Organic-soluble cytosine derivatives for studying base-pair interactions in nonaqueous solution

The presence of higher order G-quadruplex-DNA (G4) structures in human cells in guanine-rich DNA sequences has been well established. In cases of G4 helicase impairment or G4 stabilization via association with G4 ligands, it has been determined that arrest and/or termination of DNA transcription/replication occurs because these structures act as physical barriers to both helicase and polymerase mobility along the DNA. Truncation of transcription or replication may result in the loss of genetic material expression leading to disorders. To ameliorate G4-induced barriers, agents that destabilize or "unwind" the quadraplex structures are sought. Earlier work has shown that the cytosine analogue phenylpyrrolocytosine has potential G4-destabilizing ability. It was hypothesized that the unwinding activity was related to its ability to base pair with guanosine. To facilitate the design and evaluation of the hydrogen bonding ability of cytosine analogues, solubility in aprotic, and lowpolarity solvents, e.g., deutero-chloroform (CDCl3), is needed; then, determination of association constants (Ka) with guanine via 1H nuclear magnetic resonance (NMR) spectroscopy or isothermal titration calorimetry (ITC) will be possible. Herein, we report on the synthesis of three new chloroform-soluble, cytosine analogues: N1-cyclohexylmethyl-7-phenylimidazolocytosine, N1-cyclohexylmethyl-7-(6-(methoxycarbonyl)pyridin-2-yl)imidazolocytosine, and N1-cyclohexylmethyl-7-(6-carbamoylpyridin-2- yl)imidazolocytosine. We further report the binding interactions of each of these analogues with the guanosine derivative 2',3',5'-O-tris(tert-butyldimethylsilyl)guanosine through the use of both ITC and 1H NMR titration experiments in chloroform and compare the binding to persilyated ribo-cytidine.