Chemical shifts in X-ray photoelectron spectroscopy caused by pH-induced protonation on methionine and cysteine aqueous solutions

Full text: In the early 70s, Siegbahn and Siegbanh published the first results of experimental electronic states for liquids, however, restricted to liquid samples with low vapor pressure (pv), as formamide. With the development of microjet technique proposed by B. M. Winter and Faubel [3] significantly has increased the possibility to analyze other liquids with higher vapor pressure, such as liquid water (pv = 4.6 Torr). Liquid water is the medium where the main biological reactions occur, therefore, data of electronic states of biological molecules in vapor-liquid interface is of great importance for understanding the subject. We present the first results of preliminary characterization of R4000 Scienta electron analyser in the study of liquid-vapor interface of methionine and cysteine aqueous solutions, using the microjet technique. We obtain by using X-ray photoelectron spectroscopy the N1s, C1s and S2p chemical shifts of cysteine and methionine in aqueous solutions at different pH values. These shifts are associated with the sequential protonation of the amino and carboxyl groups, which affects both charge state and hydrogen bonding to the surrounding water molecules. (author)