Far Ultraviolet Electric Linear Dichroism. II. Pulsed Electric Dichroism Apparatus and Dichroic Spectra of Collagen in Aqueous Solution in the 230–187 nm Wavelength Region

A Pulsed electric linear dichroism (ELD) apparatus is described for studying the optical properties and solution conformation of biologically important macromolecules whose chromophores absorb light in the far ultraviolet region. This apparatus is capable of generating a rectangular-wave electric pulse field with a duration of 5 μs–5 ms and an amplitude of 0.2–8 kV over the 360–185 nm region. A special feature to this apparatus is its ability to record both parallel and perpendicular dichroism signals simultaneously and independently. To prove the instrumental performance, some dichroic spectra of calfskin collagen solution (pH 3.16 and concentration 0.03 g dm−3) in 0.001 mol dm−3 H3PO4 were measured in the 230–187 nm region, where the peptide chromophore shows a broad absorption spectrum. The reduced dichroism of collagen was positive but not constant throughout the wavelength region. A detailed analysis revealed that the peptide spectrum contains three overlapping absorption bands centered at 219, 200, and 189 nm. These transition moments were found to be inclined at ±(50.6–51.0°) (219 nm), ±(48.8–49.5°) (200 nm), and ±(56.0–55.8°) (189 nm) relative to the orientation axis of a triple-stranded collagen helix.