pH dependence of soft X-ray absorption spectra of nucleotides in aqueous solution

Shimada, Hiroyuki*; Minami, Hirotake*; Sakuma, Ichiro*; Okuizumi, Naoto*; Ukai, Masatoshi*; Fujii, Kentaro; Yokoya, Akinari; Fukuda, Yoshihiro*; Saito, Yuji  

In order to investigate the role of core-excited states induced by X-ray absorption of DNA under biological environments, X-ray absorption near edge structure (XANES) at N K-edge region were measured for various nucleotides in aqueous solutions. The experiments were performed at BL-23SU in SPring-8 using a liquid microjet apparatus. pH of the sample solution was controlled by addition of NaOH. Total yield of electrons ejected from the sample solution by irradiation was measured as a function of X-ray energies. Under neutral conditions, the XANES spectrum for GMP shows two peaks (A) at lower energy and (B) at higher energy with similar intensities in the nitrogen pre-K edge region. In contrast, under basic condition, the intensity of peak (A) increases, while that of (B) decreases. The pH dependence was also observed for other nucleotides. These results show that specific sites of N atoms in the nucleobase moieties could be (de)protonated depending on the pH conditions.