Tropocollagen (TC) and hydroxyapatite (HAP) interfaces are one of the main load bearing entities in bone family of materials. Atomistic interactions in such interfaces occur in a variety of chemical environments under a range of biomechanical loading conditions. It is challenging to investigate such interactions using traditional analytical or using classical molecular simulation approaches owing to their limitations in predicting bond strength change as a function of change in chemical environment. In the present work, 3D ab initio molecular dynamics simulations are used to understand such atomistic interactions by analyzing tensile strain dependent deformation mechanism and strength of two structurally distinct idealized TC-HAP interfaces in hydrated as well as unhydrated environments. Analyses suggest that the presence of water molecules leads to modification of H-bond density at the interfaces that also depends upon the level of strain. TC molecules become stiffer in the presence of water due to the presence of H-bonds. Bond forming-and-breaking cycle change as a function of H-bond density lies at the heart of TC-HAP interfacial shear deformation. Consequently, interfaces with TC molecule placed flat on the HAP crystal surface experience significantly higher shear stress during deformation in comparison to the interfaces with TC molecule placed with their axes perpendicular to the HAP surface.