Supramolecular Structure and Polymorphism of Alkali Metal Salts of Guanosine 5′-Monophosphate: SEM and NMR Study

Scanning electron microscopy images of the Na+, K+, and Rb+ salts of guanosine 5'-monophosphate (5'-GMP) in the presence of the corresponding metal chloride have shown the formation of exceptionally large molecular aggregates. These are much larger than those previously reported in solution. Each cation system produced a solid with a different morphology. The SEM samples were prepared from concentrated aqueous (D2O) solutions containing various amounts of 5'-GMP and metal chloride, and they approached the limit of solubility for the 5'-GMP under these conditions. Straight or slightly curved free standing rods composed of bundles of parallel stacks of G-quartets were formed from solutions of 0.85-1.0 M Na-2(5'-GMP) containing 0.25-0.50 M NaCl. The rods had varying lengths of 6000-40 000 nm and an average diameter of 2000 nm. Calculations estimate this diameter to correspond to approximately 650 parallel stacks of G-quartets. Alignment of the individual G-quartet stacks into bundles and rods occurred as a result of phosphate charge neutralization by the high concentration of Na+ ions. The SEM image of the K+ system showed the presence of two types of morphologies, a rodlike lattice formation interpreted to be formed of stacked G-quartets, and irregular twisting fibers of varying diameter. In conjunction with the H-1 NMR data, the latter are proposed to be composed of continuous helices of doubly hydrogen-bonded guanines having the same H-bonding motif as the planar G-quartets. The Rb+ system had some similarities to both the Na+ system and the K+ system. H-1 NMR. spectra were different for each cation system, corresponding to the differences observed by SEM imaging of the solids. Polymorphism has been observed in telomeric sequences but has not been extensively explored in 5'-GMP.