Most proteins lack convenient intrinsic chromophores, and much has been learned by the incorporation of unnatural chromophores such as fluorophores into proteins. However, in some cases the incorporation of these unnatural probes into proteins may unduly influence the properties of the system to be studied. Phosphate compounds are ubiquitous in biology and could serve as intrinsic infrared probes in biological systems such as phospholipid membranes and nucleotide binding proteins. Because vibrational transitions are sensitive to the local electric field through the vibrational stark effect, it is possible to utilize infrared probes to study electric fields. Using model phosphate compounds I have measured the shift in frequency of the phosphate absorption in response to an applied electric field. This sensitivity to electric field is called the stark tuning rate. I am also using the GTPase Ras to measure the stark tuning rate of the nucleotide phosphate vibrations. Once the response of the probe has been calibrated in this way we can measure biologically relevant changes in electric field. For instance, by performing time-resolved infrared absorption measurements on Ras we can study how the electric field in the active site changes during nucleotide hydrolysis. The binding of the GTPase activating protein to Ras results in an increase in the hydrolysis rate of 5 orders of magnitude, and electric field measurements may help explain how this remarkable rate enhancement is brought about.