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In the original article, few equations and units were published incorrectly. [ABSTRACT FROM AUTHOR]

Graphical abstract The complexes of calcium phosphate and calcium carbonate in blood serum have physiological activities. Along with the lower-affinity albumin-bound calcium and free calcium, they compose a local chemical system that could play a role in maintaining the calcium level in blood serum. Highlights • First X-ray absorption spectrum of calcium in blood serum. • Direct evidence for the constituents of the complexed calcium fraction in blood serum. • Serum phosphate and carbonate as bidentate and unidentate ligands, respectively. • Spectral evidence for the weak calcium-albumin interaction. • A local chemical system consisting of low-affinity constituents in blood serum. Abstract The total calcium (tCa) in blood serum comprises free Ca2+ ions (fCa), protein-bound calcium (prCa), and complexed calcium by small anions (cCa). The cCa fraction, in addition to fCa, has been indicated to have some physiological activity. However, there is little evidence for the structure of its constituents. Here we report an ex vivo detection of the cCa constituents by synchrotron X-ray absorption near-edge structure spectroscopy. We collected the data directly on rat blood serum and, by making use of the reference samples, derived a spectrum that exhibits the features of cCa constituents. Among the features are those of the complexes of calcium phosphate and calcium carbonate. The detected complexes in the cCa fraction are mainly Ca(η2-HPO 4)(H 2 O) 4 and Ca(η1-HCO 3)(H 2 O) 5 +, in which HPO 4 2− and HCO 3 − serve as bidentate and unidentate ligands, respectively. The remained H 2 O molecules on the coordination sphere of Ca2+ enable these complexes to behave partially like aquated Ca2+ ions in protein-binding. Besides, as the dominant part of prCa, albumin-bound calcium (albCa) exhibits a spectrum that closely resembles that of fCa, indicating weak interactions between the protein carboxyl groups and calcium. The weak-bound cCa and albCa, along with fCa and the relevant anions, compose a local chemical system that could play a role in maintaining the calcium level in blood. [ABSTRACT FROM AUTHOR]