Packing Density and Structural Heterogeneity of Insulin Amyloid Fibrils Measured by AFM Nanoindentation

Abstract

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.

Regulation of Endothelial Cell Adherence and Elastic Modulus by Substrate Stiffness

“…Parameter s indicates the Poisson's ratio and was set to 0.5 due to the assumption of incompressibility for hydrogels (Sunyer et al 2012). A previously published algorithm (Guo & Akhremitchev 2006;Nikkhah et al 2011) was used to obtain the young's modulus of substrates based on d parameter in Hertz's model. The values of AFM head movement (z) were subtracted from deflection (d) of the cantilever to obtain indentation depth (d).…”

Immunofluorescence-guided atomic force microscopy to measure the micromechanical properties of the pericellular matrix of porcine articular cartilage

“…From the Hertz model, the presented modulus values can be converted from n ¼ 0.04 to a different assumed value of n by multiplying by the conversion factor (1 2 n 2 )/ (1 -0.04 2 ). Probe -surface contact was identified using contact point extrapolation, a method that uses the indentation portion of the approach curve to determine the probe -surface contact point on soft substrates on the basis of an applied mathematical model [53]. Hertzian contact mechanics provided excellent fits to the experimental data for all force -indentation curves (r 2 .…”

Nanomechanics of Amyloid Materials Studied by Atomic Force Microscopy

“…An early AFM indentation measurement was conducted on insulin amyloid fibrils adsorbed on mica surface (Guo & Akhremitchev, 2006). The obtained force curves showed no adhesion between the tip and fibril, by which it was rationalized that the Hertz model could be used to fit the force curve.…”

“…The preferred method is using electron microscopy to image the tip after the measurements (Guo & Akhremitchev, 2006). The resulting image might have a low contrast due to the low electron density of silicon nitride, which can be improved by sputtering a layer of metal on the tip.…”

“…In this situation, the substrate also contributes to the mechanical response of the tip-sample system, resulting in a deviated Young's modulus. This effect can be corrected by calculating correction factors using the uncorrected Young's modulus and normalized force (Akhremitchev & Walker, 1999;Guo & Akhremitchev, 2006). After all the calibration and approximation, the force can be linearly fitted against indentation by equation 8 and the Young's modulus is obtained.…”