Collagen fibres in articular cartilage (AC) exhibit a predominant direction of average alignment1 . Water molecules bound to these oriented fibres undergo anisotropic molecular reorientation. This results in an incomplete averaging of the intra-molecular proton-proton magnetic interactions and gives rise to residual dipolar couplings2 . In magnetic resonance imaging (MRI) experiments, this is seen as anisotropic spin relaxation of protons throughout a sample. Because of this, relaxation data has the ability to provide information about the organisation and microstructure AC. However, interpretation is difficult because the effective proton transverse relaxation rates (R2) depend on the angle of the ordered collagen fibres to the static magnetic field3 . Understanding the water - collagen interactions that are responsible for spin relaxation anisotropy will lead to a better interpretation of MRI results and the potential conversion of R2 data into spatially-resolved maps of collagen fibre alignment in AC.
This study extends upon previous analytical findings of the rotational diffusion of water around collagen fibres aligned with a static magnetic field4 . NAMD5 was used to simulate the interaction of water with a fixed collagen molecule with the goal of quantifying the nature of both the translational and rotational dynamics of the bound water molecules. From the simulations, the average rotational correlation times and spectral densities of the proton-proton vector of water molecules were calculated and used to investigate the effective relaxation rates of the water in the simulation volume.