Hydrolysis amino acid ester chelates

Amino acid esters act as chelates to Co111 for example, the /3-alanine isopropyl ester is known as both a chelate and as an /V-bonded monodentate,983 and the mechanism of hydrolysis of the ester, which is activated by coordination, to yield chelated /3-alanine has been closely examined. 

Rate Constants (km0, kon) for Intermolecular Hydrolysis of Some Free and Co(III)-Chelated Amino Acid Esters at 25°C, I = 1.0 M... 

These chelates are structurally similar to that postulated above for the metal ion-catalyzed hydrolysis of oj-amino esters the position of the protons in the transition state is different, but this is a completely arbitrary distinction. A further distinction is that the metal ion is facilitating attack in this instance not by a polarization of the substrate molecule, but rather by the positioning and fixation of the hydroxide ion at the reaction site. It is not clear which of these two representations—for the amino acid esters involving polarization or for the carboxylate esters involving fixation of the hydroxide ion—is the correct interpretation. It is conceivable that both are correct. A similar explanation will account for the large effect of calcium ions on the alkaline hydro ysis of acetylcitric and benzoyl-citric acids (53). 

The metal-accelerated hydrolysis of amino acid esters or amides comprises one of the best investigated types of metal-mediated reaction (Fig. 3-7). One of the reasons for this is the involvement of chelating ligands, which allows chemical characterisation of intermediates and products in favourable cases, and allows detailed mechanistic studies to be made. The reactions have obvious biological relevance and may provide good working models for the role of metals in metalloproteins. 

In the case of inert cobalt(m) complexes it is possible to isolate the chelated products of the reaction. Let us return to the hydrolysis of the complex cations [Co(en)2(H2NCH2C02R)Cl]2+ (3.1), which contain a monodentate iV-bonded amino acid ester, that we encountered in Fig. 3-8. The chelate effect would be expected to favour the conversion of this to the chelated didentate AO-bonded ligand. However, the cobalt(iu) centre is kinetically inert and the chloride ligand is non-labile. When silver(i)... 

Figure 3-9. The stepwise hydrolysis of an amino acid ester. The labilisation of the chloride by interaction with silver(i) is a crucial prerequisite to the formation of the reactive chelated AO-bonded ligand.

The ability of a metal ion to increase the rate of hydrolysis of a peptide has enormous implications in biology, and many studies have centred upon the interactions and reactions of metal complexes with proteins. However, hydrolysis is not the only reaction of this type which may be activated by chelation to a metal ion, and chelated esters are prone to attack by any reasonably strong nucleophile. For example, amides are readily prepared upon reaction of a co-ordinated amino acid ester with a nucleophilic amine (Fig. 3-11). In this case, the product is usually, but not always, the neutral chelated amide rather than a depro-tonated species. 

There are a number of useful synthetic applications of these reactions of chelated amino acid esters (Fig. 3-12). For example, if the attacking nucleophile is not a simple amine, but is another amino acid ester or an O-protected amino acid, then peptide or polypeptide esters are formed in excellent yields. This may be developed into a general methodology for the metal-directed assembly (and, in the reverse reaction, the hydrolysis) of polypeptides. 

We saw in Chapter 3 that the hydrolysis of chelated amino acid esters and amides was dramatically accelerated by the nucleophilic attack of external hydroxide ion or water and that cobalt(m) complexes provided an ideal framework for the mechanistic study of these reactions. Some of the earlier studies were concerned with the reactions of the cations [Co(en)2Cl(H2NCH2C02R)]2+, which contained a monodentate amino acid ester. In many respects these proved to be an unfortunate choice in that a number of mechanisms for their hydrolysis may be envisaged. The first involved attack by external hydroxide upon the monodentate A-bonded ester (Fig. 5-62). This process is little accelerated by co-ordination in a monodentate manner. 

Metal chelating amino acid derivatives of cellulose were recently obtained via modification of cellulose with 2,4-toluenediisocyanate, followed by treatment with amino acid ester derivatives [58,59]. Diisocyanates are able to crosslink cellulose chains and/or to yield reactive cellulose isocyanate, depending on the reaction conditions. Sato and his coworkers [60] examined the optimum conditions for the reaction between cellulose and 2,4-toluenediisocyanate and succeeded in introducing 0.30 mol of free isocyanate group per glucose unit. Cellulose isocyanate was further converted into isothiocyanate [61]. This derivative has also been synthesized by condensation of cellulose with 2,4-diisocyanototoluene, followed by hydrolysis and thiophosgene treatment [61]. 

When chelation contributes to the binding of the substrate to (or stabilization of the product by) the metal ion as in the catalysis by various metal ions (M = Cu, Ni, Mg, etc.) of the hydrolysis of amino acid esters according to the mechanism ... 

A number of amino acids esters that can form chelates with metal ions have been found to be readily hydrolyzed, whereas the hydrolysis of esters having only one oxygen donor atom per molecule was found to be comparatively unaffected by metal ions. In the examples of ester hydrolysis described below, it is postulated that a metal chelate ring system is formed and undergoes hydrolysis via a nucleophilic process. 

