Lipids
Lipids play a certain role in mineralization, and mineralization inducing molecules are very important in atherosclerosis.[53,182,183] Atherosclerosis is an accumulation of intimal plaques that appear in the intima of the arteries, and the plaques consist mainly of lipoproteins, oxidized lipoproteins, various cellular debris, and finally, groups of apatite deposits.[135,184] The apatite formed by calcification of the medial arteries of the arterial intima is embedded in the lipid deposits.[126] There are individual and regional differences in atherosclerosis, which makes it impossible to understand the pathogenesis.
Other proteins
A portion of the vertebrate body is biomineralized by proteins other than collagen fibers. The organic matter in the enamel consists of AMBN, AMELX, ENAM, MMP20, ALP, and KLK4. When AMELX is secreted by ameloblasts to the outside of cells, it exists in the form of a monomer or nano aggregate.[43,44,185,186] The inside of nano aggregate is a hydrophobic core, and the outside is a hydrophilic retractable fragment.[187–190] In dependence on temperature, pH, and protein concentration, AMELX self-assembles to form a linear morphology that provides nucleation sites for hydroxyapatite (Figure 5 ).[191–193] This linear morphology may be the key to controlling the directional growth of hydroxyapatite.[194–196] AMBN and ENAM are all present in enamel nucleation and crystallization, but their specific roles are not yet clear.
Interestingly, AMBN, AMELX, and ENAM were not found in the mature enamel. It was found that the expression and secretion of MMP20 and KLK4 are spatiotemporal throughout the process of enamel mineralization.[28,38,197] MMP20 and KLK4 induce the degradation of enamel matrix proteins, providing space for hydroxyapatite crystal growth and making enamel harder.[40,197] When Raman spectroscopy is used to analyze enamel, the Amide I, II, and III bond is more of a characteristic peak of enamel matrix protein.[58,198–200] A typical example is the markedly enhanced intensity of the Amide III in enamel after MMP20 knockdown, indicating that the absence of MMP20 expression results in enamel matrix proteins being blocked in the enamel.[145]
Other minerals
Calcium oxalate stones are formed under pathological conditions. Its chemical formula is CaC2O4 or Ca (COO)2, fundamentally different from hydroxyapatite and carbonate.[137,201–203] The Raman characteristic peaks of calcium oxalate are also distinct. Characteristic bands for calcium oxalate monohydrate are assigned to the C-O asymmetric stretch (1630 cm–1), C-O symmetric stretch (1485 and 1460 cm–1), and C-C stretch (895 cm–1), and O-C-O in-plane bending (500 cm–1).[139,204–206] The pathological environment is more complex, and the embedding of different organic matter also causes changes in the Raman spectroscopy of biomineralization. The exact trend of the changes is not clear. In addition to calcium oxalate stones, there are xanthine stones, cystine stones, ammonium magnesium phosphate stones, uric acid (urate) stones, and calcium phosphate stones.[207–209]