Gu-Sheng Yu

Isolation of preresonance and out-of-phase dual circular polarization Raman optical activity

Abstract We report the first isolation of preresonance Raman scattering behavior in Raman optical activity (ROA) and first measurement of out-of-phase dual circular polarization (DCPII) ROA. Using a series of molecules of increasing delocalized electronic structure, we observe a correlated increase in the degree to which the DCPI and ICPu ROA backscattering spectra differ from one another. Through this comparison, ROA is shown to be a sensitive way of detecting the presence of presonance Raman scattering intensity using a single Raman excitation wavelength.

Comparison of IR and Raman forms of vibrational optical activity

Natural vibrational optical activity consists of two principal forms. The IR form is known as vibrational circular dichroism (VCD) and is simply the extension of electronic circular dichroism into the IR vibrational region of the spectrum. The Raman form, known as Raman optical activity (ROA), is a new form of optical activity that has no counterpart in the classical forms of optical activity. In this paper, the similarities and differences of the IR and Raman forms of vibrational optical activity will be examined. Although both VCD and ROA were discovered and confirmed in the period from 1973 to 1975, each field has evolved independently with key advances in theoretical description, instrumentation and application coming at different times over the past 20 years. The current relative strengths and weaknesses of VCD and ROA will be discussed, and specific examples of VOA spectra of (-)-alpha-pinene and the amino acid L-alanine, for which overlapping VCD and ROA data are available, will be presented.

Raman optical activity of biological molecules

Abstract Recent progress in the use of Raman optical activity (ROA) to study the stereochemical properties of biological molecules is described. The basic experimental, theoretical, instrumental and calculational methodology of backscattering ROA is presented together with examples of applications to amino acids and peptides in aqueous solution.

Backscattering dual circular polarization raman optical activity in ephedrine molecules

Abstract We report the first measurements of Raman optical activity (ROA) in ephedrine molecules. In-phase dual circular polarization (DCP I ) ROA spectra have been recorded for (1S,2R)-ephedrine, (1S,2R)-norephedrine, (1S,2S)-pseudoephedrine and (1S,2S)-norpseudoephedrine as hydrochloride salts in H 2 O solution. The spectra are interpreted in relation to the small changes in molecular structure among these four molecular species.

Quantitative Comparison of Experimental Infrared and Raman Optical Activity Spectra

Infrared vibrational circular dichroism (VCD) and vibrational Raman optical activity (ROA) have been measured and compared quantitatively over the frequency range from 835 to 1345 cm−1 for trans-pinane, cis-pinane, α-pinene, and β-pinene. For these molecules in this region of spectral overlap between VCD and ROA, the average ratio of VCD or ROA to its parent vibrational intensity favors ROA by a factor of two to three. Several vibrational modes in each molecule yield both large VCD and large ROA, while several other modes show little propensity toward significant VCD or ROA intensity. Beyond this general property of a few strongly chiral and strongly achiral vibrational modes, little additional correlation between VCD and ROA intensity is found. This quantitative compilation of VCD, infrared, ROA, and Raman intensities provides an experimental basis for computational intensity studies of VCD, ROA, and their theoretical comparison.

Chapter 3 Instrumental methods of infrared and Raman vibrational optical activity

Abstract We present the basic concepts and methods for the measurement of infrared and Raman vibrational optical activity (VOA). These two forms of VOA are referred to as infrared vibrational circular dichroism (VCD) and Raman optical activity (ROA), respectively The principal aim of the article is to provide detailed descriptions of the instrumentation and measurement methods associated with VCD and ROA in general, and Fourier transform VCD and multichannel CCD ROA, in particular. Although VCD and ROA are closely related spectroscopic techniques, the instrumentation and measurement techniques differ markedly. These two forms of VOA will be compared and the reasons behinds their differences, now and in the future, will be explored.

New experimental methods and theory of Raman optical activity

Recent advances have led to dramatic improvements in the measurement of Raman optical activity (ROA). These include the use of new scattering geometries and improvements in polarization modulation methodology and multichannel-detector technology. Several new ROA experimental configurations are selected to illustrate the capabilities of a new ROA instrument constructed recently at Syracuse University. Both general and simplified theoretical expressions are used to identify fundamental aspects of various ROA experiments and to determine preferred configurations for the measurement of ROA. The authors find for their instrument that the optimum configuration is in-phase dual circular polarization (DCPI) ROA in backscattering geometry, as illustrated using (+)-fenchone. DCPI- ROA backscattering spectra are presented for aqueous solutions of L-alanine, glycyl-L-alanine and D-mannose. The large couplet arising from the bands between 800 and 900 cm-1 in the ROA spectrum of alanine is interpreted in terms of coupled methyl rocking and the methyl carbon-carbon stretching internal coordinates. The feasibility of applying ROA to the study of biological molecules is demonstrated by the high quality of the ROA spectra obtained for these molecules.

COMPARISON OF FOURIER-TRANSFORM VIBRATIONAL CIRCULAR DICHROISM AND MULTICHANNEL-DETECTED RAMAN OPTICAL ACTIVITY

Infrared, vibrational circular dichroism, dual circular polarization Raman optical activity and Raman spectra have been used in conjunction for investigation of the vibrational modes of trans- and cis-pinane, and α- and β-pinene with a view of evaluating the usefulness of these techniques. It has been concluded that vibrational circular dichroism and Raman optical activity provide highly complementary and non-redundant information, of comparable spectral quality.

Raman Optical Activity of Small Peptides and Terpene Molecules

Raman optical activity (ROA), the circular intensity difference between right and left circularly polarized incident and/or Raman scattered radiation, provides a unique conformational probe of biological molecules in aqueous solution.1,2 In the classical measurement of ROA, the polarization of the incident laser radiation is modulated between the two circular polarization states, keeping the scattered linear polarization fixed, resulting in ICP-ROA. More recent methods developed in our laboratory include modulation only of the Raman scattered radiation, denoted SCP-ROA, or dual circular polarization modulation of both the incident laser and the Raman scattered radiation, either in- or out-of-phase, denoted DCPI- and DCPII-ROA, respectively. The depolarized backscattering DCPI-ROA measurement has been found to be particularly advantageous for biological samples. One emphasis of our recent research is to explore the sensitivity of ROA to locallized vs. longer range vibrational modes and to local vs. long-range stereochemistry. In addition, comparison of results from the various forms of measurement of ROA provides information on the relative importance of magnetic dipole vs. electric quadrupole mechanisms for ROA intensity. Two classes of molecules have been surveyed. Studies of simple peptides provide the opportunity to investigate the degree of localization of ROA features on specific residues and the effects of vibrational coupling among residues.3