Spectra and structure of complexes formed by sodium fusidate and potassium helvolate with beta- and gamma-cyclodextrin.

The complexation of two steroid antibiotics of the fusidane family, sodium fusidate and potassium helvolate, by beta-CD and gamma-CD has been studied by using 1D and 2D-NMR techniques. Both guests form 1:1 complexes with gamma-CD and 1:2 (guest:cyclodextrin) complexes with beta-CD. Thus, both antibiotics behave as monotopic and ditopic guests when they are complexed by gamma-CD and beta-CD, respectively. Both steroids enter into the cavity of the gamma-CD by the side chain, reaching the central region of the steroid (rings C and D), whereas the A and B (partially) rings remain outside. For beta-CD complexes, ROESY spectra show a remarkable absence of interactions of the protons of the C and D rings, whereas clear interactions corresponding to the side chain, and A and B rings are observed. The obtained equilibrium constants (see previous paper) are discussed in terms of the structures proposed for the complexes. NMR spectra of sodium fusidate are revised, and a full assignment of the 1H and 13C NMR spectra is presented for potassium helvolate.