A Spectroscopic Approach to Unravel the Local Conformations of a G-Quadruplex Using CD-Active Fluorescent Base Analogues.

The formation of a stable G-quadruplex (GQ) can inhibit the increased telomerase activity that is common in most cancers. The global structure and the thermal stability of the GQs are usually evaluated by spectroscopic methods and thermal denaturation properties. However, most biochemical processes involving GQs might require local conformational changes at the guanine tetrad (G4) level. These local conformational changes of individual G4 layers during protein and drug interactions have not yet been explored in detail. In this study, we monitored the local conformations of individual G4 layers in GQs using 6-methylisoxanthopterine (6MI) chromophores, which are circular dichroism (CD)-active fluorescent base analogues of guanine, as local conformational probes. A synthetic, tetramolecular, parallel GQ with site-specifically positioned 6MI monomers or dimers was used as the experimental construct. Analytical ultracentrifugation studies and gel electrophoretic studies showed that properly positioned 6MI monomers and dimers could form stable GQs with CD-active fluorescent G4 layers. The local conformation of individual fluorescent G4 layers in the GQ structure was then tracked by monitoring the absorbance, fluorescence intensity, thermal melting, fluorescence quenching, and CD changes of the incorporated probes. Overall, these studies showed that site-specifically incorporated fluorescent base analogues could be used as probes to monitor the local conformational changes of individual G4 layers of a GQ structure. This method can be applied to explore the details of small molecule-GQ interaction at the level of the individual G4 layers, which may prove to be useful in designing drugs to treat GQ-related genetic disorders, cancer, and aging.