Borate complexation

In aqueous solution, riboflavin has absorption at ca 220—225, 226, 371, 444 and 475 nm. Neutral aqueous solutions of riboflavin have a greenish yellow color and an intense yellowish green fluorescence with a maximum at ca 530 nm and a quantum yield of = 0.25 at pH 2.6 (10). Fluorescence disappears upon the addition of acid or alkah. The fluorescence is used in quantitative deterrninations. The optical activity of riboflavin in neutral and acid solutions is [a]=+56.5-59.5° (0.5%, dil HCl). In an alkaline solution, it depends upon the concentration, eg, [a] J =—112-122° (50 mg in 2 mL 0.1 Ai alcohohc NaOH diluted to 10 mL with water). Borate-containing solutions are strongly dextrorotatory, because borate complexes with the ribityl side chain of riboflavin = +340° (pH 12). 

Separations of polysaccharides by fractionation on a preparative scale were also examined. Stemming from earlier work in his laboratory on the isolation of acidic polysaccharides by precipitation as their insoluble Cetavlon salts, Stacey and coworkers showed that it was possible to fractionate neutral polysaccharides by selective precipitation with Cetavlon after the formation of borate complexes. 

CHROMIUM ALKYLS WITH NITROGEN- AND OXYGEN-LIGANDS 3.1. Tris(pyrazolyl)borate complexes... 

Fig. 4.3 Ranges of isomer shifts observed for Fe compounds relative to metallic iron at room temperature (adapted from [24] and complemented with recent data). The high values above 1.4-2 mm s were obtained from Co emission experiments with insulators like NaCl, MgO or Ti02 [25-28], which yielded complex multi-component spectra. However, the assignment of subspectra for Fe(I) to Fe(III) in different spin states has never been confirmed by applied-field measurements, or other means. More recent examples of structurally characterized molecular Fe (I)-diketiminate and tris(phosphino)borate complexes with three-coordinate iron show values around 0.45-0.57 mm s [29-31]. The usual low-spin state for Fe(IV) with 3d configuration is 5 = 1 for quasi-octahedral or tetrahedral coordination. The low-low-spin state with S = 0 is found for distorted trigonal-prismatic sites with three strong ligands [30, 32]. Occurs only in ferrates. There is only one example of a molecular iron(VI) complex it is six-coordinate and has spin S = 0 [33]...

Pal, B. C., Novel application of a sugar-borate complexation for separation of ribo-, 2 -deoxyribo-, and arabinonucleosides on cation exchange resin, /. Chromatogr., 148, 545, 1978. 

Yamamoto, A., Inoue, Y., Kodama, S., and Matsunaga, A., Capacity gradient anion chromatography with a borate complex as eluent, ]. Chromatogr. A, 850,... 

Rabinovich D (2006) Poly(mercaptoimidazolyl)borate Complexes of Cadmium and Mercury 120 143-162... 

Properties of nickel poly(pyrazol-l-yl)borate complexes such as solubility, coordination geometry, etc., can be controlled by appropriate substituent groups on the pyrazol rings, in particular in the 3- and 5-positions. Typical complexes are those of octahedral C symmetry (192)°02-604 and tetrahedral species (193). In the former case, two different tris(pyrazolyl)borate ligands may be involved to give heteroleptic compounds.602,603 Substituents in the 5-position mainly provide protection of the BH group. Only few representative examples are discussed here. 

There has been particular recent interest in zinc nitrate complexes as coordination models for bicarbonate binding in carbonic anhydrase. The mono- or bidentate coordination modes have been studied with tris-pyrazolyl borate complexes and can be rationalized in the context of the enzyme activity.433 Caution in this comparison is introduced by ab initio calculations on these model systems demonstrating both monodentate and bidentate coordination energy minima for nitrate binding to zinc 434... 

Gorin, P.A.J. and Mazurek, M. "C Magnetic Resonance Spectroscopic Evidence for Formation of Borate Complexes of Polyhydroxy Compounds," Carbohydrate Research. 27(1973), 325-339. 

Solution Studies of Poly(pyrazolyl)borate Complexes. 116... 

Recently, Lipton et al. [25] have used zinc-67 NMR to investigate [Zn(HB(3,5-(CH3)2pz)3)2] complexes which have been doped with traces of paramagnetic [Fe(HB(3,4,5-(CH3)3pz)3)2]. The low-temperature Boltzmann enhanced cross polarization between XH and 67Zn has shown that the paramagnetic iron(II) dopant reduces the proton spin-lattice relaxation time, Tj, of the zinc complexes without changing the proton spin-lattice relaxation time in the Tip rotating time frame. This approach and the resulting structural information has proven very useful in the study of various four-coordinate and six-coordinate zinc(II) poly(pyrazolyl)borate complexes that are useful as enzymatic models. 

Although the poly(pyrazolyl)borate complexes of iron(II) have been well known for many years, [1] it is only recently that the complexes with the tris(l-pyrazolylmethane ligand, HC(pz)3, [45-48] have been studied in detail. It should be noted that poly(pyrazolyl)methane ligands, such as the tris(l-pyrazolylmethane ligand, are neutral, whereas the poly(pyrazolyl)bo-rate ligands, such as the tris(l-pyrazolyl)borate ligand, HB(pz)3", are monoanions. As a consequence, the metal(II) poly(pyrazolyl)methane complexes are dications and often have quite different properties from those of the analogous metal(II) poly(pyrazolyl)borate molecular complexes. But, in spite of these differences there are often very close structural similarities between the dicationic complexes and the neutral complexes. Therefore the study of the pyrazolylmethane complexes will parallel that of the borate complexes discussed above. 

Mochizuki et al. also demonstrated that the uncatalyzed autoxidation of catechins is suppressed in the presence of a sufficient amount of borate buffer (44). This was interpreted by considering the adduct formation between borate ion and the catechins. The oxidation resumes on addition of Cu(II) in the presence of the borate ions. This may indicate that Cu(II) forms more stable complex(es) with the catechins than the borate ion, but direct oxidation of the catechin-borate complex by Cu(II) cannot be excluded either. 

Addition of some sodium borate to increase the fructose —> mannitol hydrogenation selectivity due to fructose-borate complex formation [21, 22]. 

L-Amino acid oxidase has been used to measure L-phenylalanine and involves the addition of a sodium arsenate-borate buffer, which promotes the conversion of the oxidation product, phenylpyruvic acid, to its enol form, which then forms a borate complex having an absorption maximum at 308 nm. Tyrosine and tryptophan react similarly but their enol-borate complexes have different absorption maxima at 330 and 350 nm respectively. Thus by taking absorbance readings at these wavelengths the specificity of the assay is improved. The assay for L-alanine may also be made almost completely specific by converting the L-pyruvate formed in the oxidation reaction to L-lactate by the addition of lactate dehydrogenase (EC 1.1.1.27) and monitoring the oxidation of NADH at 340 nm. 

Solid-phase borate complexation coupled with RP-HPLC has been employed for the measurement of polyhydroxyflavones in human blood plasma, vegetables and redwine. The chemical structures of polyhydroxyflavones included in the investigation are shown in Fig. 2.87. Vegetables were homogenized, centrifuged and the supernatant was applied for analysis. Human plasma was heparinized before analysis. The outer skins of onion were... 

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