Factor tolerances

Brewers and bakers dried yeasts are used as dietary supplements. They contribute some protein and trace minerals, and some B vitamins, but no vitamin C, vitamin B 2 or fat-soluble vitamins. The glucose tolerance factor (GTE) of yeast, chromium nicotinate, mediates the effect of insulin. It seems to be important for older persons who caimot synthesize GTE from inorganic dietary chromium. The ceU wall fraction of bakers yeast reduces cholesterol levels in rats fed a hypercholesteremic diet. 

Rice bran is the richest natural source of B-complex vitamins. Considerable amounts of thiamin (Bl), riboflavin (B2), niacin (B3), pantothenic acid (B5) and pyridoxin (B6) are available in rice bran (Table 17.1). Thiamin (Bl) is central to carbohydrate metabolism and kreb s cycle function. Niacin (B3) also plays a key role in carbohydrate metabolism for the synthesis of GTF (Glucose Tolerance Factor). As a pre-cursor to NAD (nicotinamide adenine dinucleotide-oxidized form), it is an important metabolite concerned with intracellular energy production. It prevents the depletion of NAD in the pancreatic beta cells. It also promotes healthy cholesterol levels not only by decreasing LDL-C but also by improving HDL-C. It is the safest nutritional approach to normalizing cholesterol levels. Pyridoxine (B6) helps to regulate blood glucose levels, prevents peripheral neuropathy in diabetics and improves the immune function. 

Chromium deficiency may be related to the glucose tolerance factor (Herold and Fitzgerald 1994). The determination of this deficiency, however, is questioned, because the lack of accuracy of the Cr determination in the earlier publications. 

The tolerance factor t for perovskites AMX3 is a value that allows us to estimate the degree of distortion. Its calculation is performed using ionic radii, i.e. purely ionic bonding is assumed ... 

In this equation rA is the radius of the cage site cation, rB is the radius of the octahedrally coordinated cation, and rx is the radius of the anion. The factor l is called the tolerance factor. Ideally, t should be equal to 1.0, and it has been found empirically that if t lies in the approximate range 0.9-1.0, a cubic perovskite structure is stable. However, some care must be exercised when using this simple concept. It is necessary to use ionic radii appropriate to the coordination geometry of the ions. Thus, rA should be appropriate to 12 coordination, rB to octahedral coordination, and rx to linear coordination. Within this limitation the tolerance factor has good predictive power. 

The enthalpies of formation of selected perovskite-type oxides are given as a function of the tolerance factor in Figure 7.17. Perovskites where the A atom is a Group 2 element and B is a d or / element that readily takes a tetravalent state [19, 20] show a regular variation with the tolerance factor. Empirically, it is suggested that the cations that give t close to 1 have the most exothermic enthalpies of formation. When t is reduced, the crystal structure becomes distorted from cubic symmetry and this also appears to reduce the thermodynamic stability of the... 

Before dealing with these structure t es in detail, the clear-cut dependence of the occurence of these distorted perovskites on the radius ratio of the ions in question should be mentioned. The tolerance factor defined by Goldschmidt [115),... 

The ionic radii for calculation of the tolerance factor t have been taken from Ahrens (2). An increase of 6% has been taken into account for the A-ions in 12-coordination (246). Only the Me + ionic radii of Zn and Cd were slightly modified, as indicated by the molecular volumes of their compounds relative to others (rzn = 0.71 A, red = 0.86 A, instead of 0.74 and 0.97 A resp.). By similar reasons, the ionic radii of both, NHj+ and T1+ were enlarged as compared to that of Rb+ (1.47 A) (riiH4 = 1-48 A, rn = 1.49 A, instead of 1.43 and 1.47.A resp.). A fluoride ionic radius of rj- = 1.33 A has been accepted. 

The two mentioned ternary fluorides of cadmium with their tolerance factors of 1.00 and 0.88 resp. mark quite accurately the field of existence of cubic perovskites. As may be seen from the following Table 25 the tolerance factors of all cubic fluoroperovskites of the transition metals hitherto known lie within the range of these limits. 

A group of 8 ternary fluorides containing the transition metal ions Cr2+ and Cu + crystallizes in a tetragonedly distorted perovskite lattice. This distortion is caused by the Jahn-Teller effect displayed by the configurations d% d (Cr +) and d d (Cu2+) resp., rather than by geometrical reasons. As for their space requirements the ions Cr + and Cu + are very close in size to Mn2+ and Co + resp. and as a consequence the corresponding compounds do not differ in their tolerance factors. 

This type of orthorhombic perovskite structure appears, if the tolerance factor of Goldschmidt is smaller than t — 0.88. The example of the compound NaMnFs [t = 0.78), showing doubled lattice constants a and h (287), is likely to mark the lower limit of the field in which orthorhombic fluoro-perovskits of the GdFe03-t3q>e may occur. Fluoroperovskites which have a smaller tolerance factor than t = 0.78 never have been observed so far, nor do fluoride structures of the ilmenite type seem to exist, which might be expected for ya = Me, corresponding to 1=1/1/2=0.71. 

There exists quite a number of hexagonal oxidic perovskites 183, 332), but there seem to be only three types in the case of ternary fluorides. Their occurrence again clearly depends on the tolerance factor wich thus proves to be useful in classifying the hexagonal perovskites also. After having described their structures in detail they will be further discussed under a common point of view. 

No other ternary fluoride of this stracture has become known until now. According to the value of its tolerance factor only CsZnFa t = 1.07) might adapt the same structure type. This compound, though repeatedly described in the literature 191, 215, 297) does not seem to exist, however (8). Instead of it the ternary fluoride Cs4ZnaFio is easily formed (10). 

The structure described for CsNiFs might be expected too for CsMgFs with its tolerance factor of < = 1.10. But as in the case of CsZnFj informations on this compound could not be verified. Only a ternary fluoride Cs MggFio has been found (10). 

Chromium was recognized as an essential trace element in 1955.1190 Rats fed a chromium-deficient diet developed an impaired tolerance for intravenous glucose, which could be reversed by an insulin-potentiating factor present in brewer s yeast, meat and various other foods. The insulin-potentiating factor was found to be a complex of chromium(IH)1191 and such substances have been termed Glucose Tolerance Factor(s) (GTFs). Chromium was demonstrated to be essential for humans in 1975.1192 There are several reviews of the chemistry of chromium(III) and its relationship to glucose tolerance.1193-1196... 

Essential to ascidians ("sea squirts"), which concentrate in a miilionfold from sea water. Essential to chicks and rats. Deficiencies cause reduced growth, impaired reproduction and sirvival of young, impaired tooth and bone metabolism and feather development.4 May be a factor in manic-depressive illness/ Essential involved in gkicose metabolism and diabetes potentiates effect of insulin. Presence in glucose tolerance factor from brewer s yeast questioned/... 

Glucose tolerance factor chromium in 888 Glucose transporters 415, 416 GLUT1, topology diagram 416 GLUT4, response to insulin 416 Glucosidase... 

Finally, it should be noted that this chemistry may have biological relevance. Several metalloenzymes are believed to contain more than one metal ion bound at the active site. One relevant example is the glucose tolerance factor (GTF) which is important for the metabolic degradation of glucose (398-401). GTF is a low-molecular-weight protein which contains chromium(III). Its structure is not known, but it has been suggested that the active site contains a dinuclear chromium(III) complex (401). The fact that hydroxo-bridged dinuclear chromium(III) complexes exhibit reactions which are often very fast compared with those observed for the parent mononuclear species seems to support such a proposal. 

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