Thermal degradation of dentine collagen heated between 100o.C and 300o.C: electron spin resonance, ultraviolet-visible and infrared spectroscopy

Abstract

By heating the dentine makes possible to observe loss of water, changes in collagen conformation, rise of ESR signals and tissue browning; these changes start at 100OC and are present up to 300OC, before organic matrix elimination. Some of the changes were observed reported in the literature but their origins are not totally understood. In this work we correlate the changes observed with Electron Spin Resonance (ESR) spectroscopy, Ultraviolet-Visible (UV-Vis) and Infrared (IR) spectroscopy clarifying the chemical origin of all observed results. We analyse heated bovine dentine between 100OC and 300OC with exposure time of 30 minutes; the samples were cooling down before analysis in the spectrometers. Increasing the heating temperature we observed by infrared spectroscopy the loss of water (3800-2500 cm−1 ) and conformational change of collagen molecule (fingerprint region: 1201 cm−1 ; 1281 cm−1 ; 1234 cm−1 and 1334 cm−1 ); all infrared bands decrease linearly with the temperature. The heated dentine shows ESR signals that were identified at g values between 2.005 and 2.007 and line width of about 0.8 mT. The ESR intensity increases exponentially with the temperature according the Arrhenius equation. Over the evaluated temperature range the dentine becomes brown and this colour is assigned to the optical density (O.D.) increase at ultraviolet and blue spectral range. The O.D. increases with the temperature, an empirical equation was determined: O.D.= (103.08/λ) K, where λ is the wavelength and K is a constant that change for different temperatures: 3.4 at 100OC, and 10.2 for samples heated at 300OC. The correlation between infrared absorption bands, rise of ESR signals and tissue browning was done. The reduction of water was correlated (correlation coefficient (R) equal 0.85) with the collagen infrared bands; these collagen bands were also correlated with the increase of ESR signals (R=-0.94) and tissue browning (R=-0.87); as well as the ESR signals is correlated with tissue browning with R=0.92. This work shows that there is an association between water loss, collagen conformation, ESR signals and tissue browning after heating the dentine to temperatures up to 300OC, indicating that all observed results are assigned to changes in the structure of collagen molecule.