ISSN 1062-8738, Bulletin of the Russian Academy of Sciences. Physics, 2015, Vol. 79, No. 2, pp. 227–232. © Allerton Press, Inc., 2015. Original Russian Text © P.V. Seredin, D.L. Goloshchapov, Tatiana Prutskij, Yu.A. Ippolitov, 2015, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2015, Vol. 79, No. 2, pp. 250–255. 227 INTRODUCTION Among the problems of modern stomatology, establishing the mechanisms responsible for the for- mation of caries is one of the main lines of inquiry, and preventing the development of the disease in its initial stages is a problem of great interest. It is well known that the first sign of the carious process is a spot whose size and color is subject to considerable variation over time, due to transformations in the structure and chemical composition of the enamel in the region of the spot. It has been established that at the level of unit cells, intense ion substitutions occur in the apatite crystals that constitute tooth enamel. These substitu- tions lead to variation in the phase composition of the mineral complex of enamel apatite and thus to upset- ting of the dynamic equilibrium in the mechanism of mineral exchange, a prerequisite to the formation of caries [1, 2]. The formation of enamel apatite occurs according to the well-known reaction (1) i.e., dicalcium phosphate dihydrate (DCPD) → octa- calcium phosphate → hydroxyapatite. The chemical transformations that occur in the tooth enamel during the emergence of caries are con- siderably more complex. Substituted hydroxyapatite (HAP)—the main component of the mineral constit- uent of tooth enamel—can behave in a complex man- ner when it dissolves. This is due not only to HAP’s capability for isomorphic substitutions but also to the possibility of a broad class of calcium phosphates whose solubility is higher than that of tooth-enamel ⋅ → ⋅ → 4 8 4 10 4 Ca(HPO HO Ca (PO HO Ca (PO OH 2 2 6 2 6 2 ) 2 ) 5 )( ), HAP participating in formation processes. In a num- ber of studies, it was assumed on the basis of theoreti- cal concepts that DCPD and fluorapatite (FAP) would precipitate at the initial stages of caries development during enamel demineralization under the effect of acids [3–5]. Analysis of the solubility diagrams of phosphates [6, 7] shows that hydroxyapatite is one of the most stable form of calcium phosphate (exclud- ing fluorapatite) at pH close to neutral. According to the diagram in [6], DCPD becomes more stable in an acidic medium beyond the limits of the singular point at which the two solid solutions (enamel and DCPD) are in equilibrium. Based on theoretical calculations, it was shown that the probability of such phase trans- formations increases at low pH (4.3–5.5). However, the chain of such transformations was not still con- firmed experimentally by direct investigations of the development of the carious process. It should be noted that during the development of the carious process, the elemental composition of the affected region (i.e., the degree of substitution in it) varies with the depth of caries’ penetration into the enamel [8]. This means that caries is not simply a chain of chemical transformations during HAP disso- lution. Mineral reprecipitation is likely during the selective dissolution of minerals, testifying to the inverse growth of HAP crystals on the affected seg- ments of hard tooth tissues. Analyzing the above, it is clear that the role of organics in the development of caries is underestimated, since the organic constituent alone can inhibit the growth of HAP crystals. The aim of this work was to investigate the emer- gence of phase transformations in the hard tissues of the human tooth at the initial stages of the carious pro- Investigating Phase Transformations in Hard Tissues of the Human Tooth during the Carious Process by Means of Raman Microspectroscopy and Luminescence P. V. Seredin a , D. L. Goloshchapov a , Tatiana Prutskij b , and Yu. A. Ippolitov c a Voronezh State University, Voronezh, 394036 Russia b Instituto de Ciencias, Benemérita Universidad Autónoma de Puebla, 72050 Puebla, PUE, Mexico c Voronezh State Medical Academy, Voronezh, 394000 Russia e-mail: paul@phys.vsu.ru Abstract—Investigations of the regions of the nascent enamel carious process confirm assumptions on the formation of weak phosphates during the interaction between enamel apatite and organic acids (products of the biological activity of microorganisms). The described approach for determining regions of hard tooth tis- sue with an incipient carious process could form the basis for a new approach to caries diagnosis. DOI: 10.3103/S1062873815020240