Radiolytic Degradation of Cystathionine Irradiated with Monochromatic Soft X-rays at the K-shell Resonance Absorption of Sulfur.

The degradation of cystathionine was investigated using irradiation with monochromatized synchrotron X-rays at 2472 eV and 2466 eV. 2472 eV corresponds to the energy of the K-shell resonance absorption peak of the sulfur atom. The energy at 2466 eV was adopted as a reference. Cystathionine has a sulfur atom which joins two amino acid residues. This form is useful for analyzing the effect of degradation that originates from photoexcitation in the sulfur atom. The degradation products of cystathionine were detected by high performance liquid chromatography (HPLC). Of the products present, alpha-aminobutyric acid was produced threefold on irradiation with 2472 eV X-rays, as compared with the value for irradiation at 2466 eV and the same exposure. Almost the same amount of glycine was produced at the two irradiation energies. The yields of these two products were analyzed on the basis of the amount of cystathionine degraded and the number of photons absorbed by sulfur and the other elements in cystathionine. Cleavage of a bond was shown to depend on whether the bond includes an atom that absorbs the X-ray photon.