| Popis: |
The metal binding properties of the marine pyridoacridine alkaloid ascididemin was studied with a wide range of spectroscopic and spectrometric techniques. MSn electrospray ionization mass spectrometry showed that ascididemin binds Cu2+ and forms either 2 : 1 or 1 : 1 complexes depending on the concentration of the metal ion. Analysis of the MSn results showed that there is the possibility of a one electron reduction of Cu2+ to Cu+ on complexation to ascididemin. It was hypothesised that this one electron transfer was dependent on the type of salvation ligands present after ascididemin forms a complex with the metal ion. It looks as if ligands like C1-, OH- and CH3COO- do not favour the one electron transfer from ascididemin to copper and hence complexes solvated by these ligands have copper in the +2 oxidation state. Neutral ligands like H2O and CH3OH favour the one electron transfer and complexes solvated by these ligands have copper in the +1 oxidation state. Complexes between ascididemin and copper which were detected in MSn with no ligands of salvation had copper in the +1 oxidation state. UV results showed multiple isosbestic points for the gradual formation of the 2 (ascididemin) : (copper) complex when increasing concentrations of the metal ion (0-1 equivalents) were added to 2 equivalents of ascididemin. The fluorescence of ascididemin is quenched by the metal ions Cu2+, Co2+ and Ni2+ in a concentration dependent manner. However, Zn2+ does not quench the fluorescence of ascididemin which led to the suspicion that the metal does not form complexes with ascididemin. Detailed fluorescence spectroscopy studies revealed that Cu2+ is capable of competing out (binds preferentially) Co2+ and Ni2+ in binary solutions containing copper and either of the metals. In more complex mixtures simulated by a combination of all four metal ions (Cu2+, Co2+, Ni2+ and Zn2+) it seemed as if the effect of copper is not obvious at very low concentrations (10-9 -10-15 M). However, at higher concentrations (10-6 M or higher) copper is able to compete out all the three metals and the use of ascididemin as a chemosensor for copper is possible in this concentration region. Cyclic voltammetry studies showed that the formation of complexes between ascididemin and the divalent metal ions is able to decrease the amount of current formed (Ipc) at the glassy carbon electrode by reduction of Cu2+ to Cu+. This effect was attributed to either a decrease in the diffusion rates of mass transport for the complexes relative to the metal ions or a decrease in the concentration of the metal ion Cu2+ due to ascididemin complexation. |
