NMR studies of proton exchange kinetics in aqueous formaldehyde solutions

Aqueous solutions of formaldehyde, formalin, are commonly used for tissue fixation and preservation. Treatment with formalin is known to shorten the tissue transverse relaxation time T₂. Part of this shortening is due to the effect of formalin on the water T₂. In the present work we show that the shortening of water T₂ is a result of proton exchange between water and the major constituent of aqueous solutions of formaldehyde, methylene glycol. We report the observation of the signal of the hydroxyl protons of methylene glycol at 2 ppm to high frequency of the water signal that can be seen at low temperatures and at pH range of 6.0 ± 1.5 and, at conditions where it cannot be observed by the single pulse experiment, it can be detected indirectly through the water signal by the chemical exchange saturation transfer (CEST) experiment. The above finding made it possible to obtain the exchange rate between the hydroxyl protons of the methylene glycol and water in aqueous formaldehyde solutions, either using the dispersion of the spin-lattice relaxation rate in the rotating frame (1/T_{1 ρ)} or, at the slow exchange regime, from the line width hydroxyl protons of methylene glycol. The exchange rate was ∼10 ⁴ s^-1 at pH 7.4 and 37 C, the activation energy, 50.2 kJ/mol and its pH dependence at 1.1 C was fitted to: k (s^-1) = 520 + 6.5 × 10⁷[H⁺] + 3.0 × 10⁹[OH ^-].