Apolipoprotein A-I is amenable to a number of specific mutations associated with hereditary systemic amyloidoses. Amyloidogenic properties of apoA-I are determined mainly by its N-terminal fragment. In the present study Forster resonance energy transfer between tryptophan as a donor and Thioflavin T as an acceptor was employed to obtain structural information on the amyloid fibrils formed by apoA-I variant 1-83/G26R/W@8. Analysis of the dye-fibril binding data provided evidence for the presence of two types of ThT binding sites with similar stoichiometries (bound dye to monomeric protein molar ratio 10), but different association constants (similar to 6 and 0.1 mu M-1) and ThT quantum yields in fibril-associated state (0.08 and 0.05, respectively). A p-strand-loop-p-strand structural model of 1-83/G26R/W@8 apoA-I fibrils has been proposed, with potential ThT binding sites located in the solvent-exposed grooves of the N-terminal beta-sheet layer. Reasoning from the expanded FRET analysis allowing for heterogeneity of ThT binding centers and fibril polymorphism, the most probable locations of high- and low-affinity ThT binding sites were attributed to the grooves T16_Y18 and D20_L22, respectively. (C) 2013 Elsevier Inc. All rights reserved.