The structure and function of the photosystem II PsbO extrinsic protein is under intense research, being an essential part of the biomolecular engine that carries out water oxidation and oxygen production. This paper presents a structural analysis of the isolated PsbO protein by FTIR spectroscopy, reporting detailed secondary structure quantification and changes in the secondary structure content of the protein attributed to the effect of calcium (Ca(2+)). Measurements in H(2)O and D(2)O have allowed us to see the effect of calcium on the conformation of the protein. The results indicate that (i) the protein presents a major content of beta-structure (i.e., beta-sheet, beta-strands, beta-turns) as detected by the infrared bands at 1624-1625, 1678-1679, 1688-1689 cm(-1), which account for about 38% in water and 33% in heavy water, in the presence of calcium; and (ii) the amount of this beta-structure fraction increases 7-10% in the absence of calcium, with a concomitant decrease in loops and nonordered structure. The thermal denaturation profile of the protein in the presence of calcium showed low stability with T(m) approximately 56 degrees C. This profile also shows a second phase of denaturation above 60 degrees C and the appearance of aggregation signals above 70 degrees C. Our observations indicate that calcium is able to modify the conformation of the protein at least in solution and confirm that PsbO is mainly a beta-protein where beta-sheet is the major ordered secondary structure element of the protein core.