The amine-imine creatinine tautomerism observed in both the solution and gas phases

• The biological small creatinine molecule was studied in the solution and gas phases. • Determination of size parameters for creatinine species by light transport in NIR. • In the creatinine amine-imine equilibrium, the imine form led to enolization. Creatinine is mainly eliminated from the bloodstream by the kidneys and it is a product of muscle metabolism. The level of serum creatinine is the most commonly used and widely accepted measure of kidney function in clinical medicine. Herein, we aim to understand the creatinine amine-imine equilibrium and the species populating its solutions by means of spectroscopic and spectrometric techniques. Results from both solution and gas phases were complementary regarding the displacement of the creatinine amine-imine equilibrium toward the imine form at compound concentrations above 5 × 10 −5 M. We found that the enolization of the creatinine imine form is favored in a polar protic solvent (water) and by increasing the creatinine concentration. We also introduced, for the first time, the old concept of light transport in the near-infrared for the creatinine in solution and determined the creatinine particle size parameters to obtain information regarding the developed creatinine amine-imine equilibrium.