A possible calcium binding site in D1 protein: A fluorescence and FTIR study of the interaction between lanthanides and a synthetic peptide.

A peptide ranging from residue 229 to 240 of the D1 protein of Photosystem (PS) II was synthesized and lanthanides were used as candidates of calcium. Fluorescence and FTIR spectroscopy were used to test the conformational adaptation after lanthanide additions. Fluorescence spectroscopy showed that the synthetic peptide provides lanthanide binding site, and that glutamic acids are involved in lanthanide binding. Resolution enhancement techniques were combined with band curve-fitting procedures to quantitate the FTIR spectral information from the amide 1 bands. The relative areas of these component bands indicate that lanthanide induced a substantial decrease in the amount of unordered structure and turns, while a corresponding increase in the amount of α-helix and 'open loop' was also observed. This indicates that a relatively compact structure of the synthetic peptide is formed if lanthanides are applied. The results may reflect on the physiological and biochemical function of calcium in PS II, including preventing D1 from trypsin digestion.