الفهرس 
Chemistry of Amino Acids and Peptides: A Global ViewOnly 14 pages are availabe for public view
 |  | from | 241 |  |  |
| | | from | 241 | | |
Abstract
In the present thesis five analogues of Polymyxin E1 antibiotic were synthesized via Fmoc-modified solid phase peptide synthesis (SPPS) strategy using Biotage® Initiator+Alstra™ microwave peptide synthesizer. Among all the research studies discussing the synthesis of various analogues of polymyxin antibiotics, this work represents the first one focusing on the replacement of five toxic Dab residues in both the cyclic heptapeptide moiety and the tripeptide tail of Polymyxin E1, with a natural amino acid, Arg. The only Dab4 residue involved in lactam cyclization was substituted with another natural amino acid, Lys. Additionally modifications of the tripeptide tail part have been introduced into two analogues.
These compounds were cyclized using different techniques, either side-chain to C-terminal in solution via L-Lys4, L-Thr10, in analogues (I-III) or side-chain to side-chain cyclization on polymeric support via L-Lys4, L-Asp10 in analogues (IV &V) to investigate the influences of different cyclization conditions on the purity and yield of the final products. On polymeric support cyclization has emerged as a powerful approach for the synthesis of cyclopeptidomimetics, which perform the cyclization reaction while the linear peptide is still anchored on the resin.
The use of HMBA base labile linker resembles the key factor in cyclization step in both solution and solid phase cases owing to its ability to partial acid deprotection of peptide chain using TFA while keeping peptide chain still attached to the polymeric support. Therefore HMBA linker enables side-chain to side-chain cyclization on polymeric support through partial acid deprotection of Nε-terminal Boc of Lys and the OtBu of β-carboxylic side chain of Asp in one step in analogues (IV &V) and it was reflected in our results as there are 27% increase in the yield and 13% increase in purity, as shown in the HPLC diagrams and was demonstrated by the better consistency of the compounds. Additionally on polymeric support cyclization assisted with microwave irradiation is clearly accelerated the rate of cyclization reaction which completed within only 10 minutes in contrast to time-consuming conventional cyclization.
In this work, the use of different coupling agents was also investigated. Cyclization was performed using three different coupling reagents, EDC.HCl condensing agent in case of analogues (I-II), BOP in case of analogue (III) and DIC condensing agent in case of analogues (IV&V) under heterogenous conditions. DIC is the most convenient coupling agent for cyclization on polymeric support in analogues (IV&V) as its diisopropyl urea is slightly more soluble in dichloromethane, which therefore, renders it easier to wash off from the solid support but whenever cylization in solution would be the only option available, the coupling agent of choice would be BOP as its use improved the purity and yield of the synthesized compound (III).
The manual synthesis of analogue I using slow batch technique allowed to optimize the synthetic methods of polymyxin E1 analogues and it was clearly in case of difficult residues (Arg,L-Leu,D-Leu) coupling which necessitated the recoupling with additional fivefold excess of activated Fmoc-amino acid residues in order to obtain the required sequences and suppress the formation of failure sequences. Also deprotection reactions varied between 35 to 75 mins, particularly increasing in case of the difficult sequences.
The results of spectroscopic analysis of synthesized compounds were in accordance with the predicted results.
Pharmacophore mapping approach was applied to describe some preliminary structure features insights on our new candidates. Concerning to identify the possible correlations between the obtained molecular modeling data and the determined antibacterial activities of the candidates were equivalent linked. The molecular modeling approach indicated that our synthesized compounds showed low affinities to the generated hypothesis, consistent with the absence of antibacterial activity following biological evaluation. The loss of antibacterial activity of synthesized analogues is mainly related to the size of lactam ring as an extended ring to 25 atom in analogues (I,II) and 26 atom in analogues (IV&V) while decrease to 22 atom in case of analogue III may disturbed the scaffold required for electrostatic and hydrophobic LPS contact points.
Suggestion for new effective analogues could be proposed based on the molecular modeling approach and our current work. Therefore it should comply indispensable characteristics including 23 atom ring size, three cationic charges mainly within heptapeptide ring and lipophilic Fmoc group.