Side-chain effect on conformation of ionizable polypeptides in aqueous solution

Backbone Cyclization and Dimerization of LL-37-Derived Peptides Enhance Antimicrobial Activity and Proteolytic Stability

Can antimicrobial activity and peptide stability of alpha-helical peptides be increased by making them into dimers and macrocycles? Here, we explore that concept by using KR-12 as the starting point for peptide engineering. KR-12 has previously been determined as the minimalized antimicrobial fragment of the human host defense peptide LL-37. Backbone-cyclized KR-12 dimers, tethered by linkers of two to four amino acid residues, were synthesized and their antimicrobial activity, proteolytic stability and structures characterized. A modified KR-12 sequence, with substitutions at previously identified key residues, were also included in the screening panel. The backbone cyclized KR-12 dimers showed improved antimicrobial activity and increased stability compared to monomeric KR-12. The most active cyclic dimer displayed 16-fold higher antibacterial activity compared to KR-12 against Pseudomonas aeruginosa and Staphylococcus aureus, and 8-fold increased fungicidal activity against Candida albicans. It also showed increased hemolytic and cytotoxic activity. Enhanced antimicrobial activity coincided with increased membrane permeabilization of liposomes with one distinct discrepancy: monomeric KR-12 was much less disruptive of liposomes with bacterial lipid composition compared to liposomes from fungal lipid extract. Circular dichroism showed that the four-residue linked most active cyclic dimer had 65% helical content when bound to lyso-phosphatidylglycerol micelles, indicating that the helical propensity of the parent peptide is maintained in the new macrocyclic form. In conclusion, the current work on KR-12 suggests that dimerization together with backbone cyclization is an effective strategy for improving both potency and stability of linear antimicrobial peptides.

Kinetics of helix-coil transition of polypeptides in solution by the relaxation methods

Relaxation phenomena were studied in aqueous solutions of poly (alpha-L-glutamic acid) (Glu)n and poly (alpha-L-lysine) (Lys)n under various conditions using the electric field pulse method with detection by conductivity change. The relaxation time for the (Glu)n has a maximum value at the midpoint of the helix-coil transition. Some possible mechanisms are discussed and the observed relaxation phenomenon is attributed to the helix-coil transition. In the case of (Lys)n, the relaxation time as a function of pH exhibits two maxima. One is assigned to a proton transfer reaction and the other to the helix-coil transition. Using the Schwarz's theory the rate constants of the helix growth step for both (Glu)n and (Lys)n are estimated. The difference in the activation parameters for (Glu)n and (Lys)n is discussed.