Exploratory XPS Evidence for Ca–N Environments at the Organic–Mineral Interface in Bone

ABSTRACT Bone derives its remarkable mechanical properties from the nanoscale relationship between its organic matrix and mineral components. While interfacial interactions are known to affect bone mechanics, direct evidence of chemical bonding at this interface has been lacking. This exploratory study uses X‐ray photoelectron spectroscopy (XPS) to generate hypotheses about potential chemical bonding between the organic matrix and mineral components in human bone. Femoral tissue samples from five cadaveric donors with varying bone mineral density (BMD) were analysed. Core‐level electronic states of nitrogen and calcium were examined to explore potential chemical bonding signatures. XPS analysis revealed signatures consistent with calcium–nitrogen (Ca–N) interactions in all samples, with a 2.4‐fold higher frequency in normal bone compared to osteoporotic bone. Normal bone exhibited a component‐derived Ca–N:N–Ca ratio of 17:8 ± 0.75, while osteoporotic bone showed 1:1 ± 0.06. The N–Ca interaction showed binding energies of 398.4–398.9 eV, while Ca–N signatures in Ca 2p 3/2 appeared at 346.9–347.0 eV. These preliminary XPS‐based observations are consistent with potential chemical bonding between the organic matrix and mineral components. We propose this interaction may occur between Ca 2+ in extrafibrillar mineral and an iminic nitrogen moiety in the organic matrix between mineralized fibrils. The reduction of these signatures in osteoporotic bone suggests they may contribute to bone strength, though the causal relationship remains to be established and further complementary techniques are needed for confirmation.