Ultraviolet, Rotatory, Infrared Spectra

Physical Properties of the Ansamycins A. Ultraviolet, Rotatory, Infrared Spectra 

With the exception of the maytansinoids (which are colorless) 61 to 63, 150), the ansamycins are all yellow to orange crystalline compounds, soluble in polar organic solvents and insoluble in water and non-polar organic solvents 2, 22, 36, 54, 122, 123, 127, 128, 130, 158, 159). The streptovaricins tend to form firmly bound solvates and this tendency gave some difficulty in obtaining accurate microanalyses 108). 

The electronic spectra of the ansamycins are complex, but most show high extinction coefficients ( 25,000) at wave lengths below 230 nm, with maxima at longer wave lengths for the naphthoquinonoid antibiotics than for the benzoquinonoid compounds, as would be expected. Ultraviolet spectra of streptovaricin complex 84), rifamycins B 123), S 122) and SV (722), and geldanamycin (22) are shown in Fig. 28. That of maytansine 62) has not been published, but it is reported to have maxima at 223 (c 29,800), 243 (27,100 sh), 254 (27,200), 282 (5,690), and 290 nm (5,520). It is of some interest that the spectra of the streptovaricins resemble that of rifamycin S but not that of rifamycin B this hampered early recognition of the antibiotics similarity. 

Specific rotations of the natural streptovaricins are all strongly positive at the sodium D line, with +400° to +700° 47, 103), and this has 

Specific rotations of the other ansamycins are less exceptional (Table 1). Rifamycin B has a small negative rotation (—11°) 123) while rifamycin Y is, surprisingly, strongly positive ( + 325°) 68). Other rifamycins, O (727), S 128) and SV 128), have varying rotations, with only rifamycin S 

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