Metal complexing assays

Metal complexing assays (i.e. Jerumanis method) are generally more specific than redox-based assays, because the color of the complexes depends on a specific pattern of substitution on the phenolic rings [108]. Procyanidins form chelates with metal ions via their ortho-diphenolic groups on the B-rings [112],... 

Up to this point, we have discussed the use of metal complexes in assay and imaging. It remains to consider how they can be used as sensors analogous to those described at the beginning of this chapter. 

Metal complexation — One of the most insidious and widely occurrent sources of analytical variation in IEC is product complexation with metal ions. Most proteins can form complexes with metals, regardless of whether or not they are metalloproteins.1 Participant metal ions can derive from the cell culture production process, purification process buffers, or even stainless steel chromatography systems. Complexation can alter retention times, create aberrant peaks, and substantially increase peak width. To the extent that metal contamination of your sample is uncontrolled, so too will be the performance of your assay. 

Whatever the scope of your investigations into selectivity, it is critical that you address the issue of product complexation in the sample, particularly com-plexation with metals. Complexation problems are by no means universal, but they are more common than generally realized. If you confirm the occurrence of product complexation, it is important to discover and characterize its source. Even though this may not fall strictly within the usual bounds of assay development responsibilities, the potential for the problem to affect purification, formulation, and even pharmacokinetics demands that this potential source of variation be addressed. Seek to eliminate it. If you cannot eliminate it, then reduce it. If you cannot reduce it, then at least try to maintain it within defined limits. 

Particularly noteworthy examples are Entries 8 and 9 in Table 3.19 these represent a diastereoselective RCM in which the stereoselectivity is controlled by the catalyst [886]. Entries 17, 23 and 24 (Table 3.19) illustrate the use of RCM for the solid-phase synthesis of lactams [894]. RCM induces both ring closure to the lactam and cleavage from the support. Although elegant at first glance, the usefulness of this methodology will be limited if the products must be used without further purification (as is usually the case for compound libraries prepared by parallel synthesis). Because relatively large amounts of catalyst are required, the crude products will only be acceptable for assays in which transition metal complexes do not interfere. 

Although a-D-mannosidase from mammalian, plant, and molluscan sources is dependent upon zinc for its catalytic activity, the addition of this ion has a marked effect in the enzyme assay only at those pH values where the active, protein-metal complex dissociates appreciably despite the presence of substrate. (Dissociation, which is greater at lower values of pH, is lessened in the presence of substrate.) The presence of zinc ion in the assay (0.1 mM) is thus of particular importance in the case of the limpet enzyme, where the pH of optimal activity is 3.5. Jack-bean and rat-epididymal a-D-mannosidase are both assayed at pH 5, and up to 10% activation may be observed with zinc. 

With the exception of the enzyme from the limpet, P. vulgata, a-D-mannosidase from most of the important sources shows optimal activity at pH values lying between 4 and 5. For the enzyme from jack-bean meal39 and that from rat epididymis,80 we employed a pH of 5 for routine assays. If this is not the actual optimum, it is close to it on the broad pH-activity curves, and, at this pH, the addition of Zn2+ and other cations has relatively little effect in the assay, thus simplifying the study of the various metal complexes that can be formed by the enzyme protein. 

The position with regard to the limpet enzyme is more complicated.46 Although zinc has little effect in the assay at pH 5, this pH is so far removed from the sharp optimum at pH 3.5 (Cl- present) that the enzyme displayed less than 20% of its activity. Furthermore, a large proportion of the enzyme seemed to be firmly complexed with toxic cations derived from the limpet, and these were not displaced by addition of an excess of zinc during an assay at pH 5 (see Fig. 1). At its relatively low pH of optimal activity, the limpet protein-metal complex was readily dissociable, even in the presence of substrate, and, consequently, on assay at this pH, Zn2+ seemed to displace toxic cations, allowing the enzyme to display its full, potential activity. 

