Because collagen is a biological material, it is not surprising that its elastic modulus is dependent on pH. If the pH of the surrounding solution is too acidic or too basic, collagen's elastic modulus decreases. Specifically, Silver et al. found that for pHs approaching the pKa of carboxylic acid (3.5) or amine , the elastic modulus of collagen begins to decrease. [4] It is hypothesized that the charged pairs on the outside of the triple helix stabilize the molecule, and that the mechanism of elastic energy storage is related to the separation of the charged pairs. For a pH within the range of 3.5 to 10.6, the amine groups of collagen will be protonated and the carboxylic acids will not, maximizing the charge-charge interactions on the surface of the triple helix.