The role of hydroxyl channel in defining selected physicochemical peculiarities exhibited by hydroxyapatite

Abstract

Mysteries surrounding the most important mineral for the vertebrate biology, hydroxyapatite, are many. Perhaps the Greek root of its name, απαταo, meaning ‘to deceive’ and given to its mineral form by the early gem collectors who confused it with more precious stones, is still applicable today, though in a different connotation, descriptive of a number of physicochemical peculiarities exhibited by it. Comparable to water as the epitome of peculiarities in the realm of liquids, hydroxyapatite can serve as a paradigm for peculiarities in the world of solids. Ten of the peculiar properties of hydroxyapatite are sketched in this review piece, ranging from (i) the crystal lattice flexibility to (ii) notorious surface layer instability to (iii) finite piezoelectricity, pyroelectricity and conductivity to protons to (iv) accelerated growth and improved osteoconductivity in the electromagnetic fields to (v) high nucleation rate at low supersaturations and low crystal growth rate at high supersaturations to (vi) higher bioactivity and resorbability of biological apatite compared to the synthetic ones, and beyond. An attempt has been made to explain this array of curious characteristics by referring to a particular element of the crystal structure of hydroxyapatite: the hydroxyl ion channel extending in the direction of the c-axis, through a crystallographic column created by the overlapping calcium ion triangles.

Calcium orthophosphates (CaPO 4 ): Occurrence and properties

“…Other examples of the computer simulations of the structure and properties of HA are available elsewhere (de Leeuw 2010;Corno et al 2010;Slepko and Demkov 2011;Aquilano et al 2014Aquilano et al , 2015. Finally, an attempt was performed to explain an array of the curious characteristics of HA by referring to the hydroxyl ion channels extending in the direction of the c-axis, through a crystallographic column created by the overlapping calcium ion triangles (Uskoković 2015).…”

Apatite Biominerals

“…In addition, other functional roles have been attributed to apatite biominerals like homeostasis or the inactivation of toxic elements. Several reviews have been published on this subject with different approaches focusing on crystal structures, and calcium phosphate precursors [6], the role of OH-channels [7], the role of unstable amorphous precursors [8], the role of polyphosphates [9], general view including the organic matrices [10], apatite biominerals and biomimetic processing and materials [11], and mineralogical oriented reviews [12]. This review aims to present and discuss the specific characteristics of biomineral apatites and illustrate how these characteristics and the resulting mineral properties have been employed in the adaptation for specific biological functions, in the case of vertebrate hard tissues (bone, dentine and enamel).…”

Estudo, caracterização e propriedades de ortofosfatos multifuncionais

“…[15]. Entretanto, é certo que se tornaram essenciais para a ocorrência da vida como a conhecemos e sua importância para nós é tão vital quanto a das moléculas de água, assim como indaga USKOKOVIC [16]. As funções exercidas por estes biominerais são complexas e específicas, e atualmente estão presentes em uma infinidade de seres vivos.…”

“…Uma destas funções em específico é a utilização da estrutura dos ortofosfatos de cálcio na estocagem de minerais (Ca 2+ , PO4 3-, K + , Na + , etc. ), os quais são liberados de acordo com a necessidade do organismo [16].…”

“…Entretanto, a presença de impurezas na rede cristalina da HA (como CO3 2-) e vacâncias iônicas estabilizam a fase hexagonal na temperatura ambiente. Isto faz com que na natureza, os raros minerais geológicos de HA assumam a estrutura hexagonal, bem como a HA biológica em sistemas fisiológicos, pois ambos são versões impuras da HA [16]. Em laboratório, a síntese da HA em condições atmosféricas também leva à inclusão de impurezas e estabilização de vacâncias na rede cristalina, fazendo com que a estrutura hexagonal seja adotada nestes casos [34].…”