A NMR protocol is introduced that
permits accurate measurement
of minute, remote chemical shift perturbations (CSPs), caused by a
mutation-induced change in the electric field. Using protein GB3 as
a model system, <sup>1</sup>H<sup>N</sup> CSPs in K19A and K19E mutants
can be fitted to small changes in the electric field at distal sites
in the protein using the Buckingham equation, yielding an apparent
dielectric constant ε<sub>a</sub> of 8.6 ± 0.8 at 298 K.
These CSPs, and their derived ε<sub>a</sub> value, scale strongly
with temperature. For example, CSPs at 313 K are about ∼30%
smaller than those at 278 K, corresponding to an effective ε<sub>a</sub> value of about 7.3 at 278 K and 10.5 at 313 K. Molecular
dynamics simulations in explicit solvent indicate that solvent water
makes a significant contribution to ε<sub>a</sub>