Vibrational Optical Rotatory Dispersion Research Articles

Terahertz chiroptical spectroscopy of an α-helical polypeptide: a molecular dynamics simulation study.

Vibrational spectroscopy has provided incisive information on the structure of biological molecules. Here, using a molecular dynamics simulation method, infrared vibrational circular dichroism and vibrational optical rotatory dispersion spectra of a right-handed α-helix in the terahertz (THz) frequency range are calculated. Both the autocorrelation function of an electric dipole moment and the cross-correlation function of electric and magnetic dipole moments of the α-helix are calculated and Fourier-transformed to obtain THz absorption and optical activity spectra, which reveal characteristic features of the helical polypeptide structure. The anharmonicity and delocalized nature of the low-frequency modes in the THz frequency domain are taken into account to obtain statistically convergent results on the THz optical activity spectra. In addition, the magnitude of the THz vibrational optical activity signal of the α-helix is directly compared with those of typical, previously studied mid- and near-infrared chiral molecules. We anticipate that THz chiroptical spectroscopy that has not yet been demonstrated experimentally would provide highly important and complementary information on protein structure and dynamics.

In-situ vibrational optical rotatory dispersion of molecular organic crystals at high pressures

Organic structures respond to pressure with a variety of mechanisms including degradation, intramolecular transformation and intermolecular bonding. The effects of pressure on chiral organic structures are of particular interest because of the potential steric controls on the fate of pressurized molecules. Despite representing a range of opportunities, the simultaneous study of high pressures on different forms of chiral structures is poorly explored. We have combined synchrotron-source vibrational optical rotatory dispersion, micro-Fourier transform infrared spectroscopy and the use of a diamond anvil cell to simultaneously monitor the effects of pressure on the two enantiomers of the simple amino acid, alanine.

Infrared Optical Activity: Electric Field Approaches in Time Domain

Vibrational circular dichroism (VCD) spectroscopy provides detailed information about the absolute configurations of chiral molecules including biomolecules and synthetic drugs. This method is the infrared (IR) analogue of the more popular electronic CD spectroscopy that uses the ultraviolet and visible ranges of the electromagnetic spectrum. Because conventional electronic CD spectroscopy measures the difference in signal intensity, problems such as weak signal and low time-resolution can limit its utility. To overcome the difficulties associated with that approach, we have recently developed femtosecond IR optical activity (IOA) spectrometry, which directly measures the IOA free-induction-decay (FID), the impulsive chiroptical IR response that occurs over time. In this Account, we review the time-domain electric field measurement and calculation methods used to simultaneously characterize VCD and related vibrational optical rotatory dispersion (VORD) spectra. Although conventional methods measure the electric field intensity, this vibrational technique is based on a direct phase-and-amplitude measurement of the electric field of the chiroptical signal over time. This method uses a cross-polarization analyzer to carry out heterodyned spectral interferometry. The cross-polarization scheme enables us to selectively remove the achiral background signal, which is the dominant noise component present in differential intensity measurement techniques. Because we can detect the IOA FID signal in a phase-amplitude-sensitive manner, we can directly characterize the time-dependent electric dipole/magnetic dipole response function and the complex chiral susceptibility that contain information about the angular oscillations of charged particles. These parameters yield information about the VCD and VORD spectra. In parallel with such experimental developments, we have also calculated the IOA FID signal and the resulting VCD spectrum. These simulations use a quantum mechanical/molecular mechanical molecular dynamics (QM/MM MD) method and calculate the electric dipole/magnetic dipole cross-correlation function in the time domain. Although many quantum chemistry calculation approaches can only consider a limited number of geometry-optimized conformations of chiral molecules in a gas phase, this computational method includes the solute-solvent interactions and the inhomogeneous distributions of solute conformers in condensed phases. A subsequent Fourier transformation of the chiral response function produced a theoretical VCD spectrum in the entire mid-IR frequency range. Directly comparing theory and experiment, we demonstrate quantitative agreement between frequency-tunable femtosecond IOA measurements and QM/MM MD simulations of (1S)-β-pinene in CCl(4) solution. We anticipate that these direct IOA measurement and calculation methods will be applied to the studies of equilibrium chiroptical properties and structure determinations. These methods provide tools to investigate ultrafast structural dynamics of chiral systems with unprecedented time resolution.

Special issue “Advances in Chiroptical Methods”

Special issue “Advances in Chiroptical Methods”

Observation and calculation of vibrational circular birefringence: A new form of vibrational optical activity

We report the first mid-infrared observation of vibrational circular birefringence (VCB) arising from individual chiral molecules. VCB can also be called vibrational optical rotatory dispersion (VORD) and is the Kramers-Kronig transform of vibrational circular dichroism (VCD). The method of measurement involves a simple change in the optical set-up and electronic processing of a VCD spectrometer such that the VCB spectrum appears at twice the polarization modulation frequency as a pseudo vibrational linear dichroism (VLD) spectrum. VCB spectra are also calculated with density function theory (DFT) for the first time using a commercially available program for rotational strengths where the calculated intensities are convolved with the real, dispersive part of a normalized complex Lorentzian lineshape rather than the imaginary, absorptive part, normally used for IR and VCD intensity calculations. Comparison of the measured and calculated VCB, VCD, and IR spectra of (+)-R-limonene and (-)-S-alpha-pinene show close agreement and confirm the validity of the new VCB measurements.

Vibrational circular dichroism and vibrational optical rotatory dispersion in molecular crystals. I. Fundamental tones

A theory is advanced of the vibrational circular dichroism (VCD) and of the vibrational optical rotatory dispersion (VORD) in molecular crystals for the frequency region of the intramolecular vibrations. The substantial role of the symmetry of the crystal lattice is shown. The contributions of the fundamental tones of nondegenerate and degenerate vibrations in VORD and VCD are discussed. The effect of the separate molecule and of the intermolecular interaction on the gyrotropy of the crystal is estimated. The most favorable conditions for experimental examination of VCD are commented upon, as well as the information obtained from the VCD spectra.

Vibrational circular dichroism and vibrational optical rotatory dispersion in molecular crystals. II. Selection rules

The selection rules for the contribution of the electric and magnetic dipole and electric quadrupole moments of the optical active vibrations to the gyrotropy of molecular crystals are derived by group-theoretical analysis. The peculiarities of nondegenerate and degenerate vibration modes are discussed. The selection rules for k → 0 case are tabulated for all 32 crystal classes, taking into account the time-reversal symmetry and Wigner co-representation theory.