Packing density and structural heterogeneity of insulin amyloid fibrils measured by AFM nanoindentation

A nanoindentation approach based on atomic force microscopy was applied to test the elastic properties of insulin amyloid fibrils. Fibrils exhibited a nearly elastic response to the compressive load. The results, corrected for the finite sample thickness effect, reveal that the fibril Young's modulus is considerably lower than the modulus of protein crystals, suggesting lower packing density in amyloid fibrils. Variation in elasticity among and within fibrils has been studied, showing that the Young's moduli of insulin fibrils have a relatively wide distribution of values, ranging from 5 to 50 MPa. Amyloid fibrils with higher modulus were found to be more wear-resistant during AFM scanning. The measured distribution of elasticity values of different fibrils together with wear-resistance tests indicates structural heterogeneity among fibrils, whereas the structure of individual fibrils appears to be homogeneous. The relative simplicity of the method used in this study can facilitate rapid collection of quantitative information related to the packing density and heterogeneity of fibrils formed by different proteins.