Protein fluctuations are sensed by stimulated infrared echoes of the vibrations of carbon monoxide and azide probes

The correlation functions of the fluctuations of vibrational frequencies of azide ions and carbon monoxide in proteins are determined directly from st imulated photon echoes generated with femtosecond infrared pulses. The asym metric stretching vibration of azide bound to carbonic anhydrase II exhibit s a pronounced evolution of its vibrational frequency distribution on the t ime scale of a few picoseconds, which is attributed to modifications of the ligand structure through interactions with the nearby Thr-199. When azide is bound in hemoglobin, a more complex evolution of the protein structure i s required to interchange the different ligand configurations, as evidenced by the much slower relaxation of the frequency distribution in this case. The time evolution of the distribution of frequencies of carbon monoxide bo und in hemoglobin occurs on the approximate to 10-ps time scale and is very nonexponential. The correlation functions of the frequency fluctuations de termine the evolution of the protein structure local to the probe and the e xtent to which the probe can navigate those parts of the energy landscape w here the structural configurations are able to modify the local potential e nergy function of the probe.