Characterization of the ionization and spectral properties of sulfonephthalein indicators. Correlation with substituent effects and structural features

Casula, Rita; Crisponi, Guido; Cristiani, Franco; Nurchi, Valeria Marina; Casu, Mariano; Laı̈, Adolfo

doi:10.1016/0039-9140(93)80097-b

Cited by 33 publications

(34 citation statements)

References 25 publications

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“…[22], it has time to follow an analogous H-abstraction route of oxidation of the nonphenolic substrate. In fact, phenol red is more acidic (pK 2 ¼ 7.42) [25] than simple phenols (pK a ¼ 9-10) [26]. This is a result of the delocalization of the negative charge of the anion onto the adjacent quinoid ring (see Fig.…”

Section: A Mechanistic Parallel Between N-hydroxy and Phenolic Mediatorsmentioning

confidence: 99%

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First evidence of catalytic mediation by phenolic compounds in the laccase‐induced oxidation of lignin models

D’Acunzo

Galli

2003

European Journal of Biochemistry

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The sulfonephthalein indicator, phenol red, exhibits an unusually slow rate of oxidation by laccase from Poliporus pinsitus, in spite of the fact that it is a phenol and therefore a natural substrate for this phenoloxidase enzyme. Nevertheless, after prolonged exposure to laccase (24 h) phenol red is oxidized by more than 90%. We found that phenol red, which can be oxidatively converted into a resonance-stabilized phenoxy radical, performs as a mediator in the laccase-catalyzed oxidation of a nonphenolic substrate (4-methoxybenzyl alcohol) and also of a hindered phenol (2,4,6-tri-tert-butylphenol). In particular, phenol red was found to be at least 10 times more efficient than 3-hydroxyanthranilate (a reported natural phenolic mediator of laccase) in the oxidation of 4-methoxybenzyl alcohol. Other phenols, which do not bear structural analogies to phenol red, underwent rapid degradation and did not perform as laccase mediators. On the other hand, several variously substituted sulfonephthaleins, of different pK 2 values, mediated the laccase catalysis, the most efficient being dichlorophenol red, which has the lowest pK 2 of the series. The mediating efficiency of phenol red and dichlorophenol red was found to be pH dependent, as was their oxidation E p value (determined by cyclic voltammetry). We argue that the relative abundance of the phenoxy anion, which is easier to oxidize than the protonated phenol, may be one of the factors determining the efficiency of a phenolic mediator, together with its ability to form relatively stable oxidized intermediates that react with the desired substrate before being depleted in undesired routes.Keywords: laccase; phenolic mediators; lignin models; radicals; acidity.Laccases (EC 1.10.3.2) are multicopper oxidases, produced by micro-organisms and plants, which participate in nature in both the biosynthesis and degradation of lignin [1]. In the latter case, laccase operates in conjunction with other ligninolitic enzymes, such as lignin peroxidase and manganese peroxidase. The latter two enzymes, which are stronger oxidants than laccase, are able to oxidize most aromatic constituents of lignin, whereas laccase oxidizes directly only the phenolic subunits, which are easier to oxidize but relatively less abundant (15%). It has been speculated [1], however, that laccase may react indirectly with nonphenolic lignin components through mediation by phenolic species present in its natural environment. These Ônatural mediatorsÕ could be metabolites [2], or even lignin fragments [3] generated by the other ligninolitic enzymes, and would open up alternative, possibly radical, routes to the oxidation of nonphenolic components [1]. It is quite reasonable to argue that natural mediators may participate in the laccase-catalyzed oxidation of nonphenolic lignin subunits in those micro-organisms that only rely on laccase for their ligninolitic action [4,5], and a number of substances, such as phenolic acids [6,7] and 3-hydroxyanthranilate (HAA) [8], have indeed been proposed as natural laccase ...

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Section: A Mechanistic Parallel Between N-hydroxy and Phenolic Mediatorsmentioning

confidence: 99%

“…As a result, the kinetic curve is biphasic and the data cannot be expressed simply in terms of phenol red concentration. [25], [26], [29] and [30].…”

Section: Kinetics Of the Laccase-catalyzed Oxidation Of Phenol Redmentioning

confidence: 99%

First evidence of catalytic mediation by phenolic compounds in the laccase‐induced oxidation of lignin models

D’Acunzo

Galli

2003

European Journal of Biochemistry

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“…The pK a1 values are generally below 2 , therefore most of these molecules presents only one transition interval in aqueous solutions in the range of pK a2 , i.e., between 3.5 and 8.5 (Scheme 13) [23].…”

Section: Sulfonephthaleinsmentioning

confidence: 99%

Colour maps of acid–base titrations with colour indicators: how to choose the appropriate indicator and how to estimate the systematic titration errors

2016

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To recognise the end point of acid-base titrations, colour indicators are still frequently used. Colour indicators can cause systematic errors, and the theoretical evaluation of these errors is given in many textbooks. Random errors caused by the transition range of an indicator can also be estimated with the help of pH-logc i diagrams. However, very often, undergraduate students have problems to correctly interpret the colours of the different species of an indicator (its acidic and alkaline forms) and colour changes during titration. Frequent questions are of the type: ''Is it still blue, or is it already red?'' The aim of the present text is to show how carefully a suitable colour indicator has to be chosen, and what colour changes happen at the inflection point of a titration. Typical acid-base titrations like the alkalimetry of hydrochloric acid, sulphuric acid, oxalic acid, and the acidimetry of borax at two concentration levels using eleven common colour indicators are compared with potentiometric end point determination. Photographs were taken of solutions containing the indicators during the course of titration, i.e., at different pH values. These photographs were used to prepare a colour map for each indicator. Such colour maps are very helpful to find the most suitable indicator for a certain reaction, and also to estimate the maximum accuracy.

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“…The 13 C NMR spectrum of the corresponding basic form is also included, because former assignments still presented some faults. [2,5] …”

Section: Introductionmentioning

confidence: 99%

“…With respect to the monoanion and the dianion, the quinoid structures 2 and 3 have been proposed. [2,6] These proposals require that at least one of the phenyl rings is located in the plane of the three central coplanar bonds. This is in contrast to X-ray crystallographic studies of triarylcarbenium ions [7] in which only structures with twisted aromatic rings have been observed.…”

Section: Introductionmentioning

confidence: 99%

13C NMR and Crystallographic Study of Thymolsulfonephthalein

et al. 1998

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Up to now the correct structure of the undissociated thymolsulfonephthalein has not been determined unequivocally and three possible resonance structures have been proposed so far: a zwitterionic one, a quinoid one, and a lactonoid one. The present 13 C NMR and X-ray crystallographic study resolves this problem and it can be explicitly shown that the correct structure is zwitterionic with some delocalization of positive charge into the phenol rings. The correct assignment of the completely dissociated blue basic form is also included and permits the conclusion that a fourfold-charged resonance structure is as probable as a quinoid one.

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Characterization of the ionization and spectral properties of sulfonephthalein indicators. Correlation with substituent effects and structural features

Cited by 33 publications

References 25 publications

First evidence of catalytic mediation by phenolic compounds in the laccase‐induced oxidation of lignin models

First evidence of catalytic mediation by phenolic compounds in the laccase‐induced oxidation of lignin models

Colour maps of acid–base titrations with colour indicators: how to choose the appropriate indicator and how to estimate the systematic titration errors

13C NMR and Crystallographic Study of Thymolsulfonephthalein

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