SYNTHESIS OF A NEW CATIONIC PYROPHEOPHORBIDE DERIVATIVE AND STUDIES OF ITS AGGREGATION PROCESS IN AQUEOUS-SOLUTION

A new water-soluble cationic pyropheophorbide vas prepared from pyroph eophorbide a by a Curtius rearrangement, The self-aggregation process of this chlorhydrate of aminopyropheophorbide was investigated from th e changes observed in its spectroscopic properties when passing from e thanol to aqueous solutions The formation of aggregates was characteri zed in tile absorption spectrum by a broadening of the Qy band, which is slightly shifted to the red, a significant increase in signals of c ircular dichroism and a drastic quenching of the fluorescence intensit y at 674 nm, Analysis of the nonlinear variation of the fluorescence i ntensity with the dye concentration in Tris-buffered solutions shows t hat dimerization is the dominant aggregation process, The dimerization equilibrium constant (at 25 degrees C, pH 7.5 in 5 mM Tris buffer, 10 mM NaCl) was determined to be 3.10(6) M-1. NMR measurements performed in DMSO at various dye concentrations showed that only the chemical s hifts of some atoms are affected by these changes, indicating the part of the ring involved in the overlap of the two molecules in the dimer . A molecular modeling study of the packing arrangement of the aminopy ropheophorbide molecules in aggregates performed by both semi-empirica l quantum chemical PM3 and force field MM2 methods provided a structur e consistent with these spectroscopic data.