Carboxylate stretching bands in the D2/S1 FTIR spectrum disappear by metal chelator in Ca2+-depleted OEC

Abstract

A mid range S2/S1 difference FTIR spectrum shows vibrational feature arising from carboxylate stretching modes. These bands are believed to be ascribed to acidic amino acid(s) that ligates the Mn-cluster and/or Ca2+. We studied effects of Ca2+-depletion and chelating agents on the spectrum under strictly controlled conditions in terms of Ca2+ concentration. The carboxylate bands were normally induced even after Ca2+-depletion, indicating that the Ca2+ is not directly involved in the change of the ligation structure of the Mn-cluster upon S2 formation. Interestingly, these characteristic bands disappeared in the presence of metal chelators (EDTA and EGTA), and were restored by the addition of Ca2+. On the other hand, the redox and magnetic properties of the S2 state Mn-cluster were little affected by the addition of chelator as depicted by the formation of the normal S2QA- thermoluminescence band and the normal S2 multiline signal in the presence of EDTA. The results may suggest that a chelator interacts with the Mn-cluster as a replacement with the native carboxylate ligand in the absence of Ca2+, but its ligating mode does not change upon the oxidation of the Mn-cluster.