Fluorescence intensity loss

Fluorescence Intensity Loss Constantsa from Polymer 1 Films... 

The importance of phenol formation by the proposed pathway was probed by irradiating l,3-diphenoxy-2-methyl-2-propanol (5) under the same conditions. Compared to 3, the rate of phenol formation was approximately 2 times slower. Since the 11-transfer step in Scheme II is not available to 5, the results provide support for the scheme as an important, but not sole, pathway for phenol formation. Irradiation of and 5 with an air purge resulted in faster rates of phenol formation (ca. 5-fold) relative to N2. These findings parallel the accelerated fluorescence intensity loss from polymer 1 films in air as compared to the results in vacuo (see Table I). 

During the course of these studies, it was found that fluorescence intensity from the polymeric films rapidly decreased on continued excitation in a fluorescence spectrophotometer (ca. 30% loss in 1 min for L). Herein, we (1) elaborate further upon the fluorescence loss studies, (2) provide direct evidence for RET from fluorescence lifetime measurements, and (3) present preliminary findings on the photochemistry of model compounds for polymer 1. The results support the conclusion, from previous studies, that the effectiveness of added stabilizer decreases with time due to formation of a photoproduct(s) from the polymer which competes in RET, and is less able to dissipate the resulting excitation energy.1... 

Shell Chemical Company), exhibits a maximum at 300 nm, corresponding to that of the model chromophore anisole. The fluorescence intensity decreases monotonically with increasing concentration of 2,4-dihydroxybenzophenone (DHB) and, furthermore, decreases with time on continued excitation (274 nm) in the spectrophotometer. The fluorescence loss with time may be resolved into two exponential decays. Initially, a relatively rapid fluorescence loss is observed within 20 sec, followed by a slower loss. Loss constants for the initial (k ) and secondary (kj) exponential decays for 1.5 ym films (on glass slides) containing varying concentrations of DHB are provided in Table I (entries 1-3). The initial loss constants are seen to decrease more markedly with increasing DHB concentration than the secondary constants. 

On continued excitation in the phase fluorimeter, the fluorescence lifetime of polymer 1 films also decreased with time. The lifetime decrease was exponential with an average loss constant of 8.2 1.2 x 10 lf sec-1 (1.5 pm thick film) from measurements at different sites on the film. These findings constitute direct evidence for RET from the polymer to a photoproduct(s) in support of the fluorescence intensity measurements. 

With increasing concentration of DHB, the photoproduct forms more slowly, as evidenced by decreasing loss of fluorescence intensity (Table I, entries 1-3). Nevertheless, the concentration of photoproduct(s) and RET from the polymer to photoproduct(s) are expected to increase with time, and stabilization of the polymer will eventually depend upon the capability of the photoproduct(s) to dissipate excitation energy imparted in the RET process. The observed decrease in stabilization efficiency by DHB (based on film discoloration) with exposure time in an accelerometer indicates that DHB is more effective than the photoproduct(s) in dissipating the light energy. Similar spectroscopic studies on polystyrene have led to the same conclusion in this case, as well.6... 

The F/P ratio of the purified, labeled protein may be determined by measuring the absorbance at 345 and 280nm. Ratios between 0.3 and 0.8 usually produce labeled molecules having acceptable levels of fluorescent intensity and good retention of protein activity. AMCA-labeled proteins may be lyophilized without significant loss of fluorescence. The addition of bovine serum albumin (15mg/ml) or another such stabilizer is often necessary to retain solubility of the freeze-dried, labeled protein after reconstitution. 

Diazirine fluorescence provides additional support for RIES.22 Excited di-alkyldiazirines (but not alkylhalodiazirines) fluoresce, and the fluorescence intensity increases with decreasing temperature, suggesting the existence of a barrier to nitrogen loss from the excited diazirine.22 For example, dimethyl-diazirine (35) and 35-df fluoresce upon pulsed laser excitation, with emission due to the excited diazirines. 

Notice that the fluorescence intensity is directly proportional to the optical density only over a narrow range. Use of the above equation corrects for this apparent loss in fluorescence intensity. 

The effect of pH on the fluorescence of some DNS- (5-dimethylaminonaphthalene-l-sulfonyl-) derivatives has been examined [32]. Fig. 2.7 shows the change in fluorescence intensity of DNS-4-methylthio-3,5-xylenol with variation in the pH of the solution. This may be explained in terms of Fig. 2.8 which shows possible resonance structures of DNS-derivatives. When protonated, as in D, the naphthoquinone-type resonance is prohibited, resulting in a loss of fluorescence. 

To determine the concentration of the TPP-extracted GFPuv, fluorescence intensity was correlated to the concentration of native GFPuv, since denatured GFPuv cannot fluoresce, and the loss in fluorescence intensity was a measure of denatured GFPuv concentration. 

The denaturation of GFPuv in buffer solutions at various pH values was measured by the loss of fluorescence intensity and expressed in the decimal logarithm of the decrease in native GFPuv concentration vs the time of exposure at constant temperature (Fig. 1). To estimate D-values at constant heating temperatures and pH values (Table 1), the interval of GFPuv concentrations considered was between 3.5 (acetate, pH 5.18) and... 

At 90°C and 95°C, a slight drop in fluorescence intensity was observed (Fig. 1) without interfering with the determination of D-value. For lower temperatures, the loss of fluorescence intensity decreased gradually. For the same temperatures, the thermal stability of GFPuv was observed to gradually increase proportionally withpH and was shown to be dependent on the buffer used. 

This is designed to avoid loss of fluorescent intensity due to light scattering. At this detection point, the two DNA molecules are well resolved, as shown in Figure 9.29C. Such a DC separation for the two DNA molecules cannot be... 

The fluorescence intensity of CGTase tryptophan residues decreases (Figure 7.13) after treatment of the enzyme with increasing concentrations of TNM and purification of the modified enzyme on co-polymer. Tryptophan fluorescence intensity of the 8 mM-TNM-modified CGTase (0.03 /uM 1 tryptophan) is similar to that of free L-tryptophan (0.034 /xM 1). The loss of tryptophan fluorescence observed during nitration may then be related to elimination of this energy transfer. 

Calibration of the system The scopoletin concentration is adjusted in such a way that its fluorescence exceeds the substrate-induced fluorescence at least tenfold. Any marked decrease in fluorescence intensity therefore demonstrates an oxidation of scopoletin by H202 via horseradish peroxidase. Hence, after suitable calibration of the system, the rate of loss of fluoresence can be used to measure the rate of hydrogen peroxide production. 

A disadvantage of the back end monochromator is its inability to separate Ka 2 from Kai. For very precise unit cell determinations this separation is desirable. For this purpose a front end or primary beam monochromator is used (Figure 12c). This monochromator is the only t)q)e that can be used with area detectors. Their disadvantages include a high intensity loss, the need for precise alignment and nonremoval of fluorescent radiation. 

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