Lan-Chi Koenigsberger

Solubility phenomena (i.e. dissolution and precipitation reactions) are the physicochemical basis of numerous biological processes. These include, for instance: • gas solubilities in respiratory and photosynthetic processes; the solubility of volatile anaesthetics; • the crystallisation, both in biologically controlled and pathological processes, of biogenic minerals in a variety of body fluids; the resorption of mineralised tissue; • the accumulation, due to their higher solubility, of lipophilic substances, such as pesticides, in liquid fat contained, e.g. in adipose tissue or human milk; • the incorporation of metal ions such as strontium (including the radioactive 90Sr isotope) in bone, by co-precipitation and solid-solution formation. Since Sr stabilises bone apatite crystals (i.e. decreases solubility), it may retard the resorption of the calcified matrix and thus have therapeutic potential in the prevention and treatment of osteopenic disorders [1]. In dental enamel, a combination of strontium and fluoride was reported to be more effective in stabilising the apatetic structure than each element alone [2]. This results in an improved crystal resistance to degradation by bacterial acids and hence may be useful for the prevention of dental caries [3]. All of these solubility phenomena are governed by the laws of thermodynamics and kinetics. The human body is essentially an isothermal system (a notable exception, related to gout, has been reported in the literature – see below). Thus, the pertinent in vitro measurements have almost always been performed at 37_C. However, reactions in body fluids are complicated by the presence of organic complexing agents which affect the speciation of metal ions (i.e. their distribution among these complexes). Computer speciation modelling of biofluids, which has a long history [4], has also to be considered in the modelling of solubility equilibria. The presence of organic macromolecules such as proteins provides templates or matrices which control the crystallisation of biogenic minerals and modify their morphologies. The mineral phase in organic/inorganic composites (such as teeth or bone) may form nanosized crystals, which exhibit unusual dissolution and crystallisation behaviour [5]. A new biomineralisation mechanism invoking liquid precursors and their importance for normal and pathological biomineralisation processes has been proposed [6]. These and other aspects of normal and pathological biomineralisation processes have been reviewed recently [7].

Biomineralization, which refers to the complex processes by which organisms form minerals, is frequently associated with a high degree of regulation on different hierarchical levels [1,2]. ‘Biologically controlled’ mineralization, in which extra-, inter- and intracellular activities direct the nucleation, growth and morphology of minerals that form ‘normal’ biomaterials such as bone and teeth [1,2], is fundamentally different from ‘biologically induced’ mineralization, which occurs as a result of interactions between biological activity (affecting e.g. the pH and composition of secretion products) and the environment [1,2]. Since there is little control of the biological system over the type and habit of minerals deposited, these vary as greatly as the environments in which they form and are often poorly defined, heterogeneous and porous [1,2]. Biologically induced mineralization is commonly associated with various bacterial activities and with epicellular mineralization in marine environments, occasionally leading to the complete encrustation of organisms, that sink subsequently and form sediments [1,2]. However, its characteristic features are also typical for uncontrolled ‘pathological’ crystallization resulting in painful or even life threatening conditions such as calculi formation (renal, biliary, pancreatic or sublingual), development of gout or arteriosclerosis, tissue calcification associated with cancer, etc.