Bimolecular quenching constant

RNA. Both proteins contain no tryptophan S8 contains three tyrosines, and S15 contains two tyrosines. The tyrosine emission of these two proteins represents a case in which the quantum yield is higher in the native than the denatured protein. The average fluorescence lifetime, however, was little affected by denaturation no change was observed for S8, and the average lifetime decreased about 12% for S15. The collisional quenchers 1 and Cs + had essentially equivalent access to the tyrosines of both proteins, either in the native or denatured state, and comparison of the bimolecular quenching constants with that of free tyrosine suggested that the tyrosines were all well exposed. The mechanism for the reduction in quantum yield upon denaturation, apparently a static interaction, has not been elucidated. 

In addition to the partition coefficient, the bimolecular quenching constant (km) is obtained from quenching experiments. 1"1 "7-IIX i and, in principle, this can be used to obtain the lateral diffusion constant of the quencher by using the Smoluchowski equation ... 

This expression is known as the Stern-Volmer equation and Ksv as Stern-Volmer constant. Ksv is the ratio of bimolecular quenching constant to unimolecular decay constant and has the dimension of litre/mole. It implies a competition between the two decay pathways and has the ch".acter of an equilibrium constant. The Stern-Volmer expression is linear in quencher concentration and Ksv is obtained as the slope of the plot of 4>f°If vs [Q], if the assumed mechanism of quenching is operative. Here, t is the actual lifetime of the fluorescer molecule in absence of bimolecular quenching and is expressed as... 

Later in this chapter we will describe a series of recent experiments in which non-racemic excited states were obtained from racemic mixtures by adding an optically active quencher molecule. The chiral quencher changes the excited state lifetimes of the enantiomers, and makes them unequal. We designate the new lifetimes as k+ and k, and relate them to the concentration of chiral quencher, [QaJ, and bimolecular quenching constants, k and k as follows... 

Bimolecular quenching constant obtained from Stem-Volmer plots, by using p-N02C6H4CH2Br as quencher for more details see text. 

Since the bimolecular quenching constant is an upper limiting value for the rate constant that should be used for calculating the energy transfer efficiency (10), the a values are upper limiting values for act-... 

Ksv characterizes the accessibility of the fluorophore to the quencher while the bimolecular quenching constant kq characterize the diffusion properties of the quencher. 

Like the triplet-state lifetimes of the photosensitizer, the bimolecular quenching constants with oxygen are also affected by the presence of micelles. The experimentally determined quenching constants for the para-substituted TPPs in micelles (5.9 X lO M s ) were smaller than in cyclohexane (1. 9 X 10 M s ) despite the longer triplet-state lifetimes in micelles. A possible reason for the smaller rate constants in micelles is restricted molecular motion of the porphyrins within the micelles due to porphyrin-micelle interaction [7]. This suggestion agrees with NMR relaxation measurements, which clearly reveal that the porphyrins in micelles are less mobile than in organic... 

Thus from the plot of this ratio versus the quencher concentration and by knowing 6a separately, the bimolecular quenching constant, kq, can be determined. The... 

C Wlmnne the bimolecular quenching constants for the acoesdUe end inacoesdble leaidtoes ... 

In order to calculate the bimolecular quenching constants, it is necessary to know the oxygen concentration in... 

Figure 9.25. Rado of bimolecular quenching constants for a fluorophore on tte surface of a macromolecule ikf g) and when free In solution IS a foncticHi of the size of the macromolecule. Revised and r Nrinted, with permission, from Ref. SS, Copyright 0198S,Blo ysioU Society.

Interpretadon of Apparent Bimolecular Quenching Constants Calculate the apparent Hmolecular quenching constant of 2-(9-anthroyloxy) palmitate (2-AP) for quenching by co r or dimethylaniline (Figure 9.5). Assume that the unquenched lifetiine is 10 ns. Interpret tltese values with respect to the maximum value of possible in aqueous solution. 

Oxygen Bimolecular Quenching Constant in a Membrane Use the data in Figure 9.36 to.caiariate 1 for oxygen... 

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