in the excited state than in the ground state with respect to dissociation from the amino group. The same is true for the dication. Fo¨rster cycle calculations, however, indicate that dissociation from the heterocyclic nitrogen is more difficult in the excited state. It is shown that when hydrogen-bonding interactions are present, solvent relaxation effects after excitation are best studied in non-activating solvents rather than in rigid hydrogen-bonding solvents.

Prototropic and metal complexation equilibria of nalidixic acid in the physiological pH region

Abstract The dissociation of nalidixic acid in the near neutral pH region is shown to be due to a single prototropic equilibrium involving the carboxyl group exclusively. Precise dissociation constants as well as stability constants for the binding of the nalidixate anion by several divalent metal ions were determined. The magnitudes of the formation constants of the metal nalidixate complexes support physiological significance of the 1:1 complexes and the lack of importance of the 2:1 complexes.

PROTOLYTIC EQUILIBRIA IN THE LOWEST EXCITED SINGLET STATES OF 2-HYDROXYQUINOLINE AND 4-HYDROXYQUINOLINE

Abstract— Fluorescence spectra of 2‐ and 4‐hydroxyquinoline in a variety of solvents indicate that the lactam‐lactim tautomerism favoring the lactam structure in the ground state also favor the lactam structure in the lowest excited singlet state. Fluorimetric titrations show that the cations of both isomers become slightly more acidic in the lowest excited singlet state and that the anion of the 2 isomer becomes slightly more basic in the excited state. The excited anion of the 4 isomer is either identical in basicity to the ground state anion or does not equilibrate with the quinolone during the lifetime of the excited state. The Forster cycle is found to give a pKα* values in generally poor agreement with those obtained from fluorimetric titrations. The reasons for this are discussed.

Electronic absorption and fluorescence spectrophotometry of quinacrine

Abstract The absorption and fluorescence spectra of quinacrine and its dealkylated derivative, 2-methoxy-6-chloro-9-aminoacridine, were studied as a function of pH and Hammett acidity. The relative stability and high quantum yield of the dication derived from quinacrine make it the most desirable species for fluorimetric determination. This species predominates in the region pH 8-H o -6. The alkylamino side-chain of quinacrine appears to interfere with excited state protonation and dissociation of the dication. Moreover, in the dication, the steric interaction between the side-chain and the 9-amino group may prevent coplanarity between the 9-amino group and the acridine ring, thereby favoring the aminoacridine over the iminoacridan structure.

Acid–base chemistry of omeprazole in aqueous solutions

Omeprazole is a potent anti-acid drug. Its absorption and mode of action are closely related to its prototropic behavior. In the present study, omeprazole samples from different sources and in different forms were studied spectrophotometrically to obtain pKa values. In the neutral to alkaline pH region, two consistent p Ka values of 7.1 and 14.7 were obtained from various samples. The assignment of these pKa values was realized by comparison with the prototropic properties of N(1)-methylated omeprazole substituted on the nitrogen at the 1-position of the benzimidazole ring, which was found to have a pKa of 7.5. The omeprazole pKa of 14.7 is assigned to the dissociation of the hydrogen from the 1-position of the benzimidazole ring and the pKa of 7.1 is assigned to the dissociation from the protonated pyridine nitrogen of omeprazole. The results presented are at variance with those of earlier work.