One of the most striking indications of the importance of Zn2+ for a-D-mannosidase activity was obtained with preparations that had been inactivated by incubation with EDTA. On addition of an excess of Zn2+ to the assay mixture, complete activity was regained instantaneously, regardless of the extent of prior inactivation. (When the EDTA-inactivated enzyme described in Fig. 4 was assayed in the presence of Zn2+, the points followed the line for the Zn2+-stabilized enzyme.) Again, no other cation that we have examined can replace Zn2+, leaving little doubt as to the identity of the activating cation in the original material. It also follows that EDTA must withdraw Zn2+ from the protein-metal complex. Had EDTA merely formed a... 

Today, testing for antiproliferative activity is usually done in colorimetric cell culture assays, such as the MTT assay (MTT 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide). As described in the introduction, the combination of cell-targeting peptides with cytotoxic metal complexes has the potential of providing more specific drug candidates with less side effects. However, this concept has so far only rarely been realized. 

The ability of metal complexes to unwind DNA has been put forth as an important criterion for proving an intercalative binding mode and has been observed with other complexes of phen, dppz, and phi (21, 28,30,33). The enzyme topoisomerase can be used to determine if small molecules unwind DNA, according to published procedures (28). We find that by using this assay, Ru(tpy)(dppz)OH22+ unwinds DNA by 17°, which is consistent with intercalative binding. 

The use of the HPLC method to assay the activity of an enzyme may require some modifications in the composition of the reaction mixture. For example, the presence of metals in the reaction mixture can cause problems, since a metal complex may form and produce new peaks on the chromatogram. Complications associated with the requirement for termination of the reaction and for dealing with the small amounts of product that appear during early stages of the reaction may require changes in the reaction conditions as well. 

Well glass plates were loaded with different phosphines, metal complexes, aniline, and 1,3-cyclohexadiene. A coloro-metric assay for unreacted aniline (a red color arising from the acid catalyzed reaction of aniline with furfural) was used to screen successfiil catalysts, that is, those that did not turn red. Catalysts derived from [Pd(7r-allyl)Cl]2 and PPhs were most active. Increasingly, such high-throughput methods are being used in catalyst discovery and optimization. 

A novel ECL assay for the determination of 2,4- and 3,4- diaminotoluene (DAT) isomers is based on reaction of these molecules with Au" and Cu +, respectively, in aqueous solution under oxidizing conditions in buffer-containing tripropylamine [72]. Luminescence is observed upon potential ramping from 0 to -1-2.8 V. The nature of the emitting species was not specified, but could involve a charge-transfer excited state of the metal complex with DAT or an oxidized form of DAT. DAT isomers were screened for ECL enhancement against 32 metals the apparent specificity of Au+ for 2,4-DAT and Cu + for 3,4-DAT is believed to be linked to the radii of each ion. This ECL approach could lead to applications in the determination of some aminoaromatics from degradation of explosives (e.g., TNT) as well as detection and quantifiation of various transition metals in water supplies. 

Metal chelating assay Percentage of inhibition of ferrozine-Fe + complex formation... 

Two further analytical methods are based on the formation of metal complexes. Reaction of dopamine with KzCrjOy gives a Cr complex which can be assayed by atomic absorption spectroscopy, carbon rod absorption spectroscopy or, if Kf CrjO has been used, by liquid scintillation spectroscopy. Since the complex still gives a reaction with fluorescamine, the side-chain is thought not to be involved in the formation of the complex. Methamphetamine hydrochloride can be precipitated as a Bi complex. Determination of the amount of Bi remaining in solution by atomic absorption spectroscopy provides an indirect method of assay for the amphetamine. ... 

A library of 83 metalloporphyrins was generated by parallel synthesis.75 The substitution pattern was varied at the meso position of the porphyrines, and the central metal ions were altered (Table 14.3). To investigate the use of the members of the library as electrocatalytic material for sensitive sensors for nitric oxide (NO), a complex assay sequence was fulfilled by means of the electrochemical robotic system (Fig. 14.24). In first step, residual water was removed... 

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