Vibrational Stark Spectroscopy (VSS)

Non-covalent interactions, namely hydrogen bonding, electrostatic, van der Waal's interaction etc. play a crucial role towards biomolecular functions. A key challenge is to understand the relative role of these interactions towards processes like molecular recognition, protein folding and enzyme catalysis within the heterogeneous molecular environments of the proteins and enzymes. Unfortunately, it is extremely non-trivial to define and measure these non-covalent interactions in a microscopic manner using experimental observables.

As IR probes are sensitive toward local electrostatics, VSS can be used for the quantitative measurement of the electric field on the IR probe exerted by the surrounding. IR spectroscopy probes the energy gap between the ground and the first excited vibrational state that depends on the electric field exerted by the environment. Since these states have different dipole moments for an anharmonic diatomic IR probe, they are stabilized to different extents by the electric field from the environment depending on the orientation between the dipole and the field. 

A slight change in the environment modifies the exerted electric field on the IR probe, thereby causing a shift in the IR transition energy that is linear with the difference of the dipole moment of the two vibrational states. Thus a linear correlation between IR frequency and electric field would exist in different environments of the IR probe, provided it is independent of the environment, which has been shown to be the case using VSS. As long as the vibration of the IR probe is local and decoupled from the rest of the molecule, the electric field estimated directly from the IR experiments can serve as a microscopic and quantitative descriptor of non-covalent interactions.

 Selected Publications:

           1. Correlating Nitrile IR Frequencies  to Local Electrostatics Quantifies Noncovalent Interactions of peptides and Proteins

       2. Measuring Electrostatic Fields in Both Hydrogen-Bonding and Non-Hydrogen-Bonding Environments Using Carbonyl Vibrational Probes

          3. A solvatochromic Model Calibrates Nitriles' Vibrational Frequencies to Electrostatic Field