| Weak electric fields cause minor changes to molecular infrared absorption spectra, called the vibrational Stark effect. It has been used here to study electric fields in proteins, and the physics of molecular vibrations.; Samples were immobilized in frozen solvents, with isotropic orientations, and analyzed with an FTIR and applied electric fields up to 1 MV/cm. An optical liquid nitrogen immersion cryostat was designed, making it convenient to use unsealed samples with direct electrical connections. Cryogen bubbling and schlieren near the sample is prevented with a jacket of liquid nitrogen at relatively low pressure around the sample chamber. Data collection methods include ones using a DC electric field, an AC field that is synchronized to the interferometer, and a step-scan method with lock-in amplification, of which the DC method has the lowest noise.; Stark effects were measured for the stretching vibration of CO bound to the myoglobin heme iron, yielding a tuning rate of 2.4/f cm−1 /(MV/cm), which is 4 times larger than for unbound CO (f is the local field correction factor). Spectra are reported as a function of pH, for various mutants, and for similar systems, resulting in a span of 60 cm−1 for the CO frequencies but similar Stark tuning rates, indicating that the frequency serves as a probe of the local electrostatic field. Using it this way, the matrix field in myoglobin parallel to the CO bond changes by about 8 MV/cm upon histidine protonation.; The physical origins of vibrational Stark effects were investigated with spectra for the C-N stretch mode of several small nitriles. Tuning rates range from 0.2/f to 0.7/f cm−1/(MV/cm), with aromatic compounds towards the high end and symmetric dinitriles towards the low end. Most quadratic Stark effects are small and negative, while transition polarizabilities are positive and significantly affect Stark lineshapes. Symmetric dinitrile tuning rates decrease with increasing conjugation of the connecting bridge, due to improved mechanical coupling. Perturbation models show that Stark tuning rates arise from a combination of bond anharmonicity and the effect of an electric field on bond strengths. Transition polarizability arises from altered partial charges on atoms in a field. |
