Aggregation Behavior of the Antibiotic Moenomycin A in Aqueous Solution

The aggregation behavior of the antibiotic moenomycin A (MoA) in aqueous solution under the influence of pH and ionic strength has been investigated using the fluorescent probe pyrene. Changes of the cmc between 0.03 and 0.7 mM correlate with the effective charge of MoA originating from three ionizable groups, the pKa of which have been determined by infrared spectroscopy. The dependence of the polaritysensitive ratio of pyrene monomer fluorescence (I3/I1) and the monomer-to-excimer ratio (Iex/Imo) on the MoA concentration were analyzed in terms of a polydispersity model. This takes into account statistical and thermodynamic aspects of detergent aggregation, electrostatic interactions between charged groups in spherical micelles, fluorescence emission properties of the probe, and their partition between water and micelles. The mean aggregation number calculated was found to be compatible with the value obtained independently by means of time-resolved fluorescence measurements. The micellization of the nonionic detergent C12E8 in water has been investigated in parallel as a reference. The fluorescent probe DPH has been found to be unsuitable to detect the cmc of MoA.