Photoinduced proton transfer in differently structured water: an EPR approach to solving a classic problem

The mechanism of proton transfer (PT), and how it is affected by water structure, is a fundamental issue in numerous chemical and biological processes. Formulated more than 200 years ago, a possible model for PT in aqueous media was proposed by Grotthuss, which continues to be actively studied and debated. In this study, we exploit electron paramagnetic resonance to investigate PT in aqueous solutions. Our proposed method employs pH-sensitive stable nitroxyl radicals and makes use of photolysis of 2-nitrobenzaldehyde to generate protons in the sub-nanosecond timescale. This approach was used to study the impact of classical chaotropic compounds on PT as studied in various aqueous solutions, i.e. 8 M urea, 6 M guanidine hydrochloride (Gdn·HCl), and potassium chloride (KCl). Our findings confirm significant impacts on PT rates. For instance, in 6 M Gdn·HCl, PT occurred 40-fold slower than in water. The method's sensitivity to water structure is demonstrated, highlighting its potential for monitoring the kinetics of PT in ice and in proteins.