Hydrolysis of amino acids

The discovery that metal ions such as Cu catalyzed the hydrolysis of amino acid esters was first reported nearly 40 years ago. Table 6.3 shows that in the absence of direct interaction of the ester grouping with the metal ion, the primary cause of any increased rates experienced... 

Table 6.3 Effect of Metal Coordination on Rate Constants for Base Hydrolysis of Amino Acid Esters at 25 °C...

Metal-ion catalysis has been extensively reviewed (Martell, 1968 Bender, 1971). It appears that metal ions will not affect ester hydrolysis reactions unless there is a second co-ordination site in the molecule in addition to the carbonyl group. Hence, hydrolysis of the usual types of esters is not catadysed by metal ions, but hydrolysis of amino-acid esters is subject to catalysis, presumably by polarization of the carbonyl group (KroU, 1952). Cobalt (II), copper (II), and manganese (II) ions promote hydrolysis of glycine ethyl ester at pH 7-3-7-9 and 25°, conditions under which it is otherwise quite stable (Kroll, 1952). The rate constants have maximum values when the ratio of metal ion to ester concentration is unity. Consequently, the most active species is a 1 1 complex. The rate constant increases with the ability of the metal ion to complex with 2unines. The scheme of equation (30) was postulated. The rate of hydrolysis of glycine ethyl... 

Reactions of the ligand which are very slow in the absence of an electron pair acceptor or metallic coordination center. Basic hydrolysis of amino acid esters is an example of such a reaction. 

Similarly, the rates of hydrolysis of amino acid esters are enhanced by the presence of metal ions (Equation 6) (6, 7, 48, 77). 

Hay, R. W., and P. J. Morris, Metal ion-promoted hydrolysis of amino acid esters and peptides . In Metal Ions in Biological Systems, Vol. 5, H. Siegel, Ed., Marcel Dekker, New York, 1976, pp. 173-243. 

Sutton, P. A., and D. A. Buckingham, Cobalt(III)-promoted hydrolysis of amino acid esters and peptides and the synthesis of small peptides , Acc. Chem. Res., 20, 357-364 (1987). 

One reason for an otherwise apparently excessive interest in Co(trien)X2+ systems is the use of ds-Co(OH)(trien)(OH2)2+ in the hydrolysis of amino acid esters, amino acid amides and peptides785 to form cis-px- and cis-/J2-Co(trien)(aa)2+ (aa = amino acid) complexes.16 In principle, a peptide could be degraded in a stepwise manner and each amino acid residue successively characterized. By the introduction of a chiral center into the backbone of the trien moiety, it was hoped to make such reactions stereoselective. Consequently, while fully A-alkylated trien systems have been widely investigated for M11 central ions, the C-alkylated trien systems have been almost exclusively the reserve of the Co111 chemist (Table 11). 

The ability of metal ions to catalyze the hydrolysis of peptide bonds has been known for 50 years, while the catalytic effect on the hydrolysis of amino acid esters was highlighted in the 1950s. As Hay and Morris point out in their review,76 the major problem with the kinetically labile systems is determining the nature of the reactive complex in solution. Such problems generally do not arise in the more inert systems and consequently reactions involving Co111 have been the more popular for study. 

THE HYDROLYSIS OF AMINO ACID ESTERS, AMIDES AND PEPTIDES 414... 

The metal ion-catalyzed hydrolysis of amino acid esters and peptides has been a subject of continuing interest over the past three decades. In the early 1930s the hydrolysis of peptides was... 

Zlojkowska, Z., H. J. Krasuka, and J. Pachecka. 1982. Enzymatic hydrolysis of amino acid derivatives of benzocaineXenobiotical 2 359-364. 

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. 

