Natural tri- to hexapeptides self-assemble in water to amyloid beta-type fiber aggregates by unexpected alpha-helical intermediate structures.

Many fatal neurodegenerative diseases such as Alzheimer's, Parkinson, the prion-related diseases, and non-neurodegenerative disorders such as type II diabetes are characterized by abnormal amyloid fiber aggregates, suggesting a common mechanism of pathogenesis. We have discovered that a class of systematically designed natural tri- to hexapeptides with a characteristic sequential motif can simulate the process of fiber assembly and further condensation to amyloid fibrils, probably via unexpected dimeric α-helical intermediate structures. The characteristic sequence motif of the novel peptide class consists of an aliphatic amino acid tail of decreasing hydrophobicity capped by a polar head. To our knowledge, the investigated aliphatic tripeptides are the shortest ever reported naturally occurring amino acid sequence that can adopt α-helical structure and promote amyloid formation. We propose the stepwise assembly process to be associated with characteristic conformational changes from random coil to α-helical intermediates terminating in cross-ß peptide structures. Circular dichroism and X-ray fiber diffraction analyses confirmed the concentration-dependent conformational changes of the peptides in water. Molecular dynamics simulating peptide behavior in water revealed monomer antiparallel pairing to dimer structures by complementary structural alignment that further aggregated and stably condensed into coiled fibers. The ultrasmall size and the dynamic facile assembly process make this novel peptide class an excellent model system for studying the mechanism of amyloidogenesis, its evolution and pathogenicity. The ability to modify the properties of the assembled structures under defined conditions will shed light on strategies to manipulate the pathogenic amyloid aggregates in order to prevent or control aggregate formation.

Synthetic cationic amphiphilic α-helical peptides as antimicrobial agents.

Antimicrobial peptides (AMPs) secreted by the innate immune system are prevalent as the effective first-line of defense to overcome recurring microbial invasions. They have been widely accepted as the blueprints for the development of new antimicrobial agents for the treatment of drug resistant infections. However, there is also a growing concern that AMPs with a sequence that is too close to the host organism's AMP may inevitably compromise its own natural defense. In this study, we design a series of synthetic (non-natural) short α-helical AMPs to expand the arsenal of the AMP families and to gain further insights on their antimicrobial activities. These cationic and amphiphilic peptides have a general sequence of (XXYY)(n) (X: hydrophobic residue, Y: cationic residue, and n: the number of repeat units), and are designed to mimic the folding behavior of the naturally-occurring α-helical AMPs. The synthetic α-helical AMPs with 3 repeat units, (FFRR)(3), (LLRR)(3), and (LLKK)(3), are found to be more selective towards microbial cells than rat red blood cells, with minimum inhibitory concentration (MIC) values that are more than 10 times lower than their 50% hemolytic concentrations (HC(50)). They are effective against Gram-positive B. subtilis and yeast C. albicans; and the studies using scanning electron microscopy (SEM) have elucidated that these peptides possess membrane-lytic activities against microbial cells. Furthermore, non-specific immune stimulation assays of a typical peptide shows negligible IFN-α, IFN-γ, and TNF-α inductions in human peripheral blood mononuclear cells, which implies additional safety aspects of the peptide for both systemic and topical use. Therefore, the peptides designed in this study can be promising antimicrobial agents against the frequently-encountered Gram-positive bacteria- or yeast-induced infections.

Self-assembly of nanodonut structure from a cone-shaped designer lipid-like peptide surfactant.

We report here the donut-shaped nanostructure formation from the self-assembly of a designer lipid-like amphiphilic cone-shaped peptide. The critical aggregation concentration was measured using dynamic light scattering in water and phosphate-buffered saline. The dynamic self-assembly of the peptide was also studied using atomic force microscopy. We have studied numerous peptides over 17 years, and this is the first time that we have ever observed the nanodonut structure from cone-shaped peptides. We propose a plausible self-assembling pathway of the nanodonut structure that was self-assembled through the fusion or elongation of spherical micelles. Furthermore, the bending of the nanostructure gives rise to the nanodonut structures as a result of the tension originating from the interaction of the cone-shaped peptide side chains. Our observations may be useful for further fine tuning the geometry and shape of a new class of designer peptides and their self-assembled supramolecular materials for diverse uses.

Synergistic effect and hierarchical nanostructure formation in mixing two designer lipid-like peptide surfactants Ac-A6D-OH and Ac-A6K-NH2.

We here report the nanostructures from combinational self-assembly of two designer lipid-like peptides Ac-A6D-OH and Ac-A6K-NH2 using dynamic light scattering (DLS) and atomic force microscopy (AFM). The synergistic phenomenon is observed by measuring the critical aggregation concentrations (CACs) of these two mixed peptides, in different molar ratios by DLS. The nanoropes were observed in AFM images at a molar ratio of Ac-A6D-OH/Ac-A6K-NH2 = 1:1, and the thin film formation with aligned nanoropes is shown at a molar ratio of 2:1. The well aligned nanoropes at the molar ratio of Ac-A6D-OH/Ac-A6K-NH2 = 2:1 indicated the competition factor between the electrostatic repulsion according to DLVO theory and the hydrophobic interaction arising from the long side chains on lysine residues. This study will further our understanding for designing new nanomaterials based on designer lipid-like peptide surfactants.