The application of thermal microscopy, differential scanning calorimetry, and Fourier transform infrared microspectroscopy to characterize deterioration and phyisochemical change in fibrous type I collagen

Modification to macromolecular structure in fibrous collagen was explored to characterize physicochemical responses of collagenous materials to deterioration and to establish new analytical methods for conservation research of skin-based museum objects. Fibres of mammalian skin collagen contracted axially due to a thermally-induced structural phase change from a semi-crystalline to amorphous, denatured state at temperatures dependent upon the level of deterioration. Thermal microscopy required microgram sample quantities to record these shrinkage temperatures. In artifact surveys, a 30°C+ range in values recorded below normal suggested enough sensitivity to distinguish subtle conformational variations. Polarized radiation was used in an isothermal FTIR microspectroscopic technique to probe changes in the oriented structure of single fibres. The amide III band (1230 - 1300 cm-1) showed strong infrared dichroism. Absorbance intensity and dichroism fell in response to conformational breakdown; the latter ranged from five (undeteriorated) to one (extremely deteriorated). Thermal FTIR showed reduced absorbance in deteriorated, but not undeteriorated, material at temperatures below the onset shrinkage temperature. Deterioration-induced loss of strength in the coordinate, hydrogen bonding that stabilizes native conformation was revealed by lowered values of specific enthalpy change (ΔhCAL) in denaturation studies by DSC. After peak-fitting to separate the two common endothermic peaks of denaturation, comparative study indicated an uneven distribution of covalent cross-links. Modelling of deterioration was attempted using thermodynamic functions of state, including entropy and Gibbs energy. Ratios of van't Hoff and molar calorimetric enthalpies quantified the level of cooperativity in the stabilization of each set of mutually-dependent structural domains represented by the two separated peak. Decreased cooperativity reflected increased heterogeneity of molecular conformation and stability due to breakdown in the first instance and to increased cross linking in the second. A comparison of van't Hoff enthalpy and cooperativity revealed the existence and quantity of a component of native collagen in severely deteriorated samples which denatured prior to analysis