Surface-Enhanced Raman Scattering Spectroscopy: Probing the Lumenal Surface of Photosystem II Membranes for Evidence of Manganese

A surface-enhanced Raman scattering (SERS) peak at 225 cm^- was reported in detergent-extracted, O₂-evolving Photosystem II (PS II) appressed membrane fragments from spinach that were treated with CaCl₂ (Seibert, M. and Cotton, T. (1985) FEBS Lett. 182, 34-38). CaCl₂ removes the 17-, 23-, and 33-kDa extrinsic proteins associated with O₂ evolution from the surface of the membrane without extracting functional Mn. The same peak has now been observed in PS II-enriched membrane fragments from Scenedesmus obliquus. The signal is related to the presence of Mn in spinach PS II membranes but not to the presence of Ca, Mg or Fe. Substitution of Br^- for Cl^- in the membranes without displacing Mn shifts the peak from 225 cm^-1 to 140 cm^-1. However, this shift is too great for the source of the SERS signals to be explained by simple Mn-Cl or Mn-Br vibrational modes. Evidence in addition to that presented in the previous publication indicates that the 33-kDa protein covers that part of the membrane that is the cause of the SERS signal. Due to altered properties of the SERS signal in the LF-1 mutant of Scenedesmus, which is affected in the D1 protein and binds less Mn than the wild type, we suggest that the 33-kDa protein shields a Mn-binding site on the D1 protein. Possible sources of the signal include (1) an Mn-O or Mn-N vibrational mode modified by chloride not present in the first coordination sphere; (2) a cooperative effect associated with the protein environment around the Mn-binding site that changes with the presence of halide; or (3) a species associated primarily with the Ag electrode surface that changes depending on the presence or absence of Mn and halide on the membrane surface. This last possibility might explain some of the results obtained with Br-treated spinach and Ca-treated Scenedesmus LF-1 mutant membranes.