Studies on the effect of dry-heat ageing on parchment deterioration by vibrational spectroscopy and micro hot table method

摘要

In this paper we aimed at the characterization of heat-induced changes on molecular organization of collagen within parchment in very low relative humidity conditions. Collagenous materials are intrinsically heat-sensitive and hygroscopic. Parchment documents and artefacts are heterogenous materials made from a matrix of collagen fibres whose surface is often interfaced with gelatine layers. Thus, as they age and deteriorate, parchments become even less thermally stable and more hygroscopic. Getting insight into the subtler structural alterations of collagen is critical to establishing appropriate temperature and relative humidity conditions for the preservation of parchment artefacts. To this purpose, we monitored the molecular changes induced by exposing parchment prepared according to traditional recipes to increasing temperature (e.g. 50°C, 80°C, 100°C, 120°C, 150°C, 180°C and 200°C) and low relative humidity (10%) by vibrational spectroscopy (Infrared spectroscopy in ATR acquisition mode and Raman spectroscopy) whereas the macroscopic changes in collagen fibers stability were evaluated through Micro Hot Table (MHT) method. Colorimetric measurements were also performed and related to parchment deterioration processes. Artificial ageing confirmed the pattern of collagen thermally induced deterioration consisting of triple helix conversion into intermediate conformations (neither native, nor properly heat-denaturated) followed by denaturation at 120 °C and progressive gelatinisation at T ＞ 150°C. The shrinkage behaviour of collagen fibers was detailed at molecular level by FTIR-ATR and Raman analyses through the multiple changes that occur in the position and intensities of the amide I, amide 11 and amide ill components. Besides, significant changes were detected in the u(C-C) region associated with proline and hydroxyproline residues and protein backbone in the Raman spectra. Moreover, a set of spectral biomarkers was unequivocally assigned to collagen gelatinization. The results of this study can therefore serve as important tools towards a better understanding of ancient parchment condition.