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Abstract: The activity of the boron cluster ‐ closo‐borates in protein amyloid fibrillization was first studied. For this, we explored the effect of dianionic compound [B12H12]2− on aggregation of amyloidogenic protein insulin by the circular dichroism spectroscopy, amyloid‐sensitive fluorescent dye based assay and transmission electron microscopy. It was shown that the presence of closo‐borate during fibrillization reaction speeds up the loss of the protein α‐helical structure. This effect of dianionic compound on protein fibrillization process is concentration‐dependent. Closo‐borate at low concentration (10 mM) enhances the intensity of insulin fibrillization, while at higher concentration (50 mM, 100 mM) fluorescent assay showed the decrease of this intensity. The morphology of fibrils formed in the presence of the boron dianion differs from that of free insulin, namely they are less branched and have the bigger diameter. Besides, closo‐borate‐induced fibrils have strong proneness to stick together forming large aggregated clots. [ABSTRACT FROM AUTHOR]