Optical rotation detection

FIGURE 1.28 Chromatograms of the HPLC enantiomer separation of Z-protected a-aminophosphinic acids (a,b) and phosphinic acid-ilr-dipeptide (c) on (a and b) a 0-9-(tert-butylcarbamoyl)quinidine CSP and (c) corresponding 0-9-(fcrr-butylcarbamoyl)qninine-CSP, respectively. Experimental conditions Column dimensions, 150 mm x 4 mm ID mobile phase, methanol-50 mM sodium phosphate buffer (80 20 v/v) (pHa 5.6) temperature, 40°C flow rate, 1 mLmin detection, UV at 250 nm and optical rotation detection (ORD). (Reproduced from M. Lammerhofer et ah, Tetrahedron Asymmetry, 14 2557 (2003). With permission.)... 

Gagne and coworkers utilized this combination to discover enantioselec-tive receptors for (-)-adenosine [12]. A racemic dipeptide hydrazone [( )-pro-aib] generated a stereochemically diverse DCL of n-mer. The dimers were composed of two chiral (DD/LL) and one achiral isomer (DL), the four trimers (DDD, LLL, DDL, and LLD), the tetramers of four chiral and two achiral isomers, etc. Two techniques were used to measure the enan-tio-imbalance that was caused by the enantioselective binding of the chiral analyte to the enantiomeric receptors (Fig. 5.11). Since the unperturbed library is optically inactive, the optical enrichment of each library component could be measured by a combined HPLC optical rotation detection scheme (laser polarimeter, LP). LP detection differentiated unselective binding (amplification but not optical enrichment) from enantioselective recognition of the analyte (amplification and optical enrichment). In this manner the LL dimer (SS) of the dipeptide was amplified and identified as the enantioselective match for (-)-adenosine. 

Detection and identification of related substances caimot be performed by LC-MS alone. In practice, additional experiments are required using various LC gradients and/or orthogonal phase systems, alternative detection techniques like evaporative light scattering detection or optical rotation detection, and other spectroscopic techniques, like UV-DAD and NMR. Obviously, this discussion focusses primarily on the role of LC-MS in the impurity profiling and identification. Developments in LC-MS and especially mass analysers are important in strengthening the power of LC-MS in stmcture elucidation. 

An experimental set-up similar to the one used in polarization spectroscopy is employed in experiments testing atomic manifestations of parity violation in the electro-wealc interaction. [9.359-9.361]. The experiments are important for testing the Standard Model hi elementary particle physics. Right-left asymmetries in atomic physics are of the order of 1 10 . A small optical rotation detectable using crossed polarizers is induced by interference between neutral weak and electromagnetic interactions in atoms. The most accurate experiments deal with heavy elements such as mercury, thalhum, lead and bismuth [9.362, 9.363]. Another way of probing the parity violation is to observe the strength of certain forbidden transitions, notably the 7 —... 

The two basic requirements of relaxation experiments are The sufficiently fast disturbance of the equilibrium and the specific observation of the transient helix-coil transition. Stationary relaxation methods, such as sound absorption and dielectric relation, do not fulfill the second requirement. The application of electric field-jump and laser temperature-jump perturbations in combination with optical rotation detection not only satisfies both requirements, but as a bonus it provides also for the use of a large variety of solvents and ionic strengths. 

Terpenes are commonly employed for diluting lemon, orange, and bergamot oils. The addition lowers the specific gravity, increases the optical rotation, and lowers the proportion of oxygenated constituents. Terpinolene, a by-product in the manufacture of terpineol, has been detected in some oils, notably citronella and spike, lavender. It can be detected by its odour in the fractionated oils. 

