OBSERVATIONS ON THE MEASUREMENT OF VIBRATIONAL CIRCULAR-DICHROISM WITH RAPID-SCAN AND STEP-SCAN FT-IR TECHNIQUES

Extensive tests to determine an optimal method for using a Bio-Rad FTS -60A spectrometer for measurement of vibrational circular dichroism (V CD) in both step-scan and rapid-scan modes are reported. In the latter case, results of ''fast-scan'' and ''slow-scan'' experiments are also presented for comparison. In step-scan mode, phase modulation (PM) du ring the polarization-modulation measurements is found to be detriment al to the signal-to-noise ratio (S/N), but is very useful for the norm al In measurements at slow step speeds. VCD S/N is improved by doing s equential spectral collection of the polarization-modulated spectrum w ithout PM and the instrument response spectra with phase modulation. S ystematic comparative studies are used to identify optimal parameter s ets for doing the step-scan VCD measurements. These conditions use mor e rapid data collection with faster steps than previously suggested, w hich permits a more realistic evaluation of S/N and reproducibility of the spectra through comparison of spectra collected in blocks. Exampl e VCD spectra are presented for small molecules in nonaqueous solution measured in short times and for proteins in aqueous solution measured over longer times scales. Near-IR FT-VCD measured in step-scan mode a t both moderate resolution and high S/N are presented. At this stage, fast-scan, slow-scan, and step-scan techniques are shown to be virtual ly equivalent in the mid-IR, but the reduction in frequency dependence of the phase correction and the new capability for VCD measurement in the near-In favor continued development of step-scan techniques.