Dipeptide formation, undesired reaction

Dioeotide Formation. Figure 2 presents the time course of the dipeptide formation reaction. The limiting reactant, Z-Tyr-OMe, is depleted rapidly, being converted via nucleophilic attack of the glycinamide to the desired dipeptide product, Z-Tyr-Gly-NH2 via trans-esterification to a transient intermediate, Z-Tyr-OHx and via hydrolysis to the undesired side product Z-Tyr-OH. Dipeptide yield, based on the more expensive amino acid substrate, Z-Tyr-OMe, is 75% after 30 minutes using an enzyme concentration of 1 /iM, Both the... 

Diketopiperazine (DKP), a cyclic dipeptide easily formed by the cyclodimerization of amino acid esters. The formation of diketopiperazines is an undesired cycliza-tion reaction by incorporation of the third amino acid by stepwise SPPS. The free amino group of the resin-bound dipeptide can attack the peptide-resin anchorage intramolecularly, resulting in the formation of diketopiperazine [J. R. Spencer etal., Int. J. Pept. Protein Res. 1992, 40, 282]. 

Peptide synthesis is quite regularly accompanied by undesired side reactions, for instance by the alkylation of side chains during deprotection. In addition to such general problems for which at least partial remedies have been found, also a special problem is encountered in solid phase syntheses the formation of deletion peptides and truncated sequences . Both incomplete acylation and incomplete deprotection result in peptide chains from which one or more amino acid residues are missing. A second kind of deletion, the loss of the C-terminal dipeptide sequence, is caused by diketopiperazine formation followed by acylation of the resulting hydroxymethyl polymer (Fig. 8). Premature chain termination (truncation) takes place if acetic acid or trifluoroacetic acid are not completely removed after deprotection and then co-activated in the following coupling step. 

The dipeptide ester L-Asp-L-Phe-OMe (Aspartame) is used in large amounts as a low-calorie sweetener. One of the most economical strategies for its synthesis involves an enzymatic step, which is run at a capacity of 2,000 t ear (Scheme 3.25). Benzyloxycarb(Miyl-(Z)-protected L-aspartic acid is linked with L-Phe-OMe in a thermodynamically cOTitroUed cmidensation reaction catalyzed by the protease thermolysin without formation of the undesired (bitter) (3-isomer. Removal of the product via formation of an insoluble salt was used as driving force to shift the equilibrium of the reaction in the synthetic direction [318, 319]. 

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