Chiroptical analytical methods

Carcinogenicity Testing Past, Present, and Future / 431 Chiroptical Analytical Methods / 445... 

CD detection for colored derivatives might just as easily be used for structural information about the carbohydrates as well as for their analysis, but so far little attention has been given to either application. Cyclic oligomers of p-D-glucose have important supporting roles to play in analytical applications that are discussed in a later section. The union of chirality in the carbohydrate moiety of a glycoside metabolite with the unsaturation in the base in such compounds as nucleosides and nucleotides, saponins and flavones, etc., is another area that will ultimately be developed for applications of chiroptical detection methods. New and imaginative ideas are needed for the analysis of carbohydrates, and CD should be one of the favored methods. 

There are many other useful analytical methods. Chromatographic methods such as gas chromatography (GC) and high-performance liquid chromatography (HPLC) are used daily for identification and estimation of the purity of a synthetic product. Chiroptical methods, such as circular dichroism (CD) spectroscopy, are also important especially in studying the relationships between absolute configuration and bioactivity of biofunctional molecules. In later chapters I will give some examples of application of CD spectroscopy in enantioselective synthesis. 

For purposes of discussion, we divide applications of CPL spectroscopy into three categories (1) efforts to develop reliable CPL "sector rules", (2) use of comparative CD and CPL studies to probe excited state geometry changes, and (3) the specific use of the selectivity and sensitivity of CPL to probe details of molecular and electronic structure, and dynamics. Since in this book we are primarily concerned with "analytical" applications of these chiroptical methods, we will emphasize here the last of these categories. 

Chiroptical methods are widely used in the elucidation of the structure of steroid molecules [1-9]. Quantitative determination and control of the optical purity of drugs can also be solved by chiroptical methods, but they are rarely applied in pharmaceutical analysis. In pharmacopoeias, the only experimental measurements that are reported are optical rotations at the sodium D-line. These figures provide no specificity and relatively low analytical sensitivities. Nevertheless several papers have appeared in the literature where steroid determinations are based on the measurement of optical rotation [1,10-17]. The purpose of this chapter is to describe the applications of circular dichroism (CD) spectral measurements to the analysis of steroid molecules with particular emphasis to pharmaceutical preparations and drug forms. 

The methods above are based on the direct measurement of the CD curves of the test compounds at one or more wavelengths. Now several difference CD (ACD) methods are described that are based upon the changes brought about in the CD curves by either physical or chemical effects. Difference chiroptical methods are generally introduced for the purpose of increasing analytical selectivity [25]. 

The ubiquity of the aromatic ring and carbonyl chro-mophores in the molecular structure of natural products means that the number of potential analyses is enormously large. Djerrasi pointed out the analytical potential of chiroptical methods as long ago as 1960, but even now, the number of investigations is small, which is explained in part by the enormity of the field of separation sciences. Most analysts would argue that the obsession with separation is because problems with interferences are minimized. On the other hand, for many of these processes, their potential was illustrated using carefully chosen synthetic laboratory mixtures, most of which were so simple they did not even begin to address the complexities that are encountered in the analysis of real samples. 

Gergely, A. A review of the application of chiroptical methods to analytical chemistry. J. Pharm. Biomed. Anal. 1989, 7, 523-541. 

This property is absent in the parent non-chiral spectroscopies. Chiroptical methods sometimes provide enhanced resolution, because of the simple fact that di-chroic bands can be positive and negative. Chiral spectroscopies give also a new dimension to the intensity parameter. The information about structure is also encoded in the sign, the absolute value and the width of spectral bands. Not only the positions of bands, but also the entire shape of the spectral pattern carries structural information on the sample. While parent spectroscopies are more oriented toward the positions of the spectral bands, chiroptical spectroscopies are primarily intensity oriented, although band positions are just as important as in the parent methods. Chiroptical spectroscopies can draw on substantial knowledge on electronic and vibrational molecular transitions that has been collected throughout the years of analytical use of the parent spectroscopies. 

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