Heterostereocomplexes prepared from D-poly(lactide) and leuprolide. I. Characterization

Heterostereocomplexes between D-PLA and L-peptides, obtained by spontaneous precipitation from acetonitrile solution, were characterized by thermal analysis and microscopic techniques. Differential scanning calorimetry showed two transition endotherms, one for the α form that melts at 178 °C and one for the β form of PLA that melts at 169 °C. A linear correlation was found between the enthalpy of both melt temperatures and the peptide concentration. The complexation was monitored by a change in morphology, which was imaged by AFM-tapping mode. The initial fibrous network of D-PLA changed to uniform disks of 100 nm in diameter and 2.5 nm in height of the heterostereocomplex. Rhodamine B labeled leuprolide was complexed selectively to D-PLA, which was chemically bound onto mica plates. Addition Of L-PLA to the complex enabled displacement of the peptide, which was observed by fluorescent spectrometry and confocal microscopy. These results provide a method, which enables one to obtain an expression for the relative interaction strength between various stereoselective polymers and polypeptides with opposite enantiomeric configuration.