The amyloidoses are a group of disorders characterized by aberrant protein folding and assembly, leading to the deposition of insoluble protein fibrils (amyloid), which provokes cell dysfunction and later cell death. One of the physiologically relevant environmental factors able to affect the conformation and hence the aggregation properties of amyloidogenic proteins/peptides is metal ions. Zn(II) promotes aggregation of most amyloidogenic peptides/proteins in vitro, including amyloid beta protein (Abeta), but the underlying mechanism is not known. To better understand this mechanism the present study focused on the partially alpha-helical conformer, supposed to be an intermediate in Abeta aggregation. This partially alpha-helical conformer is stabilized by 10-20% 2,2,2-trifluoroethanol (TFE): therefore, the influence of Zn binding on the aggregation of the amylidogenic model peptide Abeta(1-28) (Abeta28) was investigated at different TFE concentrations. The results showed a synergistic effect of Zn(II) and 10% TFE, i.e., that either Zn or 10% TFE accelerated Abeta28 aggregation on its own, but with them together an at least 10 times promotion of Abeta28 aggregation was observed. Further studies by thioflavin T fluorescence spectroscopy, transmission electron microscopy, and circular dichroism (CD) spectroscopy suggested that the aggregates of Zn-Abeta28 formed in 10%TFE contain a beta-sheet secondary structure and are more of the amyloid type. CD spectroscopy indicated that Zn binding disrupted partially the alpha-helical structure of Abeta28 in TFE. Thus, we propose that the promotion of Abeta28 aggregation by Zn is based on the transformation of the partially alpha-helical conformer (intermediate) towards the beta-sheet amyloid structure by a destabilization of the alpha-helix in the intermediate.