Point isosbestic

In determining the values of Ka use is made of the pronounced shift of the UV-vis absorption spectrum of 2.4 upon coordination to the catalytically active ions as is illustrated in Figure 2.4 ". The occurrence of an isosbestic point can be regarded as an indication that there are only two species in solution that contribute to the absorption spectrum free and coordinated dienophile. The exact method of determination of the equilibrium constants is described extensively in reference 75 and is summarised in the experimental section. Since equilibrium constants and rate constants depend on the ionic strength, from this point onward, all measurements have been performed at constant ionic strength of 2.00 M usir potassium nitrate as background electrolyte . 

Exploitation of analytical selectivity. We have seen, in our discussion of the A —> B C series reaction (Scheme IX), that access to the concentration of A as a function of time is valuable because it permits to be easily evaluated. Modern analytical methods, particularly chromatography, constitute a powerful adjunct to kinetic investigations, and they render nearly obsolete some very difficult kinetic problems. For example, the freedom to make use of the pseudoorder technique is largely dependent upon the high sensitivity of analytical methods, which allows us to set one reactant concentration much lower than another. An interesting example of analytical control in the study of the Scheme IX system is the spectrophotometric observation of the reaction solution at an isosbestic point of species B and C, thus permitting the A to B step to be observed. 

Tire fulvalenium salts 35 and 38 react reversibly in aqueous buffer solutions to yield the corresponding triarylcyclopropenols, as evidenced by the well-defined isosbestic points in their UV spectra. Tire pK values derived from spectra are compatible with extensive delocalization of charge in the heterocyclic ring (68TL5541). 

The greater intensity of the band of the metabolite at 220 mis probably due to the presence of a second, superimposed chromophore which could also account for the shift of the minimum. On the other hand, the band near 300 m/u. has the expected intensity. Its broadness and displacement towards longer wavelength are probably due to the presence of a substituent on the double bond or benzenoid ring. That the assignment to a coumaroyl chromophore is essentially correct is evidenced by the fact that both M and the model compound underwent the same type of reaction on irradiation in the near-ultraviolet (Figure 4). The observed isosbestic points imply that the photoreaction is a simple one, such as A -> B or A = B, and is obviously the well-known light-induced trans- to c/r-isomerization (7) of cinnamic acid derivatives. 

Ultraviolet absorption curves obtained at intervals during irradiation of dilute aqueous solutions until constant (ca. 40 minutes) showing three (left) and two (right) isosbestic points... 

The presence or absence of oxygen did not appear to be significant. Product analyses suggested that aromatic nitrosocompds and nitro-phenols were the main products. However, the observed products did not conform in a simple manner to the constraints of the isosbestic points observed in all the photolyses. It appeared that other products were present, some of which could have low extinction coeffs in the spectral range studied... 

Recently, this problem has been approached by reverting to reading the color of the bilirubin directly. This used to be called an "Icterus Index", in which the color of the serum was compared to a dilute dichromate solution (39). However, the new approach has been to use two wavelengths. This can be used in either of two different procedures. In one procedure the reading is made at the peak for bilirubin which is at 453 nm and at an isosbestic point for hemoglobin (40). The effect on the reading due to hemoglobin is subtracted from the bilirubin value. Commercial instruments based on this principle have not been successful. 

The work with iodide (preceding sub-section) was extended to thiosulphate , and isosbestic points and second-order kinetics were again obtained with the various Pt(IV) complexes (Table 8). Two 8203 ions are consumed per mole of Pt(IV) reduced, suggesting tetrathionate to be the product of oxidation, viz. 

In order to develop compounds that can selectively target duplex RNA, Sinha et al. [ 194] studied the interaction of berberine with two different conformations of poly(rC) poly(rG) structures. Poly(rC) poly(rG) has been shown [15,215] to exist in two conformations depending on the pH of the solution, the A-form at physiological pH and the protonated form at pH 4.3. These two conformations have been characterized to have clearly defined but distinctly different circular dichroic and absorption spectral characteristics. Both the A-form and the protonated form of the RNA induced moderate hypochromic change and bathochromic shifts in the absorption maxima peaks at 344 nm and 420 nm of the alkaloid with three isosbestic points centered around 357,382 and 448 nm. Binding of berberine to both forms enhanced the fluorescence intensity, which was higher with the protonated form than with the A-from, suggesting clear differences in the nature of orientation... 

