Optical Rotation—Polarimetry

Polarimetry. Polarimetry, or polarization, is defined as the measure of the optical rotation of the plane of polarized light as it passes through a solution. Specific rotation [ a] is expressed as [cr] = OcjIc where (X is the direct or observed rotation, /is the length in dm of the tube containing the solution, and c is the concentration in g/mL. Specific rotation depends on temperature and wavelength of measurement, and is a characteristic of each sugar it may be used for identification (7). 

Polarization is the most common method for the determination of sugar in sugar-containing commodities as well as many foodstuffs. Polarimetry is apphed in sugar analysis based on the fact that the optical rotation of pure sucrose solutions is a linear function of the sucrose concentration of the solution. Saccharimeters are polarimeters in which the scales have been modified to read directiy in percent sucrose based on the normal sugar solution reading 100%. 

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. 

The determination of the angle of rotation is called polarimetry. In some cases, it can help a chemist follow a reaction. For example, if a reaction destroys the chirality of a complex, then the angle of optical rotation decreases with time as the concentration of the complex falls. 

Part—IV has been entirely devoted to various Optical Methods that find their legitimate recognition in the arsenal of pharmaceutical analytical techniques and have been spread over nine chapters. Refractometry (Chapter 18) deals with refractive index, refractivity, critical micelle concentration (CMC) of various important substances. Polarimetry (Chapter 19) describes optical rotation and specific optical rotation of important pharmaceutical substances. Nephelometry and turbidimetry (Chapter 20) have been treated with sufficient detail with typical examples of chloroetracyclin, sulphate and phosphate ions. Ultraviolet and absorption spectrophotometry (Chapter 21) have been discussed with adequate depth and with regard to various vital theoretical considerations, single-beam and double-beam spectrophotometers besides typical examples amoxycillin trihydrate, folic acid, glyceryl trinitrate tablets and stilbosterol. Infrared spectrophotometry (IR) (Chapter 22) essentially deals with a brief introduction of group-frequency... 

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). 

An increase in the optical rotation caused by an achiral contaminant may occur as the result of the induction of a preferential chiral conformation in an achiral contaminant33. Thus, careful purification of both sample and solvent is mandatory in polarimetry, Utmost care has to be exercised, however, to avoid accidental de- or enrichments due to fractionation during incomplete recoveries (see also Section 3.1.3.). 

Light scattering techniques Optical rotation-polarimetry Refractive index Infrared spectro-photometry Infrared process analyzers Microwave spectroscopy Gamma ray spectroscopy Nuclear quadrupole moment... 

Polarimetry coupled to a liquid chromatograph has been used to determine the optical purity of drugs [113-121]. The advantages of this approach for determining optical purity is that the enantiomers do not need to be resolved and only eluates which give rise to optical rotation or circular dichro-ism are detected. This advantage makes on-line polarimetry inherently more accurate than conventional polarimetry because minor interfering compo-... 

Table VI gives, for a large group of sugars, a summary of the mutarotation constants and optical rotations measured at 0 and 20 in water or aqueous potassium hydrogen phthalate by Isbell and Pig-man. Most of the data were published in a circular now out of print Other compilations have been made by Hudson, Lowry, Osaka, Kendrew and Moelwyn-Hughes, Pratolongo (dilatometry and optical rotation, and Riiber and associates (dilatometry, refractive index, and polarimetry). " A considerable...

OPTICAL ROTATION MEASUREMENT (40) Lincomycin hydrochloride in injections was determined by measuring its optical rotation. In this test, lincomycin hydrochloride in injection was diluted with water and the optical rotation of the resulting solution was determined by polarimetry. 

Aimed at investigating the taste enhancing activity of the individual enantiomers of alapyridaine, enantiopure 1 was prepared upon reductive animation of 5-(hydroxymethyl)-2-furanaldehyde and L-alanine with Raney nickel/hydrogen. This reaction resulted in the corresponding ( S)-A -(1-carboxyethyl)-2-hydroxymethyl-5-(methylamino)furan (Figure 7). The latter was converted into the target pyridinium betain compound by mild oxidation with bromine in water/methanol to yield (+)-(iS)-l. Similarly, the reaction with D-alanine resulted in (-)-(R)-l. After purification, the presence of (5)-l and (R)-1 was proven by polarimetry, revealing optical rotations of +40.2° and -38.6°, respectively (72). 

A final proof for the stereochemistry present in alapyridaine was performed by CD spectroscopy. As shown in Figure 8, the alap5a idaine sample isolated from heated Maillard mixtures did not show any CD effect, thus confirming the formation of racemic alap)o idaine during thermal treatment as suggested by the optical rotation of 0° measured by polarimetry. In contrast, the synthetic ( S)-(+)-enantionier showed a pronounced CD effect, in particular, when measured at pH 9, thus confirming the enantiopurity of that sample. 

Methyl-3-(2-methylphenyl)quinazoline-4(3H)-thione (471), the sulfur analogue of MQ, and 2-methyl-3-(2-aminophenyl)quinazoline-4(3H)-thione (472) were baseline separated into enantiomers on various chiral supports opening a way to barrier and chiroptical property determinations. Figure 27 reports the excellent baseline separation obtained on a Chiralpak AD column. Polarimetry indicates the sign of the optical rotation of the first and second eluted enantiomer in the mobile phase (llUPl). 

Enzymes catalyzing reactions involving optically active compounds can be assayed by polarimetry. A compound is considered to be optically active if polarized light is rotated when passing through it. The magnitude of optical rotation is determined by the molecular structure and concentration of the optically active substance which has its own specific rotation, as defined in Biot s law ... 

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