Swelling and aggregation of Leonardite upon pH change and Pb II binding: An AFM study

Environmental context Natural organic materials, such as humic substances, play key roles in the binding and environmental fate of metals. We study the interaction of protons and metal ions with humic acids, and show changes to the mechanical properties of the particles and their capability to fix metal pollutants. The results will help refine current models of metal behaviour in the environment. Abstract The swelling and aggregation of Leonardite humic acid, due to acid-base and Pb II binding interactions, was studied through atomic force microscopy (AFM) tapping mode measurements and correlated with potentiometric experiments. These experiments allowed determination of parameters for the non-ideal competitive adsorption (NICA)-elastic polyelectrolyte network (EPN) model, which predicts size and electrostatic potential changes. AFM observations showed growth of agglomerates at low pH values. Height distribution analysis allowed discrimination of single particles from agglomerates. The size of individual particles increased slightly with pH increase. Agglomeration was evaluated through the dispersity, which increased at pH < 5, concomitant with a decrease of the electrostatic repulsion and an increase of protonated carboxylic groups, thus the agglomeration is attributed to both factors. In the presence of Pb II, agglomeration is observed to rise strongly with the increase in metal concentration, which is attributed to bridging of humic particles by Pb II ions. The AFM ex situ results suggest consistency between NICA-EPN predictions and experimental behaviour.Fil: Dos Reis Copello, Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Lizarraga, Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Centro de Investigaciones en Bionanociencias "Elizabeth Jares Erijman"; ArgentinaFil: Orsetti, Silvia. Universität Tübingen; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Molina, Fernando Víctor. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin

Aromaticity at position 39 in α‐synuclein: A modulator of amyloid fibril assembly and membrane‐bound conformations

Recent studies revealed that molecular events related with the physiology and pathology of αS might be regulated by specific sequence motifs in the primary sequence of αS. The importance of individual residues in these motifs remains an important open avenue of investigation. In this work, we have addressed the structural details related to the amyloid fibril assembly and lipid-binding features of αS through the design of site-directed mutants at position 39 of the protein and their study by in vitro and in vivo assays. We demonstrated that aromaticity at position 39 of αS primary sequence influences strongly the aggregation properties and the membrane-bound conformations of the protein, molecular features that might have important repercussions for the function and dysfunction of αS. Considering that aggregation and membrane damage is an important driver of cellular toxicity in amyloid diseases, future work is needed to link our findings with studies based on toxicity and neuronal cell death