The binding of heparin to amyloid proteins has been reported to increase the degree of order of the protein within the aggregates, thus favoring the fibrillation process . In a variety of proteins that are induced to form b-structure, heparin binding sites have been shown to contain clusters of basic amino acid residues capable of binding to the negatively charged heparin molecules . Moreover, clusters of basic amino acids that do not BAY-60-7550 conform to the sequences identified by Cardin and Weintraub are present in the primary structure at the end of the B helix, i.e., RLFKSH, the beginning of the E helix, i.e., LKKHG, and at the end of the G-helix, i.e., HVLHSRH . We propose that the crucial step of heparin-induced fibrillation is the recognition and binding to the basic cluster sequences localized on turn regions of the protein that are easily accessible. The observation that heparin induces amyloid aggregation of both partially folded and fully unfolded apoprotein suggests that the basic binding sequences are not necessarily located in helical structured regions. Heparin can, therefore, act as an immobilizing charged surface on which protein monomers are correctly oriented for priming an ordered polymerization that generates fibril seeds. It is also possible, as documented by our FTIR analysis, that heparin GSK1120212 directly promotes the a-to-b structural transitions within monomeric species of the W7FW14F apomyoglobin mutant and so lead to acceleration of aggregation and fibril formation. At present, it is difficult to determine whether heparin simply acts as a concentrating surface on which protein molecules become in close contact or whether it induces conformational transitions as a consequence of which the formation of cross-b structure is favored. Probably, both effects are responsible for the observed acceleration of the aggregation process. Finally, we can not exclude that heparin induces a pathway of aggregation different from that of the amyloidogenic apomyoglobin in experimental conditions similar to the physiological setting. The proposed mechanism for the heparin-induced aggregation and fibrillation of the W7FW14F mutant under aggregating, i.e., pH 7.0, and non-aggregating experimental conditions, i.e., at pH 4.0 and 2.0, is supported by the results obtained with the wildtype apomyoglobin, a protein that does not undergo aggregation and fibril formation under native conditions. In fact, we found that the addition of heparin to this protein caused aggregation and fibril formation that was much more evident on lowering the pH from 7.0 to 5.5.