Amyloid fibrils are formed during pathological processes and are thermodynamically very stable. Intermolecular interactions between the polypeptide chains dominate the formation of the cross-β structure, which represents a common motif in amyloids irrespective of their amino acid sequence. Aβ(1-40), which is directly associated with Alzheimer’s Disease, is a very well investigated amyloid system1-3, but details of the molecular-level understanding of the disease are still incomplete. The molecular contact between amino acids F19 and L34 has been confirmed in almost all structural studies, point mutations of this contact lead to drastic biological consequences4.In particular, the point mutation F19K was found to exhibit a dramatic change in the local secondary structure of the amyloid fibrils, whereas the overall fibrillar morphology remained unchanged5. Following these results, the assumption that the sidechain of K19 is rotating out of the fibril interior was made. In this work, we investigate this hypothesis using solid-state NMR and MD-simulations to study the protonation state of K19, the local secondary structure of several residues close to the mutated side, and salt bridge formation. Furthermore, we determine order parameters to provide information about the influence of the mutation on the molecular dynamics of the fibrils. The results show that the positively charged K19 side chain is expelled from the fibril interior to avoid a high energy penalty and, although the structure of the lower β-strand is altered and the D23-K28 salt bridge is no longer formed, that F20 can replace the mutated position 19 and maintain the important hydrophobic contacts within the fibril.