26 chromatography • spectroscopy
Raman optical activity offers molecular structural information
Honggang Li and Laurence A Nafie look at how new developments in instrumentation, raman optical activity has been applied to the absolute configuration of small chiral molecules.
R
aman optical activity (ROA) is, like vibrational circular dichroism (VCD),
a spectroscopic method that probes the chirality, or handedness, of molecular vibrations. ROA offers a novel alternative to X-ray crystallography, permitting absolute configuration determination for a neat liquid, oil or solution sample. ROA requires no derivatisation of the sample or growth of a pure single crystal.
Tanks to new developments in instrumentation, ROA has been applied to the absolute configuration of small chiral molecules when combining the application of ab
Right Eight traces of Raman spectra of neat S-pinene acquired simultaneously froma single experiment. They are highly polarised (top spectrum, DCPII deploarised (middle spectrum, (DCPI
), ),
middle panel, and a degree or circularity spectum in the lower panel.
and unpolarised (lower spectrum, SCP or ICP) Raman in the top panel, and SCP, DCPI
, ICP, DCPII ROA as labelled in the
initio methods to the analysis of experimental ROA spectra1
. Te
calculations can also be carried out in commercial packages such as Gaussian09 (Gaussian Inc).
In addition, ROA holds great promise for the determination of the three-dimensional structure and conformational distribution of chiral molecules in unprecedented detail. ROA combines the structural specificity of Raman spectroscopy with the stereochemical sensitivity of chiroptical spectroscopy such as circular dichroism (CD).
Prof Laurence Barron recently stated in a review that: “Te many
structure-sensitive bands in the ROA spectra of aqueous solutions of biomolecules provide detailed structural information, in the case of proteins, not only secondary structure elements such as helix and sheet but also the tertiary fold. ROA studies of unfolded and partially folded proteins are providing new insight into the residual structure in denatured proteins and the aberrant behaviour of proteins responsible for misfolding diseases. It is even possible to measure the ROA spectra of intact viruses, from which information about the folds of the major coat proteins and the structure of the nucleic acid core may be obtained2
.”
ROA spectra are typically measured with 532 nm laser excitation, although 780 nm laser excitation is also commercially available, with a spectral window of 200 cm-1 to 2500 cm-1. Te advances in the measurement of ROA over the past three decades benefit from the progress in general Raman instrumentation: multi-channel detection with charge coupled devices (CCD), holographic gratings and the advent of frequency doubled solid-state lasers. Additionally, ROA has experienced rapid development in recent years due to the implementation of an artifact reducing scheme based on the concept of the virtual enantiomer by Prof Werner Hug in 19993
, that
overcomes the ubiquitous offset problem in an ROA spectrometer.
Most recently, we have demonstrated the simultaneously acquisition of all four forms of ROA, named incident circular polarisation (ICP), scattered circular polarisation (SCP), in-phase dual circular polarisation (DCPI
) and out-of-phase dual
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