A Surface-Enhanced Raman Signal Associated with Manganese in PS II Membranes

A surface-enhanced Raman signal at about 225 cm^-1 was detected in oxygen-evolving photosystem II (PS II) membranes treated with 1 M CaCl₂ (which removes three extrinsic proteins but does not extract Mn). The signal is not seen in Tris-treated preparations (which retain none of the three proteins nor Mn) nor in NaCl-treated preparations (which retain Mn and the 33-kDa extrinsic protein). The Raman signal is unstable and has been related kinetically, both to the disappearance of CaCl₂-reconstituted O₂ evolution and to the release of about half the Mn found initially in the CaCl₂-treated material. Although the origin of the signal may be either a direct or indirect monitor of the unstable Mn pool, the frequency is consistent with Mn-N, Mn-O or Mn-Cl vibrations reported in the literature. From these data and previous studies we suggest a model in which the unstable pool of Mn is located between the 33-kDa extrinsic protein and the PS II membrane, probably at the site of a 34-kDa intrinsic protein. O₂ evolution Manganese Photosystem II 33-kDa protein 34-kDa protein Photosynthesis.