In this work, the chemical states of sulfur in two typical photographic gelatins, both of which were undoped and doped with ferric ions, respectively, were studied by x-ray photoelectron spectrometry. Sulfur contained in gelatin I may be in three chemical states: the O = S∣∣ = O group of methionine sulfone, the O = S∣∣ group of methionine sulfoxide, and the –S– group of methionine. The proportions of these three chemical states reflect the relative amount of methionine sulfone, methionine sulfoxide, and methionine in elatin to a certain extent. Under the experimental conditions of this study the addition of ferric ions could induce a part of the methionine sulfone to be reduced to methionine sulfoxide. At the same time some of the –CH2– groups of gelatin were oxidized to – CH∣ – OH groups.
Yi-heng Zhang, Jie Li, Tian-tang Yan, Shuqin Yu, Si-yong Zhuang, Bi-Xian Peng, "Study of the Chemical State of Sulfur in Photographic Gelatin by XPS" in Journal of Imaging Science and Technology, 1999, pp 49 - 53, https://doi.org/10.2352/J.ImagingSci.Technol.1999.43.1.art00006