Allowance for thermodynamic nonideality in the characterization of protein interactions by spectral techniques

► Molal concentrations should be used to characterize reactions by spectral methods. ► Rigorous expressions are adapted to accommodate experimental use of the molar scale. ► Results for the effect of sucrose on α-chymotrypsin isomerization are reinterpreted. ► Crowding effects on catalase−superoxide dismutase interactions are reexamined. ► Future spectral studies should adopt the molal concentration scale from the outset.