Electrostatic fields and electrostatic potentials serve as useful concepts to gain insight into protein structure and function. The objective of the proposed research is to establish an experimental route that provides direct measurements of local electrostatic fields in a protein of substantial pharmacological interest, dihydrofolate reductase (DHFR). The strategy employed here will utilize nonsense suppression and vibrational Stark effect (VSE) spectroscopy: non-natural amino acids bearing appropriate VSE probe moieties will be incorporated into DHFR using nonsense suppression techniques. The vibrational Stark effect, described as a shift in energy of a vibrational transition due to the presence of an electric field, is then utilized in order to transform the simple molecular functionality that exists within the non-natural amino acid into a small, highly-localized probe of local electrostatic fields. The resulting experimental measurements may afford insight into the influence of electrostatics interactions on protein structure and function and, also, may establish a direct link between experimental data and theoretical calculations which, in turn, may lead to improvements in theoretical methods for rational drug design.