Inhibitory effect of beta-cyclodextrin on ampicillin polymerization in aqueous solution.

Ampicillin polymerizes in aqueous solution to produce antigenic polymers; the polymers were separated by anion-exchange chromatography and shown to consist of a dimer, trimer, tetramer and pentamer of ampicillin by the masses found in fast atom bombardment mass spectroscopy. The presence of an intact beta-lactam ring in each of the polymers was revealed by the characteristic positive Cotton effect near 230 nm in their circular dichroism spectra. beta-Cyclodextrin was found to inhibit this polymerization by complex formation involving equimolar quantities of the constituents. Formation of this complex was complete in 10% aqueous solution within 6 hr at 24 degrees C. The positive Cotton effect arising from the beta-lactam ring decreased as the degree of polymerization increased, but was unchanged by the complex formation with beta-cyclodextrin.