Amide Hydrogen-Deuterium Exchange in Isotopically Mixed Water

Hydrogen-deuterium exchange (HDX) of protein backbone amides provides a powerful probe of conformational dynamics. However, when experiments are performed in H₂O/D₂O mixtures, quantitative interpretation is hindered by back exchange and isotope effects not captured by the classical Linderstro̷m-Lang (LL) model. We introduce a generalized Linderstro̷m-Lang (GLL) framework that explicitly accounts for forward and reverse exchange and for changes in protection upon isotopic substitution. Analytical solutions describe equilibrium enrichment (fractionation) and protection factors in mixtures, reducing to the LL model in pure D₂O. Application to HDX/NMR of the molecular chaperone DNAJB1 in 50% D₂O demonstrates that the GLL model recovers protection factors at 100% D₂O. Ignoring back exchange (i.e., using the LL model), protection factors are systematically underestimated. A particularly powerful feature of our approach is that a single HDX experiment in a mixture (e.g., 50% D₂O) simultaneously provides protection factors that report on conformational dynamics and local stability and fractionation factors that are sensitive to the local hydrogen-bonding environment.