D-amino acid-containing proteins have been found in several human tissues, and the spontaneous accumulation of n-amino acids in proteins is thought to be involved in age-dependent diseases including dementia. Tau, a microtubule-associated protein, is a major component of neurofibrillary tangles in Alzheimer's disease. Site-specific amino acid D-isomerization in Tau has been observed in the brains of patients with Alzheimer's disease. Here, we conducted amino acid D-isomerization at specific sites in microtubule-binding repeat peptides of Tau (Tau R2 and R3) and examined the effects on Tau structure and fibril formation. Our results demonstrate that amino acid D-isomerization in Tau R2 peptides decreased the rates of 6-sheet transition and fibril formation compared with those of the wild-type peptide composed of all L-amino acids. In contrast, Tau R3 peptides that had undergone amino acid D-isomerization at either Asp314, Ser316, or Ser324 showed increased rates of beta-sheet transition and fibril formation compared with those of the wild-type Tau R3 peptide. (C) 2018 Elsevier Inc. All rights reserved.