17] Spectropolarimetry of antibiotics

Abstract

Spectropolarimetry encompasses the complementary chiroptical techniques of optical rotatory dispersion (ORD) and circular dichroism (CD) and a more recent method, magnetic circular dichroism (MCD). With antibiotics, the spectra are interpreted by comparison with the spectra of model molecules chosen with great care to contain the chromophore in question in a molecule where closely analogous electronic and conformational effects are at play. When an antibiotic is absorbed to an optically active biopolymer, such as lipoprotein or DNA-RNA, the electrons in the absorbed drug find themselves in a newly asymmetric environment. This frequently leads to the generation of new optically active transitions, characteristic of the binding, allowing much useful information to be deduced about the nature of the binding. When conformational changes follow the binding, the spectral transformations may be dramatic. It is in this latter area of extrinsic spectral bands that the greatest potential of ORD-CD lies. Specific examples involving antibiotics, such as oleandomycin, spectinomycin, potassium benzylpenicillin, hetacillin and 6-Epihetacillin, tetracycline, and chloramphenicol are presented in the chapter. The spectra described are obtained using a Jasco instrument and the remarks apply specifically to data obtained in this way. The principles apply also to the Cary instrument, but one anticipates that there would be some small differences in operating procedure and strategy.