Phenolic chromophore

Chiral crown ethers possessing two chiral cw-l-phenylcyclohexane-l,2-diol moieties as well as a p-(2,4-dinitrophenylazo)phenol chromophore were prepared and with chiral alkylamines were observed to show enantiomeric selectivity <96JCS(P1)383>. 

Optical rotatory dispersion003 146 characteristics of (-)-levorphanol and (-)-morphine have been compared. The negative Cotton effects of each compound were attributed to the phenolic chromophores, and this afforded strong evidence for the configurations at C-9, C-13, and C-14 being identical in each. Circular dichroism(104) studies on other morphinan and codeine derivatives supported this conclusion. 

The total emission spectra of these compounds, consisted only of those from the Indole chromophore neither the fluorescence, nor the phosphorescence of the phenol chromophore contributed to them. From the distortion of the absorption spectra can be concluded that a weak intramolecular hydrogen bonding of the indole chromophore to the phenolic oxygen or the tt orbitals of the phenol ring exists. The low fluorescence yields of compounds [14b] and [14c] were attributed to the formation of an Intramolecular complex between the Indole group in the excited state and the phenol group in the ground state. 

FIGURE 3.21 There is no ground-state interaction between the benzoyl and phenol chromophores of these molecules, even though transient studies show that the linking chains are sufficiently flexible to allow interaction of the triplet carbonyl and the phenol. 

Measurement of NTE activity has classically been done by colorimetric determination of phenol released by hydrolysis of the substrate, phenyl valerate [89,90]. The absorbance maximum of the red phenol chromophore overlaps substantially with that of whole blood homogenates and dilution of the blood to remove the interfering absorbance decreases NTE activity below the detection limit of the colorimetric assay [88], Thus, the colorimetric assay cannot be used to assay NTE in whole blood. The problems inherent in a colorimetric NTE assay can be eliminated by using an amperometric technique to detect phenol. 

Due to the substitution of Tyr66 by Trp the chromophores of the GFP derived CFPs contain an indole instead of a phenol or phenolate [52], In consequence, the spectra of these proteins show excitation peaks at 434nm and emission peaks at 476nm to 486nm, which is intermediate between those of GFPs with neutral phenol and anionic phenolate chromophores, Fig. (12). In fact, these proteins possess double-humped rather than conventional single excitation and emission peaks. The origin of the doubled... 

A comparison of the difference spectrum of lysozyme (Fig. 152) with that of tryptophan or glycyltryptophan (Figs. 119 and 120) shows that the difference spectrum of the protein appears to be due entirely to the indole chromophore below pH 7. Neither the peaks at 280 and 287 m i, characteristic of the phenolic chromophore, nor the peaks near 260 m/i, characteristic of the benzene chromophore, are observed. These may possibly be masked by the indole spectrum. Therefore, it is not possible to obtain information about the abnormal tyrosyl groups from the difference spectrum. 

Conversely, mutations eliminating a hydrogen donor on the phenolate chromophore, such as T203I and T203V, destabilize State B in favor of A and produce Class GREEN A. These variants emit by ESPT with a relatively high quantum yield and find applications due to their large Stokes shift between absorption and emission. 

UV-VIS Just as with arylammes (Section 22 20) it is informative to look at the UV VIS behavior of phenols m terms of how the OH group affects the benzene chromophore... 

Cross-conjugated dienones are quite inert to nucleophilic reactions at C-3, and the susceptibility of these systems to dienone-phenol rearrangement precludes the use of strong acid conditions. In spite of previous statements, A " -3-ketones do not form ketals, thioketals or enamines, and therefore no convenient protecting groups are available for this chromophore. Enol ethers are not formed by the orthoformate procedure, but preparation of A -trienol ethers from A -3-ketones has been claimed. Another route to A -trien-3-ol ethers involves conjugate addition of alcohol, enol etherification and then alcohol removal from la-alkoxy compounds. 

It is generally believed that the absorption (and fluorescence excitation) peak at about 400 nm is caused by the neutral form of the chro-mophore, 5-(p-hydroxybenzylidene)imidazolin-4-one, and the one in the 450-500 nm region by the phenol anion of the chromophore that can resonate with the quinoid form, as shown below (R1 and R2 represent peptide chains). However, the emission of light takes place always from the excited anionic form, even if the excitation is done with the neutral form chromophore. This must be due to the protein environment that facilitates the ionization of the phenol group of the chromophore. This is also consistent with the fact that the pACa values of phenols in excited state are in an acidic range, between 3 and 5 (Becker, 1969), thus favoring anionic forms at neutral pH. 

Wan s group showed that the observed photodehydration of hydroxybenzyl alcohols can be extended to several other chromophores as well, giving rise to many new types of quinone methides. For example, he has shown that a variety of biphenyl quinone methides can be photogenerated from the appropriate biaryl hydroxybenzyl alcohols.32,33 Isomeric biaryls 27-29 each have the benzylic moiety on the ring that does not contain the phenol, yet all were found to efficiently give rise to the corresponding quinone methides (30-32) (Eqs. [1.4—1.6]). Quinone methides 31 and 32 were detected via LFP and showed absorption maxima of 570 and 525 nm, respectively (in 100% water, Table 1.2). Quinone methide 30 was too short lived to be detected by LFP, but was implicated by formation of product 33 that would arise from electrocyclic ring closure of 30 (Eq. 1.4). 

Polymer/additive analysis then usually proceeds by separation of polymer and additives (cf. Scheme 2.12) using one out of many solvent extraction techniques (cf. Chapter 3). After extraction the residue is pressed into a thin film to verify that all extractables have been removed. UV spectroscopy is used for verification of the presence of components with a chromophoric moiety (phenolic antioxidants and/or UV absorbers) and IR spectroscopy to verify the absence of IR bands extraneous to the polymer. The XRF results before and after extraction are compared, especially when the elemental analysis does not comply with the preliminary indications of the nature of the additive package. This may occur for example in PA6/PA6.6 blends where... 

A collection of UV spectra of plasticisers, fluorescent whitening agents (optical brighteners), UV absorbers, as well as of phenolic and aminic antioxidants was published by Hummel and Scholl [21]. UV absorbance data for isolated chromophores are listed elsewhere [22]. A general UV atlas of organic compounds is available [23]. 

A method suitable for quantification of the functional class of bis(ethanol)amine antistatics, which lack UV chromophores, consists of reaction with methyl orange [53]. Atmer 163 (alkyl-diethanol amine) has been determined as a yellow complex at 415 nm after interaction with a bromophenol/cresole mixture [64]. Hilton [65] coupled extracted phenolic antioxidants with diazotised p-nitroaniline in strongly acidic medium and carried out identification on the basis of the visible absorption spectrum in alkaline solution. The antioxidant Nonox Cl in... 

The chromophore of ECFP does not bear the deprotonable phenol that is crucial to the photophysics of most AvGFP variants, and displays a markedly different spectroscopy. It has been quickly recognized that, despite its prominent interest in biological applications, the properties of this variant are suboptimal. Indeed, while the brightness of the protein is relatively low (eM = 32,000 M 1 cm-1, fluorescence emission is both spectrally and kinetically heterogeneous. The fluorescence comprises two major decay times at 3.6 ns and 1.3 ns... 

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