Heterogeneous Tryptophan Environments in the Cyclic Peptides Tyrocidines B and C

The Influence of Cellulose-Type Formulants on Anti-Candida Activity of the Tyrocidines

Candida species are highly adaptable to environmental changes with their phenotypic flexibility allowing for the evasion of most host defence mechanisms. Moreover, increasing resistance of human pathogenic Candida strains has been reported against all four classes of available antifungal drugs, which highlights the need for combinational therapies. Tyrocidines are cyclic antimicrobial peptides that have shown synergistic activity with antifungal drugs such as caspofungin and amphotericin B. However, these cyclodecapeptides have haemolytic activity and cytotoxicity, but they have been used for decades in the clinic for topical applications. The tyrocidines tend to form higher-order structures in aqueous solutions and excessive aggregation can result in variable or diminished activity. Previous studies have shown that the tyrocidines prefer ordered association to celluloses. Therefore, a formulation with soluble cellulose was used to control the oligomer stability and size, thereby increasing the activity against Candida spp. Of the formulants tested, it was found that commercial hydroxy-propyl-methyl cellulose, E10M, yielded the best results with increased stability, increased anti-Candida activity, and improved selectivity. This formulation holds promise in topical applications against Candida spp. infections.

Oligomerisation of tryptocidine C, a Trp-rich cyclodecapeptide from the antimicrobial tyrothricin complex

Tryptocidine C (TpcC, cyclo[D-Phe¹-Pro²-Trp³-D-Trp⁴-Asn⁵-Gln⁶-Trp⁷-Val⁸-Orn⁹-Leu¹⁰]) is a broad-spectrum antimicrobial peptide in the tyrothricin complex produced by a soil bacterium, Brevibacillus parabrevis. Electrospray mass spectrometric studies reveal the oligomerisation of TpcC into dimers and higher oligomers, analogous to tyrocidine C (TrcC, Trp⁷ replaced by Tyr⁷). Ion mobility mass spectrometry (IMMS) further confirms the formation of stable peptide dimers and tetramers with diameters of 2.7 nm and 3.3 nm, respectively, calculated from collisional cross section (CCS). Molecular dynamic simulations and docking studies support the formation of amphipathic dimers, with a diameter of 2.5 ± 0.07 nm calculated from low energy model CCS. Circular dichroism and IMMS studies point towards dynamic hydrogen-bonded conformational changes up to 28-33 μM after which the structures become more static (or in equilibrium). Fluorescence studies indicate aromatic stacking of Trp residues with a CMC of 18 μM in aqueous solutions. The concentration and time dependent interaction of Trp in oligomers indicate cooperativity in the TpcC oligomerisation that leads to the formation of higher order microscopic structures. Scanning electron microscopy studies unequivocally shows that TpcC forms nanospheres with a mean diameter of 25 nm. Repeated smaller oligomeric units, possibly dimers and tetramers, self-assemble to form these nanospheres.

Structure of tyrocidine micelles in isotropic aqueous solution

Aqueous solutions of tyrocidine B, iodoacetyltyrocidine B, and diiodotyrocidine B in 40% H2O:60% ethanol (w/w) were investigated by analytical ultracentrifugation, light scattering, and small-angle X-ray scattering techniques. A reasonable hydrodynamic description of the aggregates of molecular weight 28,600 is a rod with a length of 170 A and a diameter of 30 A; this description is consistent with the X-ray scattering data. Over a broad range of concentrations, inelastic light scattering measurements and small-angle X-ray scattering experiments provide the same hydrodynamic values (e.g., Stokes radii and frictional ratios). The positions of the iodines were resolved and found at a radius (R) of 16 A. So, the iodine-iodine distance across the cross section is 34 A, indicating that labeled tyrocidine B is not affected during the aggregational process.

DNA-supercoiling is affected in vitro by the peptide antibiotics tyrocidine and gramicidin

Tyrocidine, a peptide antibiotic produced by Bacillus brevis (ATCC 8185), relaxes superhelical DNA in a biphasic manner and induces 'packaging' of the DNA at higher concentrations. This was concluded from studies using the sensitive 4,5',8-trimethylpsoralen photobinding technique [Sinden, R. R., Carlson, J. O. & Pettijohn, D.-E. (1980) Cell 21, 773-783]. Relaxed DNA is not affected by tyrocidine whereas linearized molecules become packaged. The linear gramicidin synthesized by the same strain reverses the tyrocidine-induced relaxation as well as the packaging, an observation which might be of biological relevance.

The DNA . tyrocidine complex and its dissociation in the presence of gramicidin D

The peptide antibiotic tyrocidine affects transcription in vitro by interfering with the initiation process. The complex formed between tyrocidine and native DNA is stable against nucleolytic enzymes. It is sensitive to variations in the salt concentration. Protection of DNA as a function of the tyrocidine concentration follows a curve suggesting cooperative binding of tyrocidine to DNA. The complex formation at low tyrocidine concentrations is accompanied by the presence of single stranded regions of unknown length. With single-stranded DNA, tyrocidine forms a complex which is relatively insensitive to high salt concentrations. Both native and single-stranded DNAs of the complexes become digestible in the presence of gramicidin D, another peptide antibiotic. Breakdown of the complex as a function of the gramicidin D concentration follows a curve which is the reverse of cooperative binding. Gramicidin D acts more effectively on single-stranded rather than on double-stranded DNA . tyrocidine complexes. The DNA . tyrocidine complex, which is broken down by formamide and dimethyl sulfoxide is stabilized by gramicidin D. The implications of these results are discussed with respect to a possible recognition of DNA sequences by tyrocidine and the interaction between DNA, tyrocidine and gramicidin D.