The presence of two different probes, namely, Gal C-l and Thr C7, giving rise to signals situated in two different, uncrowded regions of the spectrum, with chemical-shift differences (AdC-l up to 8.35 p.p.m., and Ad Thr C7, up to 6.7 p.p.m.), allows the facile detection of any racemiza-tion occurring at either, or both, of the asymmetric carbon atoms of the threonine. It would be difficult to distinguish a-D-Gal — L-Thr (27) from its P analog (28), or / -D-Gal — D-alloThr (58) from / -D-Gal — L-alloThr (56), on the basis of the anomeric-carbon chemical-shifts only. However, they can be differentiated on the basis of Thr Cr chemical-shift data. It should be noted that neither optical rotation nor -n.m.r. spectroscopy could have elucidated this point.85... 

A special mention in the field of enantioselective HPLC separations must be made of chiro-optical detection systems, such as circular dichroism (CD) and optical rotation (OR), which can be also used to circumvent the low UV detectability of chromophore-lacking samples [40, 61]. While sensitivity of chiro-optical detection is not always sufficient to perform enantiomeric trace analysis, the stereochemical information contained in the bisignate spectropolarimetric response is useful in establishing elution order for those compounds not available as single enantiomers of known configuration. An example of application of different online detection systems (UV and CD at 254 nm) in the enantioselective separation of a racemic sulfoxide on a commercially available TAG CSP is reported in Figure 2.12, under NP conditions. 

Renaturation in solution. When the hot (40°C) solutions are cooled again the helices are renaturated. The presence of the left handed helices can easily be detected by measuring the optical rotation of the solutions. 

Before melting and for some time after only the band at 625 cm of the AA [C4CiIm]+ cation was observed in the 600-630 cm i region. Gradually 603 cm i band due to the GA conformer became stronger. After about 10 min the AA/GA intensity ratio became constant. The interpretation [50] is that the rotational isomers do not interconvert momentarily at the molecular level. Most probably it involves a conversion of a larger local structure as a whole. The existence of such local structures of different rotamers has been found by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) [82], Coherent anti-Stokes Raman scattering (CARS) [83],... 

The pretzel-shaped molecule 96 (the first pretzelane ) was synthesized by intramolecular bridging of the two sulfonamide units of 79 with a bifunctionalized podand-like chain [54]. Again the enantioseparation of 96 was accomplished with a baseline quality separation and a large separation factor (a=5.20). Preparative separation of the enantiomers enabled the detection of the circular dichrogram of 96 (Figure 48). The optical rotation values of 79 and 96 were both determined to be [aD] = 168° (Troger base [aD] = 281°). 

Aziridines can be detected by a color reaction with 4-(4 -nitrobenzyl)pyridine that was originally developed for the detection and/or determination of a wide range of alkylating substances. 58 It can be adapted to TLC. 59 60 Upon acid hydrolysis of aziridine-2-carboxylic acid containing peptides, as required for amino acid analysis, this amino acid is recovered as serine in varying yields. 47 Configurational characterization of aziridine-2-carboxylates is performed by NMR spectroscopy 47 51 61-63 and X-ray analysis. 61 In addition, optical rotation of the related benzyl Wtritylaziridine-2-carboxylate is also used 64 with [a]D —95.5 (c 1, THF) for the 25-enantiomer 44 as reference. 

These are some examples of the use of i.r. spectra in the analysis and identification of carbohydrates in foods and natural products. Very often, these spectroscopic techniques are complementary to others, such as the study of aldobiouronic acids obtained by hydrolysis of peach-gum polysaccharides by their optical rotations and their i.r. spectra.100 However, the i.r. results appear to be sufficiently reliable to be used in the detection of traces of fructose and glucose, and to determine the d.e. (dextrose equivalent) of corn syrups, as well as the quantitative carbohydrate content in different products.101... 

Physical methods directly related to chemical composition have been used in efforts to detect adulterations of citrus products. Physical measurements include spectrophotometry (UV, fluorescence, colorimetry), gravimetric determinations (ash, specific gravity), mass spectrometry (isotope ratios), chromatography, and optical rotation. 

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