Luminescent N2-Modified Guanosines: Synthesis, Self-Assembly and Metal ion Interactions
Guanosine , Fluorescence , NMR
The objective of this thesis was to synthesize N2-modified guanosines (N2G) in order to introduce fluorescent and chelating ligands, such as diphenylamino, 2,2’-dipridylamino, 2-(2’-pyridyl)benzimidazolyl and p-pyrenylphenyl functionalities. Their photophysical properties were examined in order to gain further knowledge about the effect of guanine modification on its electronic structure. The impact of N2-modification was first studied in terms of self-assembly of the luminescent guanosines in solution and gas phase. Extensive NMR and ESI MS studies provided evidence that these N2-modified guanosines self-assemble exclusively into octamers with high-fidelity, in the presence of Group 1 and Group 2 metal ions. In addition, the first example of “empty” G-octamer (free of metal cations) was identified by ESI MS. Experimental results suggested that N2-substituents provide additional electronic and steric effects which drive the diastereoselectivity of self-assembly and provide additional stability. Hydrogen bonding of N2Gs with cytidine was monitored using fluorescence and NMR. In addition to GC base pair formation, the G-quartet-to-GC base pair structural transformation was studied using CD, fluorescence, and NMR spectroscopy. Due to the luminescent and chelating nature of some of the N2G derivatives, their interactions with a number of metal ions, such as Zn2+, Cd2+, Hg2+, La3+, Tb3+ and Eu3+ ions, were probed by using various spectroscopic methods. The overall optical response in the presence of metal ions was highly dependent on the nature of N2-substituent, and it varied from “turn-on” to “turn-off” response, clearly indicating that the modification at the N2-site of guanosine can be used to finely tune the optical response of these nucleosides. Finally, synthesis of a phosphorescent N2-arylguanosine containing the Ru2+ metal center was achieved and its photophysical and electrochemical properties were examined.