The toxic conformer of the 40- or 42-mer-amyloid β-proteins (Aβ) (Aβ40, Aβ42) with a turn at positions 22 and 23 plays a role in oligomer formation, leading to neurotoxicity as part of the pathogenesis of Alzheimer's disease (AD). A deletion mutant at Glu22 (E22Δ) of Aβ, known as an Osaka mutation, accelerates oligomerization. Although E22Δ-Aβ has not been found to be toxic to cultured neuronal cells and is instead synaptotoxic in long-term potentiation, there is no information on the toxic conformer of E22Δ-Aβ in AD. The site-directed spin labeling study of E22Δ-Aβ40 by continuous wave-electron spin resonance (CW-ESR) spectroscopy in part showed the spatial proximity between positions 10 and 35, which are characteristic of the toxic conformation of Aβ, indicating the existence of a toxic conformer of Aβ with the E22Δ mutation. To obtain structural insight, E22Δ-Aβ42 substitutes with proline (F20P, A21P, D23P, and V24P), in which proline is known as a turn inducer but is a β-sheet breaker, were synthesized. An enzyme immunoassay using the 24B3 antibody recognizing toxic conformer of Aβ was carried out. 24B3 reacted with these substitutes of E22Δ-Aβ42 as well as E22Δ-Aβ42 in a similar manner to WT-Aβ42. Notably, only A21P-E22Δ-Aβ42 exhibited strong neurotoxicity in rat primary neurons after 8 days of incubation, with potent high-order oligomerization compared with E22Δ-Aβ42. These results suggest that E22Δ-Aβ42 could enhance neurotoxicity by generating a toxic oligomer conformation with a turn near position 21.