Study of Nano-Mechanics of Collagen I Triple-helices by Computerized Processing of AFM Images
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Study of Nano-Mechanics of Collagen I Triple-helices by Computerized Processing of AFM Images Arkady Bitler1, Emanuel Perugia2 and Sidney R. Cohen1 1 Department of Chemical Research Support, Weizmann Institute of Science, P.O.B. 26, Rehovot, Israel, 76100 2 Department of Structural Biology, Weizmann Institute of Science, P.O.B. 26, Rehovot, Israel, 76100 ABSTRACT We used atomic force microscope (AFM) to acquire high-resolution images of collagen type I triple-helices under ambient conditions in tapping mode. Angles between consecutive fixed-length segments were measured and analyzed to yield persistence length and elastic constant. Changing the segment length allowed exploring the mechanics at various scales. Understanding the mechanical properties of collagen molecules could serve to elucidate mechanisms of complex mechanical properties of interest in nanomedicine and nanotechnology. INTRODUCTION The basic functions of collagen are to provide mechanical stability, elasticity, and strength to organisms. Moreover collagen stiffness controls cell differentiation, growth, and pathology. Therefore investigation of the mechanical properties of collagen starting from the elementary building unit – a collagen triple helix – is of great importance. Earlier approaches calculated Young’s modulus E from stress-strain measurements which were fitted to the appropriate model of the collagen molecule. Sasaki and Odajima [1] used wide-angle x-ray diffraction to detect changing distance between neighboring amino acids along the collagen triple-helix axis in response to a macroscopic tensile force and obtained E of 2.9 GPa . Harley and co-authors [2] investigated elastic properties of collagen molecules by Brillouin (inelastic) light scattering. They obtained E values ranging from 9GPa in 0.15 M NaCl solution to 21.5 GPa for dried rat-tail tendon collagen. Later on these values were refined by Cusack and Miller [3] who found E of 11.9 GPa. Sun and coauthors [4] studied collagen type I triple-helix elasticity using optical tweezers to stretch it and fit the data to a simple worm-like chain model. Their study derived an E value of 12.2 GPa assuming molecular radius of 0.28 nm AFM has been extensively used to characterize various mechanical properties of biological specimens in general and collagen structures specifically. AFM images were used to gain information about mechanical properties of biological molecules (especially DNA) from static AFM images from the mid 90s by the Bustamante group [5]. Following this approach, the contours of DNA molecules were extracted from AFM images. Furthermore the angles between adjacent segments were measured and persistence length was defined from these measurements. Later studies acquired nanoscale information about specific segments of increased flexibility in coiled-coil of the human DNA repair protein [6] and high flexibility of DNA on short length scales [7] using a similar approach.
The method of evaluating molecular stiffness based on AFM imaging of these molecules is a static measurement,
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