AGGREGATION BEHAVIOR OF THE ANTIBIOTIC MOENOMYCIN-A IN AQUEOUS-SOLUTION

The aggregation behavior of the antibiotic moenomycin A (MoA) in aqueo us solution under the influence of pH and ionic strength has been inve stigated using the fluorescent probe pyrene. Changes of the cmc betwee n 0.03 and 0.7 mM correlate with the effective charge of MoA originati ng from three ionizable groups, the pK(a) of which have been determine d by infrared spectroscopy. The dependence of the polarity sensitive r atio of pyrene monomer fluorescence (I-3/I-1) and the monomer-to-excim er ratio (I-ex/I-mo) on the MoA concentration were analyzed in terms o f a polydispersity model. This takes into account statistical and ther modynamic aspects of detergent aggregation, electrostatic interactions between charged groups in spherical micelles, fluorescence emission p roperties of the probe, and their partition between water and micelles . The mean aggregation number calculated was found to be compatible wi th the value obtained independently by means of time-resolved fluoresc ence measurements. The micellization of the nonionic detergent C12E8 i n water has been investigated in parallel as a reference. The fluoresc ent probe DPH has been found to be unsuitable to detect the cmc of MoA .