Bonds and Sulfhydryl Groups

Another possibility is deactivation through increased intersystem crossing,(1) which would occur via enhanced spin—orbit coupling.(28) If intersystem crossing is enhanced, then the phosphorescence quantum yield of a 

The importance of comparing time-dependent and steady-state fluorescence measurements is well illustrated by the difficulty of resolving purely static from purely dynamic quenching. In either case, the basic relationship between the steady-state fluorescence intensity and quencher concentration is the same. The Stem-Volmer relationship for static quenching due to formation of an intermolecular complex is i 

It has been pointed out by Eftink and Ghiron(60) that a general expression for the combination of static and dynamic quenching is 

Other corrections, besides those for static interactions, are important for certain quenchers. For example, acrylamide quenching is often used to help determine the relative solvent accessibility of aromatic residue side chains. In addition to a correction for static quenching,(60,66) acrylamide quenching data for tyrosine residues require both primary and secondary inner filter corrections since acrylamide absorbs both 280- and 305-nm light.(67) 

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, 

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