The morphology, structure, and dynamics of mature amyloid β (Aβ) fibrils formed by the Aβ variant, which is truncated at residue 11 and chemically modified by enzymatic pyroglutamate formation (pGlu -Aβ(11-40)), was studied along with the investigation of the toxicity of these Aβ variants to neurons and astrocytes. The fibrils of pGlu -Aβ (11-40) were more toxic than wildtype Aβ (1-40) and the longer pGlu3-Aβ (3-40) especially at higher concentration, whereas the overall morphology was quite similar. The secondary structure of pGlu -Aβ (11-40) fibrils shows the typical two β-strands connected by a short turn as known for mature fibrils of Aβ (1-40) and also pGlu -Aβ (3-40). Further insights into tertiary contacts exhibit some similarities of pGlu -Aβ (11-40) fibrils with wildtype Aβ (1-40), but also a so far not described contact between Gly and Ile . This highlights the biological importance of chemical modifications on the molecular structure of Aβ.