The rates of hydrolysis of amino acid esters or amides are often accelerated a million times or so by the addition of simple metal salts. Salts of nickel(n), copper(n), zinc(n) and cobalt(m) have proved to be particularly effective for this. The last ion is non-labile and reactions are sufficiently slow to allow both detailed mechanistic studies and the isolation of intermediates, whereas in the case of the other ions ligand exchange processes are sufficiently rapid that numerous solution species are often present. Over the past thirty years the interactions of metal ions with amino acid derivatives have been investigated intensively, and the interested reader is referred to the suggestions for further reading at the end of the book for more comprehensive treatments of this interesting and important area. 

Figure 3-7. Hydrolysis of amino acid esters and amides.

Although the above discussion has concentrated upon the hydrolysis of amino acid esters, very similar mechanisms have been demonstrated for the hydrolysis of amino acid amides. A very wide range of intramolecular reactions of this type are now known to occur by intramolecular attack by hydroxide, with most having been demonstrated at non-labile... 

Figure 5-68. The labelling experiment that distinguished between the various pathways for hydrolysis of amino acid esters. The site of the label may be determined by IR spectroscopy or other methods. Pathway A involves co-ordinated hydroxide nucleophile and pathway B, external hydroxide. Both pathways are found to be important for cobalt(m).

Similar mechanisms may be proposed for the hydrolysis of amino acid esters and amides co-ordinated to labile metal centres such as copper(n) or nickel(n), although mechanistic studies at these centres are much more difficult to perform in view of the rapidity of ligand exchange processes. Further complications arise from the formation of insoluble or colloidal suspensions of metal oxides and hydroxides at higher pH values. In general,... 

Wieland et al. that the hydrolysis of amino acid active esters is catalyzed by aqueous bicarbonate buffers [ 134] and indications of the intermediacy of N-carboxyanhydrides have been provided [127,134-136]. The proposed mechanism (Scheme 36) is very similar to the catalytic pathway of a-aminonitrile hydration analyzed in Sect. 2.1.3. 

I. A. Yamskov, T. V. Tichonova, V. A. Davankov, Pronase-catalyzed hydrolysis of amino acid amides, Enzyme Microb. Tech., 8 (1986), 241-244. 

Co(trien)(NH3)2] + has been isolated, and only the meso trans isomers (197-198) with two different axial ligands, remain to be distinguished. There is also an extensive chemistry of N and C-alkylated derivatives of (178 180) as cA-[Co(OH)(trien)(OH2)] assists the hydrolysis of amino acid esters, amino acid amides, and peptides to form cis-fi (194) and cA-jS2-[Co(OA0(trien)] + (195)(( A = amino acid) complexes. Chiral alkylated trien ligands have the potential for chiral stereospecificity in such reactions. 

Metal-Mediated Hydrolysis of Amino Acid Esters and Peptides... 

When amines are snbstitnted for hydroxide as the base, there is no change in the rate of hydrolysis. This is becanse amines do not catalyze the hydrolysis of amino acid esters instead they prefer to add directly to the carbonyl carbon to form the corresponding amides. In this reaction the rate-limiting step is not the addition of amine, bnt rather the deprotonation of the coordinated amine by another base (eqnation 8). The of the tetrahedral intermediate is approximately 7 since almost any nonsterically hindered base with a pTTa above 7 can deprotonate it. " ... 

Owing to the snccess of cobalt(III)-mediated hydrolysis of amino acid esters, the next step was to examine how these complexes reacted with peptides. If similar hydrolytic results could be obtained with peptides, then one of the potential uses of cobalt(III) complexes would be in the N-terminal determination and seqnential analysis of polypeptides. This area has been investigated by several gronps. Peptides... 

The rate of hydrolysis of peptides by cobalt(III) complexes is 10" times faster than hydrolysis with no metal present. " Unlike the hydrolysis of amino acid esters, where the rate of hydrolysis is dependent on how the ester is bonded to the cobalt(III) complex, peptides are hydrolyzed equally if they are bound to the cobalt(III) complex in a monodentate fashion (throngh the carbonyl oxygen) or in a bidentate fashion (throngh the amino nitrogen and carbonyl oxygen). 

The hydrolysis of amino acid esters by directly coordinated hydroxy or water molecules is slower than the corresponding... 

---