Quartet formation of a guanine derivative with an isopropyl group: Crystal structures of "naked" G-quartets and thermodynamics of G-quartet formation

Abstract

The formation of guanine quartets with 9-isopropylguanine (iPG) is discussed in organic solvents. Crystal structures of the iPG quartets were determined by X-ray crystallography with template cations (Na + and Ca2+) and the structure without a template cation was also obtained by virtue of the stabilization by intermolecular hydrogen bonding with water molecules of crystallization. The difference in the quartet formation of iPG in the presence and absence of a template cation was clearly demonstrated by 1H NMR measurements in CDCl 3-CH3OH mixed solvents. The quartet formation is mainly governed by the enthalpy gain due to the electrostatic interaction between the O6 oxygen in iPG and the template cations in the presence of the cations rather than the intermolecular hydrogen bonding, while desolvation of iPG is the dominant factor for the formation in the absence of cations. In the presence of Na+ and Ca2+, ΔH and ΔS values in the formation of iPG-4-Na+ and iPG-4-Ca2+ complexes were determined to be ΔH = -8.4 kcal mol-1 and ΔS = +50 cal mol-1 K-1 for Na+ and ΔH = -12.9 kcal mol-1 and ΔS = +34 cal mol-1 K-1 for Ca2+ on the basis of van't Hoff plots attained from the results of temperature-dependent UV-Vis spectroscopic measurements. This journal is © The Royal Society of Chemistry 2013.