Towards identification of a non-abelian state: observation of a quarter of electron charge at ν=5/2\nu=5/2 quantum Hall state

The fractional quantum Hall effect, where plateaus in the Hall resistance at values of coexist with zeros in the longitudinal resistance, results from electron correlations in two dimensions under a strong magnetic field. Current flows along the edges carried by charged excitations (quasi particles) whose charge is a fraction of the electron charge. While earlier research concentrated on odd denominator fractional values of $\nu$, the observation of the even denominator $\nu=5/2$ state sparked a vast interest. This state is conjectured to be characterized by quasiparticles of charge e/4, whose statistics is non-abelian. In other words, interchanging of two quasi particles may modify the state of the system to an orthogonal one, and does not just add a phase as in for fermions or bosons. As such, these quasiparticles may be useful for the construction of a topological quantum computer. Here we report data of shot noise generated by partitioning edge currents in the $\nu=5/2$ state, consistent with the charge of the quasiparticle being e/4, and inconsistent with other potentially possible values, such as e/2 and e. While not proving the non-abelian nature of the $\nu=5/2$ state, this observation is the first step toward a full understanding of these new fractional charges

Antimicrobial and cell-penetrating peptides induce lipid vesicle fusion by folding and aggregation

According to their distinct biological functions, membrane-active peptides are generally classified as antimicrobial (AMP), cell-penetrating (CPP), or fusion peptides (FP). The former two classes are known to have some structural and physicochemical similarities, but fusogenic peptides tend to have rather different features and sequences. Nevertheless, we found that many CPPs and some AMPs exhibit a pronounced fusogenic activity, as measured by a lipid mixing assay with vesicles composed of typical eukaryotic lipids. Compared to the HIV fusion peptide (FP23) as a representative standard, all designer-made peptides showed much higher lipid-mixing activities (MSI-103, MAP, transportan, penetratin, Pep1). Native sequences, on the other hand, were less fusogenic (magainin 2, PGLa, gramicidin S), and pre-aggregated ones were inactive (alamethicin, SAP). The peptide structures were characterized by circular dichroism before and after interacting with the lipid vesicles. A striking correlation between the extent of conformational change and the respective fusion activities was found for the series of peptides investigated here. At the same time, the CD data show that lipid mixing can be triggered by any type of conformation acquired upon binding, whether α-helical, β-stranded, or other. These observations suggest that lipid vesicle fusion can simply be driven by the energy released upon membrane binding, peptide folding, and possibly further aggregation. This comparative study of AMPs, CPPs, and FPs emphasizes the multifunctional aspects of membrane-active peptides, and it suggests that the origin of a peptide (native sequence or designer-made) may be more relevant to define its functional range than any given name