Structural elucidation of 4'-epiadriamycin by nuclear magnetic resonance spectroscopy and comparison with adriamycin and daunomycin using quantum mechanical and restrained molecular dynamics approach.

The structural and electronic properties of 4'-epiadriamycin, adriamycin, and daunomycin have been studied using density functional theory (DFT) employing B3LYP exchange correlation. The chemical shifts of (1)H and (13)C resonances in nuclear magnetic resonance spectra have been calculated using Gauge-Invariant Atomic Orbital (GIAO) method as implemented in Gaussian 98 and compared with experimental spectra recorded at 500 MHz. (13)C resonances of drugs have been assigned for the first time. A restrained molecular dynamics approach was used to get the optimized solution structure of drugs using inter-proton distance constraints obtained from 2D NOESY spectra. The glycosidic angle C7-O7-C1'-C2' is found to show considerable flexibility by adopting 156 degrees -161 degrees (I), 142 degrees -143 degrees (II), and 38 degrees -78 degrees (III) conformations, of which the biological relevant structure appears to be the conformer II. The observed different conformations of the three drugs are correlated to the differential anticancer activity and the available biochemical evidence exhibited by these drugs.