Conformational Studies of Biopolymers, Peptides, Proteins, and Nucleic Acids. A Role for Vibrational Circular Dichroism

Mini review: Instrumentation for vibrational circular dichroism spectroscopy, still a role for dispersive instruments

“…Ratioing Equation by Equation , for wavelengths ( λ ) having α B = (n + 1)π/2 (ie, where sin α B = 1 and cos α B = 0), and then dividing by 1.15, yields Δ A . Alternatively magnitudes can be determined by comparison with the VCD of a standard sample to provide a (quicker) secondary calibration . After averaging sequential scans, subtraction of baseline scans obtained under the same conditions and with the same sample cell (or equivalent) completes processing.…”

“…Fourier transform infrared‐based VCD instruments are best used where the desired data must cover a wide spectral range, eg, for absolute stereochemistry studies of small molecules in organic solvents. However, for measurement of a single broad band (eg, the amide I′ for proteins in D 2 O), dispersive instruments still have an advantage . In restricted spectral regions, the dispersive spectrometer can have very flat baselines, as shown by the raw VCD (single 15 min scans, uncorrected for baseline or by smoothing or averaging) presented in Figure for a poly‐L‐lysine (PLK) sample as compared to the D 2 O baseline spectra (obtained with a separate cell).…”

“…Evaluation of such data required comparison of “noise traces,” which are measures of the difference of successive blocks of scans, while the spectra are their sums . One must also take into account the difference in sampling with each technique . The dispersive instrument oversamples the signal, recording the signal many times in a resolution element, that results in spectrally narrow baseline fluctuations, which can be smoothed with no degradation of the real spectral resolution .…”

“…We obtain useful dispersive VCD baselines by using just solvent in the same or an identical sample cell, and then subtract it from the sample VCD. Part of our success is because of the use of cells consisting of two thin CaF 2 windows separated with a spacer that are clamped in a (cushioned) metal holder and can be used for temperature variation studies as well . In our experience, these thin windows tend to have less birefringence and short aqueous (low index) sample path lengths lead to less baseline problems.…”

Reaction Monitoring Using Mid‐Infrared Laser‐Based Vibrational Circular Dichroism

“…It is, in combination with quantum chemical calculations, widely used for the determination of the absolute configuration . VCD is also very sensitive towards conformational equilibria, and secondary structure, thereby complementing ECD. These properties render the application for monitoring of time‐dependent processes highly desirable.…”