Title: Identification of Redox State Based on the Difference in Solvation Dynamics.

Oxidation-reduction (Redox) reactions are crucial for many biological processes, yet there is no method available to evaluate redox states in a non-invasive, continuous manner. Here we introduce a novel approach to distinguish between reduced and oxidized states of glutathione (GSH and GSSG, respectively) using aquaphotomics near-infrared (NIR) spectroscopy and multivariate analysis. We identified clear differences in NIR spectra reflecting not only glutathione itself, but different redox states of glutathione based on the spectral features of water molecular conformations interacting with the reaction site. Molecular dynamic simulations also revealed the difference in water molecule coordination and hydration numbers around the reaction site. This approach not only sheds light on the significance of water molecules in redox reactions but also enables non-destructive, continuous assessment of redox states, with potential applications for bioreactor optimization.