Dynamic of water molecules in polyethylene glycol solutions. Study of the longitudinal relaxation time (T1) in 2D and 1H NMR

E.M. CLOP; M.A. PERILLO ; A. K. CHATTAH

Alta Gracia

Congreso; Magnetic Resonance in a Cordubensis Perspective; 2011

Famaf

Liquid water is a network of molecules microscopically connected by hydrogen bonds, in constant topological reshaping [1]. In solution, ions and hydrophilic residues can immobilize water molecules. This interaction can describe the water in solutions as a set of dynamic subsystems defined by the pattern of water adsorption energies to the different binding sites [2]. Measurements of relaxation times T1 and T2 in magnetic resonance, provides information about the time evolution of water distributions, and help to distinguish between the free liquid fractions and structured water in the presence of solutes. In this work, the dynamics of H2O in solutions of polyethylene glycol (PEG) of molecular weight of 6000 Da, is studied by analyzing the proton (1H) or deuterium (2D) spin-lattice relaxation times (T1). The PEG concentrations ([PEG]) ranged between 0 and 70 % W/V, from dilute to highly concentrated systems, simulating the conditions of molecular crowding characteristics of cell cytoplasm. The temperature was set to 37° C in all the experiments, in order to extend the analysis to real physiological systems. First, we measured 1H in a 20 MHz spectrometer (Minispec, Bruker), observing the free induction decay (FID) and conducting inversion-recovery (IR) experiments to measure T1. The FID showed no bimodal decay for any of the tested concentrations of PEG, in contrast to the observed decay in heterogeneous systems [3]. On the other hand, a bi-exponential equation could be fitted to data obtained for IR curves. A long (@3s) and a short (<0.4 ms) time could be distinguished. The long time decreased with [PEG] increment, and the proportion of protons with this T1 is coincident with the percent of water protons. Subsequently, we measured 1H T1 in a Bruker 400 MHz in PEG´s solutions with deuterated water containing residual H2O. The signals belonging to PEG or H2O (present in low proportion), showed well resolved resonances. Both peaks were integrated separately and the data were best fitted to a bi-exponential equation in each case. The IR curves showed the presence of two components both in water and in PEG. In water the mean component, showed a long T1 (@ 20 s) that decreased with [PEG], and a second component with a short T1 (0.3 to 0.1 s), in small but measurable proportions. PEG showed a majority component with short T1 (0.3-0.05 s) and a second component with a longer one (0.8-1.45 s).