Trivalent chromium and aluminum affect the thermostability and conformation of collagen very differently

Ultrasensitive differential scanning calorimetry (US-DSC) was used to directly measure the thermal transition temperature and energy change of acid soluble collagen in the presence of Cr(3+) and Al(3+) sulfates. The behavior of Cr(3+) was analogous to kosmotropes in the cation Hofmeister series and increased the stability of collagen in dilute solutions. Meanwhile, the denaturational enthalpy change (ΔH) of collagen was substantially reduced with change to increasing Cr(3+) concentration. This is likely due to the uni-point binding of Cr(3+) with carboxyl groups of collagen side chains that could decrease the hydrogen-bonding in collagen and result in the increase of protein hydrophobicity. In the case of Al(3+), the interactions between the ions and collagen showed very different properties: at low and medium ion concentrations, the stability of the collagen was decreased; however, a further increase of Al(3+) concentration led to a salting-out effect of collagen, indicating the Al(3+) is a typical chaotropic ion. This striking difference of the two ions in the stabilization of collagen can be explained in terms of the different interactions between the cations and the carboxyl groups of collagen side chains. Additionally, we studied metal ion induced conformational change by the combination of circular dichroism (CD) and atomic force microscopy (AFM). CD measurements revealed that neither metal ion interactions of collagen with Cr(3+) nor Al(3+) ions destroyed the triple-helical backbone structure of collagen in the solution. AFM results further confirmed that the dehydration of collagen by Cr(3+) is more significant than Al(3+), thus inducing the aggregation of collagen fibrils.