First observation of natural circular dichroism for biomolecules in soft x-ray region studied with a polarizing undulator

We measured natural circular dichroism (CD) for biomolecules in soft x-ray region for the first time. Natural linear dichroism (LD) for K(TiO)PO₄ (KTP) crystal was also measured at the K-edge energy of oxygen. Measurements of CD and LD spectra were performed at the soft x-ray undulator beamline BL23SU of SPring-8 in Japan. In this beamline, an APPLE-2 type variable undulator was installed as a light source. This undulator was operated under the phase modulation mode in which left and right circularly polarized light (LCPL and RCPL) was switched with about 0.1 Hz.

As a sample for CD measurement, we used sublimated film of L-, D- and DL-phenylalanine (Phe) and L- and D-serine (Ser) (thickness ∼300 nm) and measured CD (A_L–A_R) spectra in the nitrogen and the oxygen K-edge region, respectively. Here, A_L and A_R mean the abbreviation of absorption for LCPL and RCPL, respectively. For the Phe film, we observed a negative peak at about 407 eV for the L-Phe spectra. On the contrary, a positive peak for the D-Phe spectra and no spectral feature for the DL-Phe were observed. For the Ser film, we observed positive CD peaks at around 540 eV and 548 eV at the L-Ser spectra, on the contrary, negative peaks for the D-Ser. These results are the first CD spectra for biomolecules in the soft x-ray region.

We also used KTP crystal cut parallel to the plane (1,2,0) or (1,−2,0) as a sample of LD measurement and measured (A_L–A_R) spectra in the oxygen K-edge region and observed some peaks at about 530 eV. Since sign of these peaks were dependent on sample rotation angle in a plane perpendicular to the incident light, we concluded that these peaks are derived from linear anisotropy of crystal. Because of a weak linear polarization component of incident light, we could measure such LD spectra.