Design, Synthesis, and Evaluation of Amphiphilic Cyclic and Linear Peptides Composed of Hydrophobic and Positively-Charged Amino Acids as Antibacterial Agents

Abstract

Antimicrobial peptides (AMPs) contain amphipathic structures and are derived from natural resources. AMPs have been found to be effective in treating the infections caused by antibiotic-resistant bacteria (ARB), and thus, are potential lead compounds against ARB. AMPs’ physicochemical properties, such as cationic nature, amphiphilicity, and their size, will provide the opportunity to interact with membrane bilayers leading to damage and death of microorganisms. Herein, AMP analogs of [R4W4] were designed and synthesized by changing the hydrophobicity and cationic nature of the lead compound with other amino acids to provide insights into a structure-activity relationship against selected model Gram-negative and Gram-positive pathogens. Clinical resistant strains of methicillin-resistant Staphylococcus aureus (MRSA) and Escherichia coli (E. coli) were used in the studies. Our results provided information about the structural requirements for optimal activity of the [R4W4] template. When tryptophan was replaced with other hydrophobic amino acids, such as phenylalanine, tyrosine, alanine, leucine, and isoleucine, the antibacterial activities were significantly reduced with MIC values of >128 µg/mL. Furthermore, a change in stereochemistry caused by d-arginine, and use of N-methyltryptophan, resulted in a two-fold reduction of antibacterial activity. It was found that the presence of tryptophan is critical for antibacterial activity, and could not be substituted with other hydrophobic residues. The study also confirmed that cyclic peptides generally showed higher antibacterial activities when compared with the corresponding linear counterparts. Furthermore, by changing tryptophan numbers in the compound while maintaining a constant number of arginine, we determined the optimal number of tryptophan residues to be four, as shown when the number of tryptophan residues increased, a decrease in activity was observed.

Stability Is Not Everything: The Case of the Cyclisation of a Thrombin‐Binding Aptamer

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With the aim of developing an ew approacht oo btain improveda ptamers, ac yclic thrombin-binding aptamer (TBA) analogue (cycTBA) has been prepared by exploiting ac opper(I)assisted azide-alkyne cycloaddition. This approach has been extensivelya dopted in the past to improvet he general properties of peptides [9] and peptidomimetics, [10] as well as peptide nucleic acids (PNAs) [11] and glycomimetics, [12] but has only been applied in al imited extent to oligonucleotides, [13] in general, and, to the best of our knowledge, is essentially unexploited thus far on aptamers.Herein, we report the design, synthesis and biophysical characterisation of an unprecedentedc yclic TBA analogue, herein namedc ycTBA (Figure 1), whichh as been realiseda saproof of conceptt ov alidate the efficacy of cyclisation approaches appliedt oa ptamers. The 15-mer, G-rich, oligonucleotide thrombin-binding aptamer (TBA 15 or simply TBA), which contains the sequence 5'-d(GGTTGGTGTGGTTGG)-3',i s the bestcharacterisedaptamer of thrombin.

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“…This approach has been extensivelya dopted in the past to improvet he general properties of peptides [9] and peptidomimetics, [10] as well as peptide nucleic acids (PNAs) [11] and glycomimetics, [12] but has only been applied in al imited extent to oligonucleotides, [13] in general, and, to the best of our knowledge, is essentially unexploited thus far on aptamers. This approach has been extensivelya dopted in the past to improvet he general properties of peptides [9] and peptidomimetics, [10] as well as peptide nucleic acids (PNAs) [11] and glycomimetics, [12] but has only been applied in al imited extent to oligonucleotides, [13] in general, and, to the best of our knowledge, is essentially unexploited thus far on aptamers.…”

“…This approach has been extensivelya dopted in the past to improvet he general properties of peptides [9] and peptidomimetics, [10] as well as peptide nucleic acids (PNAs) [11] and glycomimetics, [12] but has only been applied in al imited extent to oligonucleotides, [13] in general, and, to the best of our knowledge, is essentially unexploited thus far on aptamers.…”

Click-Free Synthesis of a Multivalent Tricyclic Peptide as a Molecular Transporter

“…Cyclic peptides, in particular, are considered as an emerging class of antimicrobial peptides against antibiotic-resistant pathogens [ 42 , 43 ]. c [R 4 W 4 ] is a cyclic amphiphilic peptide exhibited potent antibacterial activity against MRSA (MIC 4 µg/mL) and E. coli (MIC 16 µg/mL) and showed improved antibacterial activity when co-administered with tetracycline [ 23 , 24 ]. The structure-antibacterial activity of a series of c [R 4 W 4 ] peptides revealed the requirement of R and W amino acids [ 24 ].…”

“…We also discovered that a block of four subsequent tryptophan and positive-charged arginine residues generated amphipathic antibacterial peptide c [R 4 W 4 ], which demonstrated minimum inhibitory concentration (MIC) of 4 µg/mL against methicillin-resistant Staphylococcus aureus (MRSA) and 16 µg/mL against E. coli [ 23 ]. Furthermore, the structure-activity relationship showed the importance of tryptophan and arginine residues in this pattern for antibacterial activity [ 24 ].…”