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Polarimetry monitoring

Racemization constitutes a special case of opposing first-order reactions. The equilibrium constant is unity, and the opposing rate constants are equal to one another. Racemization can be followed by polarimetry (monitoring the angle of optical rotation) or by circular dichroism (monitoring the ellipticity). The kinetic analysis can be done by either Eq. (3-15) or (3-16). The rate constant for racemization is krac = ke/2. [Pg.49]

The separation of the two stages is easier to discern when the rates of the two processes are so different, but it can also be seen in the ultrasonic spectra of metal-sulfate systems (Sec. 3.4.4). Ultrasonic absorption peaks can be attributed to formation of outer-sphere complexes (at higher frequency, shorter t) and collapse of outer-sphere to inner-sphere complexes (at lower frequency). In addition to uv spectral and ultrasonic detection, polarimetry and nmr methods have also been used to monitor and measure the strength of the interaction. There are difficulties in assessing the value of ATq, the outer-sphere formation constant. The assemblage that registers as an ion pair by conductivity measurements may show a blank spectroscopically. The value of Aq at T" K may be estimated using theoretically deduced expres-... [Pg.206]

The basic experiment consisted of simply treating an l-UNCA with a base and monitoring the formation of the d-UNCA. The most straightforward analysis is accomplished by polarimetry. For example, Boc-Phe-NCA was dissolved in THF at a concentration of 0.33 M, and 1.5 equivalents of TEA were added. The resulting solution was placed in a polarimeter cell and the optical rotation was monitored over time (Figure 1). [Pg.664]

Polarimetry is extremely useful for monitoring reactions of optically active natural products such as carbohydrates which do not have a useful UV chromophore, and samples for study do not need to be enantiomerically pure. Nevertheless, compared with spectrophotometry, the technique has been applied to relatively few reactions. It was, however, the first technique used for monitoring a chemical reaction by measuring a physical property when Wilhemy investigated the mutarotation of sucrose in acidic solution and established the proportionality between the rate of reaction and the amount of remaining reactant [50]. The study of a similar process, the mutarotation of glucose, served to establish the well-known Bronsted relationship, a fundamental catalysis law in mechanistic organic chemistry. [Pg.73]

Wan QJ, Cote GL, Dixon IB. Dual-wavelength polarimetry for monitoring glucose in the presence of varying birefringence. Journal of Biomedical Optics 2005, 10, 024029. [Pg.355]

The isomerization reaction may be monitored by polarimetry. As the conversion of D-glucose into D-fructose causes a decrease in the specific rotation namely, [a]D25 - 140° (c 0.1, water), polarimetry provides a very sensitive, control technique. [Pg.52]

In principle, any measurable property of a reacting system that is proportional to the extent of reaction may be used to monitor the progress of the reaction. The most common techniques are spectrophotometric (UV-visible, fluorescence, IR, polarimetry and NMR) or electrochemical (pH, ion-selective electrodes, conductivity and polarography). Either a "batch" method can be used, in which samples are withdrawn from the reaction mixture and analyzed, or the reaction may be monitored in situ. By far the most widely used technique involves UV-visible spectrophotometry. [Pg.373]

The field of applications for optical biosensors is wide, covering clinical, industrial control processes, veterinary, food, environmental monitoring, among others [1]. For all these applications, it is desirable to have a compact sensor of high sensitivity, fast response time and which is able to perform real-time measurements. These requirements can be achieved mainly with optical sensors, due to the intrinsic nature of optical measurements that accommodate a great number of different techniques based on emission, absorption, fluorescence, refractometry or polarimetry. [Pg.414]

The use of NMR techniques, rather than polarimetry, to monitor muta-rotation, as with the spontaneous reaction, allowed accelerations of particular conversions of sugars to be directly monitored. Saturation difference measurements indicated that three proteins from Escherichia coli, RbsD, FucU sic) and YiiL, of previously unknown function, had mutarotase activity.RbsD interconverted the (3-pyranose and (3-furanose forms of ribose, without any... [Pg.31]

Another measure of the asymmetric kinetic properties of the two bases in the alanine racemase mechanism is the qualitative behavior of the equilibrium overshoots observed. Overshoots are often observed in reaction progress curves run in deuterium oxide that are initiated with a single stereoisomer that is protiated at the Ca position (Fig. 7.3). The optical activity is monitored by polarimetry or circular dichroism (CD). At equilibrium, the signal is zero, since the product is a racemic mixture of d- and L-isomers. However, when there is a significant substrate-derived KIE on the reverse direction (product being fully deuterated in a two-base mecha-... [Pg.1142]

Polarimetry can be used to monitor the presence of an unwanted enantiomer in some drugs. Elowever, the determination of optical rotation is not a very specific test in addition to the vmwanted enantiomer, other chiral drug related impmities can affect the results. [Pg.3628]

A detector is used to monitor the separation proeess and the response is direetly proportional to the concentration. The detector can be based on variable wavelength UV/ visible light, fluorescence, polarimetry, electrochemical behavior (for materials that ean be readily oxidized), electrical conductivity, refractive index and more recently, the highly successful mass spectrometer. [Pg.731]

The three techniques mentioned here— polarimetry, spectroscopy, and pressure measurement—can all be used to monitor a reaction as it occurs in a reaction vessel. Some reactions occur slowly enough that samples, or aliquots, can be periodically withdrawn from the reaction vessel and analyzed to determine the progress of the reaction. Instrumental techniques such as gas chromatography (Figure 13.4 ) or mass spectrometry, as well as wet chanical techniques such as titration, can be used to measure the relative amounts of reactants or products in the aliquot. By taking aliquots at regular time intervals, we can determine the relative amounts of reactants and products as a function of time. [Pg.602]


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Polarimetry

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