Disulfide, quenching

Carbon disulfide quenches the fluorescence of anthracene quite efficiently,145,149 but seems to have little effect on its triplet lifetime.147 Diphenylanthracene in benzene fluoresces with a quantum yield of 0.8 and shows a high sensitivity to the oxygen concentration in photooxygenation reactions. With about 1 vol% of CS2 present, AC>2 is practically independent of [02] (> 10"5 mole/liter). In jjoth cases, where carbon disulfide was either used as solvent or was added to an otherwise strongly fluorescent solution, the quantum yields of photooxygenation followed... 

Metalation of the 5- and 6-chloropivaloylamino pyridines 225 was shown to occur cleanly giving, after dimethyl disulfide quench, high yields of... 

Competition between fluoro and N-pivaloyl DMGs was demonstrated in the metalation of 227 as evidenced by dimethyl disulfide quench experiments (Scheme 67) (83JOC3401). While exclusive ortho-to-Af-pivaloyl deprotonation was shown to occur using f-BuLi/Et20/-78°C conditions, almost equal metalation to the two DMGs was observed under n-BuLi/ THF/0°C. 

Substimted 3 bridgcd Sulfonamides. With a versatile synthesis of the Harmony isomers in hand we next turned our attention to the synthesis of the alternate isomers. The key compound here was the ester 25, needed for the direct biological comparison with Harmony . This compound was easily prepared by treating 3,4- bromothiophene with 1 equivalent of butyllithium followed by a propyl disulfide quench. The thioether 22 can then be treated with another equivdent of butyllithium and trapped with CO2 to give 23. Esterification (23 24) followed by... 

Let us now discuss some of the characteristics of this quenching with mercaptans and disulfides. Interestingly, both sulfur derivatives are equally effective in inhibiting the photoreduction and are in fact interconverted during the reaction. The same equilibrium mixture of mercaptan and disulfide is obtained regardless of which was initially added to the reaction mixture. Furthermore, there appears to be no appreciable consumption of the sulfur compounds/64 When benzophenone is irradiated in the presence of isopropanol (OD) and mercaptan, isopropanol containing two deuterium atoms is isolated,... 

From the same zirconocene intermediates, Huang and co-workers have prepared vinyl sulfides [24,35] and sulfones [26] through use of the appropriate quenching agents (Scheme 4.6). Treatment of vinyl zirconocenes with an equivalent of a disulfide in THF at 60 °C affords, after work-up and purification, (E)-vinyl sulfides in good isolated yields. Vinyl sulfones, which as a class are generally useful as Michael acceptors and Diels—Alder dienophiles, are obtained in about two hours upon treatment of (fc)-vinyl zirconocenes with various sulfonyl chlorides in THF at 40°C. 

By comparing time-resolved and steady-state fluorescence parameters, Ross et alm> have shown that in oxytocin, a lactation and uterine contraction hormone in mammals, the internal disulfide bridge quenches the fluorescence of the single tyrosine by a static mechanism. The quenching complex was attributed to an interaction between one C — tyrosine rotamer and the disulfide bond. Swadesh et al.(()<>> have studied the dithiothreitol quenching of the six tyrosine residues in ribonuclease A. They carefully examined the steady-state criteria that are useful for distinguishing pure static from pure dynamic quenching by consideration of the Smoluchowski equation(70) for the diffusion-controlled bimolecular rate constant k0,... 

R. W. Cowgill, Fluorescence and protein structure XI. Fluorescence quenching by disulfide and sulfhydryl groups, Biochim. Biophys. Acta 140, 37-44 (1967). 

Other groups within the protein may affect excited states. Disulfide bonds quench the excited states of tryptophan. For instance, at 77 K the phosphorescence lifetime of native lysozyme is low, 1.4s reduction of the disulfide bonds or denaturation gave the typical phosphorescence lifetime of 5.6 s.(49) Therefore, the absence of phosphorescence at room temperature from this protein is likely to be due to quenching of both the singlet and the triplet state. 

Sulfur-selenium phases can be prepared by cooling molten mixtures of the elements either slowly or by quenching followed by extraction with carbon disulfide, carbon tetrachloride or benzene. The crystals are obtained upon evaporation or cooling of the resulting solutions. Their colour deepens from yellow to ruby red with increasing selenium content In the older literature there has been some confusion whether to consider these phases as mixed crystals of discrete Sg and SCg molecules or as binary compounds containing SeS bonds. 

Trithiolane is attacked at one of the disulfide sulfur atoms with methyl lithium at — 78 "C. The resulting ring-opened intermediate can be quenched with an electrophile as in the synthesis of dysoxysulfone (103) (Scheme 25) <94JOC2273>. 

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