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Register a new userMolecular origin of viscoelasticity in mineralized collagen fibrils
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Bone is mineralized tissue constituting the skeletal system, supporting and protecting body organs and tissues. At the molecular level, mineralized collagen fibril is the basic building block of bone tissue, and hence, understanding bone properties down to fundamental tissue structures enables to better identify the mechanisms of structural failures and damages. While efforts have focused on the study of the micro- and macro-scale viscoelasticity related to bone damage and healing based on creep, mineralized collagen has not been explored on a molecular level. We report a study that aims at systematically exploring the viscoelasticity of collagenous fibrils with different mineralization levels. We investigate the dynamic mechanical response upon cyclic and impulsive loads to observe the viscoelastic phenomena from either shear or extensional strains via molecular dynamics. We perform a sensitivity analysis with several key benchmarks: intrafibrillar mineralization percentage, hydration state, and external load amplitude. Our results show a growth of the dynamic moduli with an increase of mineral percentage, pronounced at low strains. When intrafibrillar water is present, the material softens the elastic component but considerably increases its viscosity, especially at high frequencies. This behaviour is confirmed from the material response upon impulsive loads, in which water drastically reduces the relaxation times throughout the input velocity range by one order of magnitude, with respect to the dehydrated counterparts. We find that upon transient loads, water has a major impact on the mechanics of mineralized fibrillar collagen, being able to improve the capability of the tissue to passively and effectively dissipate energy, especially after fast and high-amplitude external loads.
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Collagen is a key structural protein in the human body, which undergoes mineralization during the formation of hard tissues. Earlier studies have described the mechanical behavior of bone at different scales highlighting material features across hierarchical structures. Here we present a study that aims to understand the mechanical properties of mineralized collagen fibrils upon tensile/compressive transient loads, investigating how the kinetic energy propagates and it is dissipated at the molecular scale, thus filling a gap of knowledge in this area. These specific features are the mechanisms that Nature has developed to passively dissipate stress and prevent structural failures. In addition to the mechanical properties of the mineralized fibrils, we observe distinct nanomechanical behaviors for the two regions (i.e., overlap and gap) of the D-period to highlight the effect of the mineralization. We notice decreasing trends for both wave speeds and Young s moduli over input velocity with a marked strengthening effect in the gap region due to the accumulation of the hydroxyapatite. In contrast, the dissipative behavior is not affected by either loading conditions or the mineral percentage, showing a stronger dampening effect upon faster inputs compatible to the bone behavior at the macroscale. Our results improve the understanding of mineralized collagen composites unveiling the energy dissipative behavior of such materials. This impacts, besides the physiology, the design and characterization of new bioinspired composites for replacement devices (e.g., prostheses for sound transmission or conduction) and for optimized structures able to bear transient loads, e.g., impact, fatigue, in structural applications.
Charge transfer in collisions of Na_n^+ cluster ions with Cs atoms is investigated theoretically in the microscopic framework of non-adiabatic quantum molecular dynamics. The competing reaction channels and related processes affecting the charge transfer (electronic excitations, fragmentation, temperature) are described. Absolute charge transfer cross sections for Na_n^+(2.7 keV) + Cs --> Na_n + Cs^+ have been calculated in the size range 4 <= n <= 11 reproducing the size dependence of the experimental cross sections. The energy dependence of the cross section is predicted for n=4,7,9. An exotic charge transfer channel producing Cs^- is found to have a finite probability.
The covalent-like characteristics of hydrogen bonds offer a new perspective on intermolecular interactions. Here, using density functional theory and post-Hartree-Fock methods, we reveal that there are two bonding molecular orbitals (MOs) crossing the O and H atoms of the hydrogen-bond in water dimer. Energy decomposition analysis also shows a non-negligible contribution of the induction term. These results illustrate the covalent-like character of the hydrogen bond between water molecules, which contributes to the essential understanding of ice, liquid water, related materials, and life sciences.
We report experimental results on the diffractive imaging of three-dimensionally aligned 2,5-diiodothiophene molecules. The molecules were aligned by chirped near-infrared laser pulses, and their structure was probed at a photon energy of 9.5 keV ($lambdaapprox130 text{pm}$) provided by the Linac Coherent Light Source. Diffracted photons were recorded on the CSPAD detector and a two-dimensional diffraction pattern of the equilibrium structure of 2,5-diiodothiophene was recorded. The retrieved distance between the two iodine atoms agrees with the quantum-chemically calculated molecular structure to within 5 %. The experimental approach allows for the imaging of intrinsic molecular dynamics in the molecular frame, albeit this requires more experimental data which should be readily available at upcoming high-repetition-rate facilities.
We report the stopping of a molecular oxygen beam, using a series of pulsed electromagnetic coils. A series of coils is fired in a timed sequence to bring the molecules to near-rest, where they are detected with a quadrupole mass spectrometer. Applications to cold chemistry are discussed.
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