Effects of solution polarity and viscosity on peptide deamidation

Deamidation kinetics were measured for a model hexapeptide (L-Val-L-Tyr-L-P ro-L-ASn-Gly-L-Ala, 0.02 mg/mL) in aqueous solutions containing glycerol (0 -50% w/w) and poly(vinyl pyrrolidone) (PVP, 0-20% w/w) at 37 degreesC and p H 10 to determine the effects of solution polarity and viscosity on reactiv ity. The observed pseudo-first order deamidation rate constants, k(obs), de creased markedly when the viscosity increased from 0.7 to 13 cp, but showed no significant change at viscosities >13 cp. Values of k(obs) also increas ed with increasing dielectric constant and decreasing refractive index. Mol ecular dynamics simulations indicated that the free energy associated with Asn side-chain motion is insensitive to changes in dielectric constant, sug gesting that the observed dielectric constant dependence is instead related primarily to the height of the transition state energy barrier. An empiric al model was proposed to describe the effects of the viscosity, refractive index and dielectric constant on kobs Analysis of the regression coefficien ts suggested that both permanent and induced dipoles of the medium affect t he deamidation tate constant, but that solution viscosity is relatively uni mportant in the range studied.