Nina Bionda

Effects of cyclic lipodepsipeptide structural modulation on stability, antibacterial activity, and human cell toxicity.

Bacterial infections are becoming increasingly difficult to treat due to the development and spread of antibiotic resistance. Therefore, identifying novel antibacterial targets and new antibacterial agents capable of treating infections by drug‐resistant bacteria is of vital importance. The structurally simple yet potent fusaricidin or LI‐F class of natural products represents a particularly attractive source of candidates for the development of new antibacterial agents. We synthesized 18 fusaricidin/LI‐F analogues and investigated the effects of structure modification on their conformation, serum stability, antibacterial activity, and toxicity toward human cells. Our findings show that substitution of an ester bond in depsipeptides with an amide bond may afford equally potent analogues with improved stability and greatly decreased cytotoxicity. The lower overall hydrophobicity/amphiphilicity of amide analogues in comparison with their parent depsipeptides, as indicated by HPLC retention times, may explain the dissociation of antibacterial activity and human cell cytotoxicity. These results indicate that amide analogues may have significant advantages over fusaricidin/LI‐F natural products and their depsipeptide analogues as lead structures for the development of new antibacterial agents.

Cyclic lipodepsipeptides: a new class of antibacterial agents in the battle against resistant bacteria

In order to provide effective treatment options for infections caused by multidrug-resistant bacteria, innovative antibiotics are necessary, preferably with novel modes of action and/or belonging to novel classes of drugs. Naturally occurring cyclic lipodepsipeptides, which contain one or more ester bonds along with the amide bonds, have emerged as promising candidates for the development of new antibiotics. Some of these natural products are either already marketed or in advanced stages of clinical development. However, despite the progress in the development of new antibacterial agents, it is inevitable that resistant strains of bacteria will emerge in response to the widespread use of a particular antibiotic and limit its lifetime. Therefore, development of new antibiotics remains our most efficient way to counteract bacterial resistance.

Glycation of a lysine-containing tetrapeptide by D-glucose and D-fructose--influence of different reaction conditions on the formation of Amadori/Heyns products.

The site specificity, extent, and nature of modification of the tetrapeptide, Leu-Ser-Lys-Leu (1), incubated with d-glucose or d-fructose in methanol, or in phosphate buffer of pH 5.7, 7.4, and 8.0 were investigated. The generated mono- and di-glycated Amadori (1-deoxy-d-fructosyl derivatives) and Heyns rearrangement products (N-alkylated glucosamine/mannosamine derivatives) were isolated and characterized by NMR and mass spectrometry. The results identified the epsilon-amino group of the Lys residue as the preferential glycation site in tetrapeptide 1. Under all conditions investigated, glucose afforded higher yields of glycation products than fructose. In the reactions carried out in buffer, glycation at pH 7.4 and 8.0 was much faster than at pH 5.7.

Solid‐phase synthesis of fusaricidin/li‐f class of cyclic lipopeptides: Guanidinylation of resin‐bound peptidyl amines

Fusaricidins/LI‐Fs and related cyclic lipopeptides represent an interesting new class of antibacterial peptides with the potential to meet the challenge of antibiotic resistance in bacteria. Our previous study (Bionda et al. ChemMedChem 2012, 7, 871–882) revealed the significance of the guanidinium group located at the termini of the lipidic tails of these cyclic lipopeptides for their antibacterial activities. Therefore, devising a synthetic strategy that will allow incorporation of guanidinium functionality into their structure is of particular practical importance. Since appropriately protected guanidino fatty acid building blocks are not commercially available, our strategy toward guanidinylated fusaricidin/LI‐F analogs include solid‐phase synthesis of a cyclic lipopeptide precursor possessing a lipidic tail with a terminal amino group followed by its conversion into corresponding guanidine. To find the optimal method for this conversion, we have examined commonly used guanidinylation reagents under the conditions compatible with standard solid‐phase peptide synthesis. Described experimental results demonstrated superiority of N,N′‐di‐Boc‐N″‐triflylguanidine in solid‐phase preparation of fusaricidin/LI‐F class of cyclic lipopeptides. The triflylguanidine reagent gave a single monoguanidinylated product in excellent yield independently of the type of solid‐support.

A practical synthesis of N α-Fmoc protected L-threo-β-hydroxyaspartic acid derivatives for coupling via α- or β-carboxylic group.

A simple and practical general synthetic protocol towards orthogonally protected tHyAsp derivatives fully compatible with Fmoc solid-phase peptide synthetic methodology is reported. Our approach includes enantioresolution of commercially available d,l-tHyAsp racemic mixture by co-crystallization with l-Lys, followed by ion exchange chromatography yielding enantiomerically pure l-tHyAsp and d-tHyAsp, and their selective orthogonal protection. In this way Nα-Fmoc protected tHyAsp derivatives were prepared ready for couplings via either α- or β-carboxylic group onto the resins or the growing peptide chain. In addition, coupling of tHyAsp via β-carboxylic group onto amino resins allows preparation of peptides containing tHyAsn sequences, further increasing the synthetic utility of prepared tHyAsp derivatives.Graphical Abstract

Direct access to side chain N,N′-diaminoalkylated derivatives of basic amino acids suitable for solid-phase peptide synthesis

A simple and efficient one-pot procedure that enables rapid access to orthogonally protected N,N′-diaminoalkylated basic amino acid building blocks fully compatible with standard Boc and Fmoc solid-phase peptide synthesis is reported. Described synthetic approach includes double reductive alkylation of Nα-protected diamino acids with N-protected amino aldehydes in the presence of sodium cyanoborohydride. This approach allows preparation of symmetrical, as well as unsymmetrical, basic amino acid derivatives with branched side-chains that can be further modified, enhancing their synthetic utility. The suitability of the synthesized branched basic amino acid building blocks for use in standard solid-phase peptide synthesis has been demonstrated by synthesis of an indolicidin analogue in which the lysine residue was substituted with the synthetic derivative Nα-(9H-fluorenyl-9-methoxycarbonyl)-Nβ,Nβ′-bis[2-(tert-butoxycarbonylamino)ethyl]-l-2,3-diaminopropionic acid. This substitution resulted in an analogue with more ordered secondary structure in 2,2,2-trifluoroethanol and enhanced antibacterial activity without altering hemolytic activity.Graphical abstract