Optimized quantum mechanics/molecular mechanics strategies for nitrile vibrational probes: acetonitrile and para-tolunitrile in water and tetrahydrofuran.
The nitrile (Ctriple bondN) 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 Ctriple bondN 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.
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA.
Beth A
BA
Lindquist
Ryan T
RT
Haws
Steven A
SA
Corcelli