Kinetics and Mechanism of the Kmn04-Oxidative Catalysed Condensation Reaction of L-Proline to a Diketopiperazine: Evidence for Delayed Autocatalytic Behaviour

The kinetics and mechanism of the KMn04-oxidative catalysed condensation reaction of L-proline, as a cyclic secondary amino acid, has been investigated spectrophotometrically. Product analysis has revealed, that in contrast to the usual reaction of α-amino acids, which results in an aldehyde species, a valuable diketopiperazine, namely cyclo(L-pro-L-pro), is obtained as the main product, providing a cheap, simple, efficient, and novel method. Conclusive evidence has demonstrated delayed autocatalytic activity involving Mn2+in this reaction, analogous to that shown by chain primary a-amino acids. It has been revealed that such activity can emerge when a certain concentration of Mn2 +is built up in the medium, to which we refer as the “critical concentration”. Moreover, the “critical concentration” for the appearance of “autocatalytic behaviour” in this oxidative process is affected by the nature of the amino acid used, as reported in previous studies, while the magnitude of the critical concentration depends on the sulfuric acid concentration. In considering “delayed autocatalytic behaviour” of Mn2+ions, rate equations satisfying experimental data for both catalytic and non- catalytic routes have been presented. The reaction shows a first-order dependence on permanganate ion concentration, both in the catalytic and non-catalytic pathways, and an apparent first-order dependence on Mn2 +ions in the catalytic pathway. The correspondence of the pseudo-order rate constants of the catalytic and non-catalytic pathways to the Arrhenius and Eyring laws verified both the “critical concentration” and “delayed autocatalytic behaviour” concepts. The activation parameters associated with the reaction pseudo-rate constants k, and k2are computed and discussed. Proposed mechanisms for both catalytic and non- catalytic routes include a diacyclperoxide intermediate, which leads to a C-2-symmetric chiral product having a diketopiperazine skeleton.

Diketopiperazines in peptide and combinatorial chemistry

Diketopiperazines (DKPs), the smallest cyclic peptides, represent an important class of biologically active natural products and their research has been fundamental to many aspects of peptide chemistry. The advent of combinatorial chemistry has revived interest in DKPs for two reasons: firstly, they are simple heterocyclic scaffolds in which diversity can be introduced and stereochemically controlled at up to four positions; secondly, they can be prepared from readily available alpha-amino acids using very robust chemistry. Here synthetic methods, conformation, as well as applications of DKPs are summarized and discussed critically.

13C-nuclear magnetic resonance studies of 85% 13C-enriched amino acids and small peptides. pH effects on the chemical shifts, coupling constants, kinetics of cis-trans isomerisation and conformation aspects

The 13C chemical shifts of several 85% 13C-enriched amino acids and small peptides were studied as a function of pH. The results show that the chemical shifts of carbon atoms of ionizable groups vary significantly within the zone of their pK. Generally with the pH GOING FROM 7 to 1 all the deltaC are shifted more or less upfield with the exception of the carbonyl group carbon of the second last residue which is shifted slightly downfield. This suggests the formation of an hydrogen bond at acid pH involving in a seven-membered ring the C=O in question and the COOH terminal. The percentage of cis and trans conformers of glycyl-L-proline and glycyl-L-prolylglycine were studied as a function of pH. The trans form is always preponderant whatever the pH. The accessibility of the carbonyl group to protonation of the proline residue strongly influences the cis-trans equilibrium. Thus, with the pH varying from 7 to 1, the trans isomer changes from 61 to 85% for glycyl-L-proline and only from 77 to 80% for glycyl-L-prolylglycine. The proton NMR studies underline the important differences existing between the two molecular forms of glycyl-L-proline. The cis conformation is characterized with regard to the trans form by the non-equivalence of the alpha-protons of the glycine residue, by a lower pK(1) and by a larger deltadeltaHalpha of the proline residue as a function of pH. These results could suggest an end-to-end interaction in the cis form of the glycyl-L-proline molecule. The 13C-13C coupling constants were also studied as a function of pH. The results show that J(Co-Calpha) of a C-terminal residue, varying from 5 to 6 Hz and reflecting thhe pK of the carboxylate group, is a linear function of delta(Co) and delta(Calpha) as in the case of the amino acids. The total variation of the electron density of those two carbons in an amino acid is approximately 40% weaker than in a C-terminal residue. The charge distribution along the Calpha-C(o) bond, however, is practically the same in both cases. Finally the ratios of the conversion rate constants of the two isomers cis-trans of glycyl-proline were calculated at different pH values; the relations between the isomer percentages and delta(Co), delta(Calpha) on the one hand and the J(Co-Calpha) on the other were established.