An instrument employing several novel features has been assembled for the measurement of magnetic and natural circular dichroism (MCD and CD) in the visible, ultraviolet and near infrared spectral regions. The instrument is fitted with two magnets: an electromagnet for routine use and a superconducting magnet for use when higher performance is required. The contributions of CD and MCD are separated by measuring the net spectrum twice; first with the magnetic field parallel to the Poynting vector of the optical beam and then with it antiparallel. The value of the net optical activity for each wavelength increment is converted to digital form and the manipulations required to separate CD and MCD and signal averaging are performed by a computer. The data can be plotted in a variety of formats on a Cal Comp plotter. The instrument is equipped with a Cary model 14 prism-grating monochromator which, unlike double quartz prism monochromators, has good wavelength dispersion from the uv to the near ir spectral regions. Performance in the ultraviolet has been considerably improved by replacing the previously used electro-optical modulator (Pockels cell) with a quartz strain-birefringence modulator. The capabilities of this instrument are illustrated by spectra for the natural CD of ribulose-1,5- diphosphate carboxylase at 22° from 195 to 245 nm, the MCD of metmyoglobin cyanide at -196° and 22° from 340 to 470 nm, and the MCD of oxyhemoglobin at 4° from 700 to 1100 nm.
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