Mechanical response of collagen molecule under hydrostatic compression Karanvir Saini ⁎, Navin Kumar Indian Institute of Technology Ropar, Punjab, India abstract article info Article history: Received 9 June 2014 Received in revised form 7 November 2014 Accepted 6 January 2015 Available online 10 January 2015 Keywords: Collagen Protein Atomistic simulations Continuum model Compression Proteins like collagen are the basic building blocks of various body tissues (soft and hard). Collagen molecules find their presence in the skeletal system of the body where they bear mechanical loads from different directions, either individually or along with hydroxy-apatite crystals. Therefore, it is very important to understand the mechanical behavior of the collagen molecule which is subjected to multi-axial state of loading. The estimation of strains of collagen molecule along different directions resulting from the changes in hydrostatic pressure mag- nitude, can provide us new insights into its mechanical behavior. In the present work, full atomistic simulations have been used to study global (volumetric) as well as local (along different directions) mechanical properties of the hydrated collagen molecule which is subjected to different hydrostatic pressure magnitudes. To estimate the local mechanical properties, the strains of collagen molecule along its longitudinal and transverse directions have been acquired at different hydrostatic pressure magnitudes. In spite of non-homogeneous distribution of atoms within the collagen molecule, the calculated values of local mechanical properties have been found to carry the same order of magnitude along the longitudinal and transverse directions. It has been demonstrated that the values of global mechanical properties like compressibility, bulk modulus, etc. as well as local mechanical prop- erties like linear compressibility, linear elastic modulus, etc. are functions of magnitudes of applied hydrostatic pressures. The mechanical characteristics of collagen molecule based on the atomistic model have also been compared with that of the continuum model in the present work. The comparison showed up orthotropic material behavior for the collagen molecule. The information on collagen molecule provided in the present study can be very helpful in designing the future bio-materials. © 2015 Elsevier B.V. All rights reserved. 1. Introduction With the emerging challenges related to the increasing energy needs, new medical applications, novel concepts in sensor and actuator design, reliability and robustness of the devices, conservation of resources and developing new bio-materials, etc., the study of biological materials has become an inevitable requirement. The advent of nano- science and nanotechnology has facilitated high level structural understanding as well as control of the matter. The use of nano-scale knowledge to understand the concepts adopted by nature for millions of years in systematically designing biological materials, and the exploitation of these concepts for technological as well as bio-medical applications hold great promise for the future. At nano-scale lengths, the biological materials are primarily consti- tuted by protein molecules. To understand the mechanical properties of biological materials, there is a need to understand the mechanical response of proteins. Such a response can be explained in terms of the softness of a protein which is closely associated with its biological function [1–4]. A measure of protein's softness is defined through its compressibility which relates the variation in hydrostatic pressure magnitude to change in its volume. The nature of volume increment under the conformation transition of macro-molecules as well as their compressibility has been widely discussed in regard to strains in the proteins [4,5]. Linear compressibility measurements have been used to understand the directional dependence of material properties on the pressure magnitudes [6]. Hence, the accurate evaluation of compress- ibility of proteins can be very useful to explain the physical mechanism to understand the structure–function relationship of proteins. Over the past decade, the application of hydrostatic pressures has become an important tool for analyzing structural properties as well as the phase behavior of biological molecules and systems [7–16]. Various studies of the high pressure effect on the proteins can be classified into two groups. The first group includes the studies of the folding/unfolding kinetics of proteins as well as activation/inactivation of enzymes under high pressure magnitudes. The second group covers the analysis of proteins in different equilibrium conditions. Under the equilibrium conditions, the studies have been done to investigate the volume changes of the protein molecule which are the result of the Materials Science and Engineering C 49 (2015) 720–726 ⁎ Corresponding author at: School of Mechanical, Material and Energy Engineering, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India. E-mail address: karans@iitrpr.ac.in (K. Saini). http://dx.doi.org/10.1016/j.msec.2015.01.032 0928-4931/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec