SELF-AGGREGATION OF GUANOSINE 5'-MONOPHOSPHATE STUDIED BY DYNAMIC LIGHT-SCATTERING TECHNIQUES

The specific self-aggregation of sodium guanosine 5'-monophosphate (5'-GMP) in aqueous solution at different NaCl concentrations was studied by dynamic light scattering techniques. Depolarized dynamic light scattering (DDLS) monitors the rotational motion of the aggregate structures. Two relaxation processes could be observed as a function of temperature and concentration. The fast mode, in the range of a few hundred picoseconds, is associated with the reorientation of stacked monomers, while the slow process is determined by the motion of specific tetramer stacks, a unique aggregate structure of 5'-GMP. The rotational motion of both species is described by hydrodynamic stick boundary conditions. It is shown that the hydrodynamic theory of Tirado and Garcia de la Torre for cylinder-symmetric molecules provides good predictions for the average dimensions of the monomer and tetramer stacks, formed at different concentrations. A comparison with the Perrin equations for biaxial ellipsoids is given. The addition of sodium chloride to the mononucleotide solutions favors the formation of stacked tetramers. The translational motion of 5'-GMP was studied by photon correlation spectroscopy. In accordance with the DDLS results, the intensity autocorrelation function is characterized by one or two relaxation processes, depending on the experimental conditions (concentration, temperature). A combination of the two transport coefficients, in connection with the hydrodynamic model of Tirado and Garcia de la Torre, provides the molecular dimensions of the aggregate structures. At about 0.10 mol/L the average stack size of the tetramers is estimated to 9 units per stack and for the monomer stacks to about 4 units. In both cases the rotational and translational diffusion coefficients correspond to a cylinder-symmetric molecule of almost equal length and diameter. At higher concentration (1.0 mol/L) of 5'-GMP the axial ratio increases to about 2.35 (stacked tetramers) and 3.4 (stacked monomers), respectively.