Molecular rotors with a dual emission band, such as DMABN or A/,A/-dimethyl-[4-(2-pyrimidin-4-yl-vinyl)-phenyl]-amine (DMA-2,4 38, Fig. 13) [64], allow to use the ratio between LE and TICT emission to eliminate instrument- and experiment-dependent factors analogous to (10). One example is the measurement of pH with the TICT probe p-A,A-dimethylaminobenzoic acid 39 [69]. The use of such an intensity ratio requires calibration with solvent gradients, and influences of solvent polarity may cause solvatochromic shifts and adversely influence the calibration. Probes with dual emission bands often have points in their emission spectra that are independent from the solvent properties, analogous to isosbestic points in absorption spectra. Emission at these wavelengths can be used as an internal calibration reference. 

By correlating the observed spectral changes with the concentrations of added cycloamylose, dissociation constants of the cycloamylose-substrate adducts may be calculated (Rossotti and Rossotti, 1961). Values of the dissociation constants determined in this manner for a variety of complexes are presented in Table II. In most cases, stoichiometries of the complexes have been shown to be 1 1 from the presence of distinct isosbestic points in the spectrophotometric titrations. In a few cases, additional spectral perturbations are observed as the cycloamylose concentration is increased, indicating more complex modes of association. Methyl orange, for example,... 

Representative absorption spectra for cresol red in a 2 1 tetraethoxysilane (TEOS) methyltrimethoxysilane (MTMOS) film after immersion in buffer solutions of varying pH can be seen in Figure 11. The absorbance for gel-immobilized cresol red at 570 nm increases with increasing pH. Isosbestic points can be observed at ca. 480 nm for gel-immobilized cresol red. 

Addition of H202 or t-BuOOH to aqueous solutions of iron(III)-TAML complexes 1 produces brownish-green colors. The spectral changes can be measured by UV/Vis spectroscopy (Fig. 10). Less than a stoichiometric amount of peroxide causes a major increase in the absorbance in the range 350-550 nm. It requires several minutes to obtain invariable spectra at pH below 8.5, but the reaction becomes much faster at pH >9. Two isosbestic points... 

Several studies have been carried out to investigate the effect of pH on azo dye decolorization. In these assays, the decrease of absorbance at the wavelength corresponding to the maximum absorption for each dye is used as the method to evaluate the effectiveness of decolorization. Unfortunately, in most cases it is not clear if the isosbestic point of each dye was taken into account, and so it cannot be well understood if the different decolorization rate at different pH is due to a physical factor or to a differently influenced metabolic activity. 

Unless standards are prepared in buffered media, positive or negative deviations may result from measurements made at 348 nm or 372 nm respectively Alternatively, measurements can be made at the isosbesticpoint, i.e. where the absorbance curves of each form intersect, and where absorbance is not a function of equilibrium concentrations but only of the overall concentration. Solutions of weak acids and bases should also be measured at their isosbestic points for the same reason. 

HIP hidden isosbestic point Q flow rate V variability calculated from the number of spectra not crossing at the HIP divided by the total number of spectra. 

Titrations of pz (95b) with Ag+ in water to determine the metal-binding affinity gives two isosbestic points at -350 and 445 nm, a near isosbestic... 

The phenylazonaphthols present a particularly interesting series. Despite the partial loss of aromaticity in the hydrazone structures that can be drawn for l-phenylazo-2-naphthol, 2-phenylazo-l-naphthol and 4-phenylazo-l-naphthol, these structures are not much higher in energy than the azo forms [50], so that there should be no significant preference for one form over the other. When the visible absorption curves of 4-phenylazo-l-naphthol are measured in a variety of solvents, they all pass through a common point (isosbestic point) as in Figure 4.1 [51,52]. l-Phenylazo-2-naphthol shows similar behaviour. This finding is clear evidence that in solutions of these compounds both tautomers are present. 

The Lewis basicity of the solvents was found to be more important for the solvatochromism than the solvent polarity, since such Si... solvent and Si... F interactions are competitive in the presence of solvents with donor atoms. In contrast, the analogous non-fluorinated polysilane, poly(methyl-/z-propylsilylene), revealed a disordered conformation in both coordinating and non-coordinating solvents. Additionally, the UV spectra of various molecular weight fractions of 87 showed an unusual molecular weight dependency an isosbestic point is apparent, suggesting an equilibrium between globule- and rod-like conformations at room temperature, which was also evident from the... 

The phase transitions for 90 were also evident in the UV-VIS spectra, as seen in Figure 26 M has an absorption maximum at 315 nm, QM at 350 nm, and multiple absorptions with peaks at ca. 315, 350, 365, and 378 nm are evident for phase I and the other phases. Isosbestic points observed between the phases indicate coexistence of two states during structural evolution, and the existence of multiple UV peaks for a phase is considered to indicate the existence of several conformations for that phase. In this report, 0 all structures were modeled with conformations between all-T and all-D thus, the conformational nature of the different phases of (Si-/z-Dec2U is not clear. Similar UV absorptions observed for structurally related polymers, such as PDHS (see below), are suggestive of comparable backbone conformations. 

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