Quenching partitioning

A solution of 1.0 mmol of 2-acetyl alkenoate in 2.5 mL of CH2C1, is added slowly to a solution of 4.0 mmol of titanium(IV) chloride in 7.5 mL of CH-CL under an atmosphere of nitrogen at — 78 °C. The mixture instantaneously turns deep red. and is stirred at — 78 °C before being quenched by the addition of 5 mL of sat. aq potassium carbonate. The mixture is then partitioned between 10 mL of bt20 and 10 mL of water. The aqueous phase is extracted with three 10-mL portions of Et2(), and the extracts are combined, washed with 10 mL of brine, and dried over anhyd potassium carbonate. Concentration under reduced pressure gives the crude product. Product analysis is by capillary GC. 

The second source of partitioning data is experimental equilibration of crystals and liquids followed by microbeam analysis of quenched run products. Starting materials can be natural rocks, or synthetic analogues. In either case it is customary to dope the starting material with the U-series element(s) of interest, in order to enhance analytical precision. Of course, doping levels should not be so high as to trigger trace phase saturation (e g.. 

Standard alkylating and cross-linking agents such as dimethylsulfate or TV-mustards, respectively, have only one opportunity to partition between various nucleophiles since their reactions are irreversible. In contrast, QMs have the potential to partition between nucleophiles multiple times as long as the resulting adducts are formed reversibly. Continual capture and release of QMs consequently can extend their effective lifetime almost indefinitely and is ultimately limited by only the competitiveness of possible irreversible reactions. For DNA, the strongest nucleophiles act reversibly so terminal quenching remains an infrequent event. 

Quenching rate constants for dienes and quadricyclenes have similar sensitivities to the electronic and structural features of the excited aromatic hydrocarbon. However, during this process quadricyclene isomerizes to nor-boraadiene with a quantum yield of 0.52, whereas dienes usually remain unchanged/10 Hammond has suggested that vibrational energy which is partitioned to the acceptor upon internal conversion of the exciplex can lead to isomerization(10a,103) ... 

Fluorescence quenching studies in micellar systems provide quantitative information not only on the aggregation number but also on counterion binding and on the effect of additives on the micellization process. The solubilizing process (partition coefficients between the aqueous phase and the micellar pseudo-phase, entry and exit rates of solutes) can also be characterized by fluorescence quenching. 

Plots of t0/t for varying [Q]T are first otained for different concentrations of lipid. Then, form the slope and intercept of a plot of k pp against am, the value of the partition coefficient is obtained. This method has been applied to the partitioning of lindane into lipid bilayers.(102 103) An example is shown in Figure 5.4 for the quenching of DPH by 5-DOXYL-decane in dimyristoyl-phosphatidylcholine vesicles(104) the increase in the partition coefficient as the... 

Figure 5.4. EfTect of the phase transition on the partition coefficient of 5-DOXYL-decane calculated on the basis of its ability to quench the fluorescence of DPH in dimyristoyl-phosphatidylcholine vesicles. The fluorescence anisotropy of the DPH is also shown. (From Ref. 104, with permission.)...

In systems where only dynamic quenching occurs, then steady-state fluorescence intensities can be measured instead of lifetimes/101 103-,07) In experiments where comparisons are being made (i.e., for a comparison of different experimental conditions or types of membrane), it is important that the lifetime of the fluorophore (r0) is not affected by the experimental conditions. Fluorescence intensities can be obtained much more rapidly and without specialized instrumentation. Blatt and Sawyer(101) have employed a relationship essentially the same as Eq. (5.20) in this way. They have pointed out that since the quenching mechanism is collisional, the partition coefficient that is derived is a partition coefficient of the quencher into the immediate environment of the fluorophore and is therefore a local Kp. It is therefore possible to investigate the partition coefficient gradient across the lipid bilayer by using a series of probes, such as the anthroylstearates,(108) located at different depths. In their method, Eq. (5.20) has the form... 

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 ... 

G. M. Omann and M. Glaser, Biosynthetic incorporation of fluorescent carbazolylun-decanoic acid into membrane phospholipids of LM cells and determination of quenching constants and partition coefficients of hydrophobic quenchers, Biochemistry 23, 4962-4969 (1984). 

R. Fato, M. Battino, M. D. Esposti, G. P. Castelli, and G. Lenaz, Determination of partition and lateral diffusion coefficients of ubiquinones by fluorescence quenching of... 

Equation (4) describes a partition between two forms of the complex one, with fraction f, that exhibits quenching and one that does not. Equation (5) corresponds to a distribution of forms with a range of quenching rates whose breadth varies inversely with n the occurrence of distributions is well-established for proteins at low temperatures [29]. In either case, the non-exponential kinbtics necessitate that conformational interconversion in the transition range is slow compared to the lifetime of the triplet state and the gating limit is applicable. 

Methyl 3-hydroxybutanedithioate (R = Me) (0.6 g 4 mmol) was added dropwise to a solution of LDA (2 eq) in THF cooled at -78°C. The mixture was stirred for 20 min. The yellow dithioester colour disappeared. Allyl bromide (1 eq) was added. Stirring was maintained for 45 min. The solution was quenched by an aqueous solution of ammonium chloride. The mixture was partitioned between ether and brine. The organic layer was washed with brine, dried with magnesium sulfate and concentrated. The crude ketene dithioacetal was left at ambient temperature for X d to achieve the rearrangement. A yellow oil was isolated by flash chromatography (elution with a 90 10 mixture of cyclohexane/ethyl acetate), yield 74%, as a 90 10 syn/anti mixture of diastereoisomers. 

Conceptually, assays for lipase activity using the colorimetric method (copper-soap procedure Basic Protocol 2) are similar to titrimetry in that liberated fatty acids are being measured however, the colorimetric method is more specific for fatty acids (Lowry and Tinsley, 1976). Quenched subsamples of emulsified acylglycerol/lipase reaction mixtures are combined with the biphasic mixture of cupric acetate/pyridine and benzene. Cupric salts of the fatty acids are formed (molar stoichiometry of fatty acid to Cu2+ of 4 2) and these soaps, which are blue in color, are partitioned into benzene to allow for quantification by measuring absorbance of the clear benzene phase at 715 nm. 

Cycloaddition is a singlet state reaction, triplet quenching yielding only stilbene isomerization. In the limit of high t-1 concentration, the quantum yield for the formation of 89 and 90 is 0.66 and no c-1 is formed. Nonradiative exciplex decay is proposed to occur by partitioning at the pericyclic minimum (Fig. 2) between products and reactants. In the limit of high c-1 concentration, 91 is formed with a quantum yield of 0.05 and the predominant exciplex decay pathway is dissociation to yield f-c, which decays to a mixture of t-1 and c-1. 

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