Interaction of redox active copper ions with beta amyloid peptides, both Aβ(1-x) and Aβ(4x) leads to formation of Aβ-Cu(II) complexes which have very different properties. Cu(II) is predominantly coordinated by Aβ(1-x) in a distorted square-pyramidal geometry. Therefore Cu(II) ions attached to Aβ(1-x) can undergo reduction to Cu(I). This process catalyses the production of reactive oxygen and nitrogen species (ROS and RNS) which are highly toxic to neurons. In contrast, Aβ(4-x) form square-planar complexes with Cu(II) ions through its ATCUN (the amino terminal Cu(II)and Ni(II)-binding motif) sequence, that is, by its N-terminal part consisting of Phe-Arg-His (Scheme 1). Because of this, Aβ(4-16)-Cu(II) undergo Cu(II)/Cu(I) reduction at very negative potential values (below -1 V), unattainable for conditions in the human body, which prevents the formation of ROS and RNS. Additionally, ATCUN-Cu(II) complexes allow Cu(II) to be oxidised to Cu(III), which is strong oxidising species, in a reversible or quasi-reversible oneelectron redox process. In our previous work we showed that in Aβ(4-16)-Cu(II) complexes in the absence of tyrosine the Cu(II)/Cu(III) reaction is reversible. , whereas when Tyr is present in the peptide, the reduction is quenched. Scheme 1 Structure of the Aβ(4-16)-Cu(II) complex investigated in this work.