Co-Aggregation of Two Anionic Azo Dyestuffs at a Well-Defined Stoichiometry

The present work investigates the formation of well-defined heteroaggregates from a binary mixture of a red and a yellow azo-dyestuff in the presence of Mg²⁺ ions. Combined static and dynamic light scattering together with laser induced liquid bead ion desorption mass spectrometry (LILBID-MS) has been applied to characterize the states of the pure red dye and the pure yellow dye as well as of their mixture in aqueous solution without Mg²⁺. These experiments indicated that a structural reorganization on a molecular scale takes place as soon as the two dyes are combined. Solutions of the combined red and yellow dye contain micelle-like mixed entities with a size of a few tenths of nanometers. Upon the addition of Mg²⁺, these micelles vanish in favor of elongated heteroaggregates, which grow by a stepwise addition of smaller building units. As unraveled by UV/vis spectroscopy, the heteroaggregates that are formed from the red and yellow azo dye in the presence of Mg²⁺ obey a stoichiometric ratio of the two components of 1:1. A new multiangle scattering instrument allowed us for the first time to follow this aggregation process at the stoichiometric ratio by time-resolved combined static and dynamic light scattering, thereby providing further aspects of the worm-like nature of the growing heteroaggregates

Laser-Induced Liquid Bead Ion Desorption Mass Spectrometry: An Approach to Precisely Monitor the Oligomerization of the β-Amyloid Peptide

In the present work, the recently developed laser-induced liquid bead ion desorption mass spectrometry (LILBID MS) is applied as a novel technique to study Aβ oligomerization, thought to be crucial in Alzheimer’s disease (AD). The characterization of the earliest nucleation events of this peptide necessitates the application of several techniques to bridge the gap between small oligomers and large fibrils. We precisely monitored in time the transformation of monomeric Aβ (1-42) into oligomeric Aβ_<i>n</i> (<i>n</i> < 20) and its dependence on concentration and agitation. The distribution shows signs of the hexamer being crucial in the assembly process. The intensity of the monomer decreases in time with a time constant of about 9 h. After a lag time of around 10 h, a phase transition occurred in which the total ion current of the oligomers goes to nearly zero. In this late stage of aggregation, protofibrils are formed and mass spectrometry is no longer sensitive. Here fluorescence correlation spectroscopy (FCS) and transmission electron microscopy (TEM) are complementary tools for detection and size characterization of these large species. We also utilized the oligomers of Aβ (1-42) as a model of the corresponding <i>in vivo</i> process to screen the efficacy and specificity of small molecule inhibitors of oligomerization. The LILBID results are in excellent agreement with condensed phase behavior determined in other studies. Our data identified LILBID MS as a powerful technique that will advance the understanding of peptide oligomerization in neurodegenerative diseases and represents a powerful tool for the identification of small oligomerization inhibitors

Influence of Surface Groups on Poly(propylene imine) Dendrimers Antiprion Activity

Prion diseases are characterized by the accumulation of PrP^Sc, an aberrantly folded isoform of the host protein PrP^C. Specific forms of synthetic molecules known as dendrimers are able to eliminate protease-resistant PrP^Sc in both an intracellular and in vitro setting. The properties of a dendrimer which govern this ability are unknown. We addressed the issue by comparing the in vitro antiprion ability of numerous modified poly­(propylene-imine) dendrimers, which varied in size, structure, charge, and surface group composition. Several of the modified dendrimers, including an anionic glycodendrimer, reduced the level of protease resistant PrP^Sc in a prion strain-dependent manner. This led to the formulation of a new working model for dendrimer/prion interactions which proposes dendrimers eliminate PrP^Sc by destabilizing the protein and rendering it susceptible to proteolysis. This ability is not dependent on any particular charge of dendrimer, but does require a high density of reactive surface groups