Fluoride protection mechanism

In the USA, water fluoridation became widely available after 1955 and fluoridated toothpastes after 1975 and caries in adolescent children has decreased by 66%. The effects of fluoride on caries are topical from the surface to the interior. Water fluoridation ensures small amounts of fluoride throughout a tooth and fluoridated toothpaste enhances the fluoride concentration at the tooth surface. Protection from caries by artificial fluoridation of water supplies and fluoridated toothpaste is cumulative. Investigations as to how fluoridation protects from caries has identified three mechanisms of caries protection (1) inhibition of demineralization, (2) enhancement of remineralization, and (3) inhibition of bacterial enolase activity reducing lactate production from ingested carbohydrates. Fluoride has little effect on bacterial growth, and gives no direct protection from gingivitis, periodontitis, or osteoporosis... 

Figure 2.8 Solid Phase RNA Synthesis. 5 -dimethoxytrityl (DMT)-deprotection of resin bound 3 -terminal nucleoside residue is effected with trichloroacetic acid (TCA) (see Fig. 2.5). Thereafter the first coupling reaction is enabled by phosphoamidite activation with tetrazole (see Fig. 2.5) followed by oxidation of the newly formed diester linkage to a phosphodiester link. The process of 5 -DMT deprotection, phosphoramidite coupling and then diester oxidation, continues for as many times as required (n-times), prior to global deprotection and resin removal under basic conditions. RNA synthesis requires that 2 -hydroxyl groups are protected during the synthesis by tc/t-butyl dimethyl silyl (TBDMS) protecting groups labile only to fluoride treatment from tetra butyl ammonium fluoride (TBAF) (mechanism shown).

Fluorine attack follows the mechanisms mentioned for the case of chlorine. Namely, the reaction is slow at lower temperatures when a fluoride protective layer is formed but at higher temperatures, the formation of liquid and, especially, volatile fluorides leads to rapid attack of the metal. 

Fluorides. Most woddwide reductions in dental decay can be ascribed to fluoride incorporation into drinking water, dentifrices, and mouth rinses. Numerous mechanisms have been described by which fluoride exerts a beneficial effect. Fluoride either reacts with tooth enamel to reduce its susceptibihty to dissolution in bacterial acids or interferes with the production of acid by bacterial within dental plaque. The multiple modes of action with fluoride may account for its remarkable effectiveness at concentrations far below those necessary with most therapeutic materials. Fluoride release from restorative dental materials foUow the same basic pattern. Fluoride is released in an initial short burst after placement of the material, and decreases rapidly to a low level of constant release. The constant low level release has been postulated to provide tooth protection by incorporation into tooth mineral. 

Tantalum is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

The three principal electrochemical methods are described by which fluorine can be directly introduced into organic compounds, namely electrolysis in molten salts or fluoride ion solutions, electrolysis in molten potassium fluoride/hydrogen fluoride melts at porous anodes, and electrolysis in anhydrous hydrogen fluoride at nickel anodes. Using examples from the past decade, it is aimed to demonstrate that electrofluorination in its various forms has proved to be an increasingly versatile tool in the repertoire of the synthetic chemist. Each method is described in terms of its essential characteristics, reaction parameters, synthetic utility, advantages and disadvantages, patent protection, and potential for commercial exploitation. The different mechanisms proposed to explain each process are critically reviewed. 

In the presence of two equivalents of silver fluoride, N-protected bis[(trimethylsi-lyl)methyl]amines lead also to azomethine ylids which can be trapped by dipolarophiles. The mechanism of the cycloaddition reaction involves sequential electron-Me3Si+-electron transfer process from the amine to silver fluoride, which forms silver metal, ruling out a fluoride-induced desilylation process. Although silver is recovered at the end of the reaction, a cheaper oxidizing reagent is still lacking.448,449... 

Numerous dioxetanes with varying atom (X) and protecting group (PG) have been synthesized over the last decade in order to study the CIEEL mechanism. The following serve as prototypical examples. The most prevalent trigger is that of a siloxyphenyl substituent such as that incorporated into dioxetane 54. Tetrabutylammonium fluoride (TBAF) is used in an aprotic solvent, such as dimethyl sulfoxide (DMSO) or acetonitrile, to desilylate to afford the unstable phenolate 55 (Scheme 11) <2002MI305>. 

This chapter describes how individuals with severe enamel fluorosis (mottled tooth enamel) became associated with fluoride in the public water supply and protection from dental caries. A comparison of caries experience with the fluoride content of public water supplies and enamel fluorosis in adolescents indicated that 1 pg fluoride/mL (1 part/million) in the water provides caries protection with minimal enamel fluorosis (sect. 1). One mechanism is the spontaneous isomorphic replacement of apatite s hydroxide anions with fluoride, which reduces enamel solubility. A second is fluoride-mediated inhibition of enolase, which retards bacterial acid production at teeth surfaces. These findings led to the use of fluoride in toothpastes, which provides better protection from caries at tooth surfaces than water fluoridation alone (sect. 2). The chapter concludes with a discussion of potentially harmful effects of fluoride ingestion (sect. 3). 

A second mechanism of protection from caries is the incorporation of fluoride into bacterial biofilms where it inhibits enolase. Enolase catalyzes the production of phospho-enolpyruvate, the precursor of lactate in glycolysis, from 2-phosphoglycerate during glycolysis (Fig. 16.7 - see also Fig. 1.7). In addition, oral bacterial uptake of mono- and disaccharides mostly utilizes the phosphoenolpyruvate transport system to transfer them into the cytosol (Sect. 15.2.2). Fluoride therefore inhibits not only lactic acid production, but also the phosphoenolpyruvate transport system-mediated uptake of saccharide substrates. In short, fluoride inhibits saccharolytic fermentation by many oral bacteria. 

Nitration of 6-substituted purines at C-2, using a mixture of tetra-n-butylammonium nitrate and trifluo-roacetic anhydride, is an exceptionally useful functionalisation of the purine ring system. The reaction works for both electron-rich (adenosine), 6-alkoxypurines and electron-poor (6-chloropurine) substrates, but full protection of all OH and NH groups is required. This is not a simple electrophihc substitution - the mechanism has been shown, using 6-chloro-9-Boc purine, to involve sequential nitration of N-7, addition of trifluoroacetoxy at C-8 and then migration of the nitro group to C-2. The final, re-aromatisation, step involves elimination of trifluoroacetic acid. Displacement of a 2-nitro group, thus introduced, by fluoride as nucleophile (see 27.5 for nucleophilic substitutions) can be made the means to synthesise 2-fluoroadenosine. ... 

---