Changes in aggregation behavior of collagen molecules in solution with varying concentrations of acetic acid

A critical aggregation concentration of 0.30-0.50 mg/mL was previously obtained for type I collagen at 0.1 M acetic acid (AA). In the present study, the aggregation behavior of collagen in solution (0.5 mg/mL) in the presence of 0.1-2.0 M AA was investigated. Circular dichroism showed that the three helix structure was maintained across the whole AA concentration range. However, the ratio of positive peak intensity over negative peak intensity varied depending on the conformational state of collagen aggregates. Ultra-sensitive differential scanning calorimetry revealed that transition temperatures Tm1 and Tm2 decreased by 8.35 °C and 7.80 °C, respectively, between 0.1 M and 2.0 M, indicating a possible relationship between the aggregation state and the thermal effect. The surrounding polarity of collagen molecules in solution containing pyrene was investigated by fluorescence spectroscopy, which demonstrated that disaggregation of collagen aggregates was enhanced with increasing AA concentration. This observation was correlated with changes in collagen fiber size observed by atomic force microscopy. Furthermore, collagen tyrosine residues were blue-shifted in an intrinsic fluorescence spectra, further indicating changes in aggregation behavior with increasing AA concentration. Finally, the dynamic response of collagen molecules to AA was analyzed by two-dimensional correlation fluorescence spectra.