A kinetic investigation of the hydrolysis of 8-acetoxyquinoline in solutions of pH between 1 and 9, and in the presence and absence of copper(II) ions, yielded some interesting results (205). In the absence of copper(II), it was found that the rate of hydrolysis was first-order with respect to 8-acetoxyquinoline, but the rate equation that fitted the kinetic data was quite complex since the ester and the ester cation reacted with both the hydrogen and the hydroxide ion. In the presence of copper(II), the hydrolysis of the ester occurred more rapidly and the rate equation was found to be first-order with respect to 8-acetoxyquinoline copper(II) and hydroxide ion. Therefore, the reaction intermediate (structure XXXVII) is presumably a 1 1 chelate of copper(II) which is attacked by hydroxide ion, just as in the case of the amino acid ester. 

Metal ion catalysis by the direct polarization mechanism can accelerate hydrolysis rates by factors of Kf or greater (Buckingham, 1977). For example, the Cu " -catalyzed hydrolysis of a-amino acid esters occurs at a rate that is six orders of magnitude faster than the uncatalyzed process (Bender and Brubacher, 1973 Hay and Morris, 1976). The metal ion chelate is thought to have the following structure ... 

The cobalt(III)-promoted hydrolysis of amino acid esters and peptides and the application of cobalt(III) complexes to the synthesis of small peptides has been reviewed. The ability of a metal ion to cooperate with various inter- and intramolecular acids and bases and promote amide hydrolysis has been investigated. The cobalt complexes (5-10) were prepared as potential substrates for amide hydrolysis. Phenolic and carboxylic functional groups were placed within the vicinity of cobalt(III) chelated amides, to provide models for zinc-containing peptidases such as carboxypeplidase A. The incorporation of a phenol group as in (5) and (6) enhanced the rate of base hydrolysis of the amide function by a factor of 10 -fold above that due to the metal alone. Intramolecular catalysis by the carboxyl group in the complexes (5) and (8) was not observed. The results are interpreted in terms of a bifunctional mechanism for tetrahedral intermediate breakdown by phenol. 

The hydrolysis of chelated amino acid esters, H2NCHRCO2R, is known to be accelerated by metal ions, most notably cobalt(III). Dramatic enhancements are also observed with copper(II). Mechanistic studies of the hydrolysis of amino acid esters with copper(II) complexes of glycyl-DL-valine and dien (H2HCH2CH2NHCH2CH2NH2) have been reported/ The hydrolysis of benzyl-penicillin (30) by copper(II) salts to give (31) has been further investigated, and it is proposed that the key step involves intramolecular attack by metal-coordinated hydroxide in an intermediate of type (32). 

Amino-acid esters may be stereoselectively hydrolysed at pH 8 by polymeric catalysts containing amino-acid derivatives chelated to Cu" or Ni" attached to matrices of cross-linked polystyrene or polyacrylamide. Selectivity results from amino-acid ester co-ordination rather than from the affinity of the catalyst for the hydrolysis product. ... 

Co(II) and Cu(II) ions can promote the hydrolysis of glycine ethyl esters at pH 7 to 8, 25 C, conditions under which they are otherwise stable. Com-plexation takes place between the metal ion (M " ) and the amino acid ester to form a five-membered metal chelate. Subsequently, catalysis occurs as a result of the coordination of the metal ion with the amino and ester functions of the amino acid. In either case the metal ion can polarize the carbonyl group, thereby promoting attack of OH". The rate of hydrolysis increases with pH showing that OH" ion participates in the mechanism. Thermodynamically the hydrolysis occurs presumably because the carboxylate anion formed coordinates more strongly to the metal cation than the starting ester. 

Many inhibitors may be present in biological samples or in buffers currently used for EIA. Pi is a competitive inhibitor of the enzyme and forms an intermediate with the enzyme which is indistinguishable from the intermediate formed during catalysis of the hydrolysis of phosphate esters (Caswell and Caplow, 1980). The K, (i.e. the for Pi) is lower than the for the substrate, typically KijK, = 0.3, i.e. a lower concentration of Pi than of the substrate is required to half-saturate the enzyme. Arsenate is a stronger competitive inhibitor than Pi, whereas phosphonates are weaker. Metal chelating products (EDTA, cysteine, thioglycolic acid) are also important inhibitors. Many amino acids show a mixed competitive or uncompetitive inhibition (Fernley, 1971). 

An asymmetric synthesis of amino alcohols by asymmetric addition of Grignard reagents to chiral a-bromoglycine esters provides a convenient synthesis of a-amino esters (Scheme 4.8, [99]). Hydrolysis of the product ester produces racemized amino acids, but reduction affords amino alcohols that can be subsequently oxidized to the amino acids with no loss of enantiomeric purity. Note that in the proposed transition structure, the phenyl effectively shields the Re face (toward the viewer) of the imine, which is chelated to the carbonyl by magnesium halide formed in the dehydrohalogenation. 

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