Optimized Quantum Mechanics/Molecular Mechanics Strategies for Nitrile Vibrational Probes: Acetonitrile and para-Tolunitrile in Water and Tetrahydrofuran

The nitrile (CN) group is a powerful probe of structure and dynamics because its vibrational frequency is extraordinarily sensitive to the electrostatic and chemical characteristics of its local environment. For example, site-specific nitrile labels are useful indicators of protein structure because their infrared (IR) absorption spectra can clearly distinguish between solvent-exposed residues and residues buried in the hydrophobic core of a protein. In this work, three variants of the optimized quantum mechanics/molecular mechanics (OQM/MM) technique for computing CN vibrational frequencies were developed and assessed for acetonitrile in water. For the most robust variant, the transferability of the OQM/MM methodology to different solutes and solvents was evaluated by simulating the IR absorption spectra of para-tolunitrile in water and tetrahydrofuran and comparing to experiment and density functional theory (DFT) calculations. The OQM/MM frequencies compared favorably to DFT for para-tolunitrile/water, and the calculated IR absorption spectra are in qualitative agreement with experiment. This suggests that a single parametrization of the OQM/MM technique is reasonable for the calculation of nitrile line shapes when the probe is attached to different chemical moieties and when the label experiences local environments of different polarity.