Tautomeric Effect of Histidine on the Monomeric Structure of Amyloid β-Peptide(1-40).

Histidine state (deprotonated, neutral, and protonated) is considered an important factor influencing the structural properties and aggregation mechanisms in amyloid β-peptides (Aβ), which are associated with the pathogenesis of Alzheimer's disease. Understanding the structural properties and aggregation mechanisms is a great challenge because two forms (the N ^ε -H or N ^δ -H tautomer) can exist in the free neutral state of histidine. Here, replica-exchange molecular dynamics simulation was performed to elucidate the changes in structure and the mechanism of aggregation influenced by tautomeric behaviors of histidine in Aβ(1-40). Our results show that sheet-dominating conformations can be found in the His6(δ)-His13(δ)-His14(δ) (δδδ) isomer with significant antiparallel sheet structures between R5-D7 and L34-G38, as well as between L17-F20 and L34-G38, implying that a new aggregation mechanism may exist to promote the generation of oligomers and/or aggregates. This work is helpful in understanding the fundamental tautomeric behaviors of neutral histidine in the process of aggregation.