Thermally Induced Irreversible Conformational Changes in Collagen Probed by Optical Second Harmonic Generation and Laser-induced Fluorescence

Irreversible thermal conformational changes induced to collagen have been studied by optical methods. More specifically, second harmonic generation (SHG) from incident nanosecond Ng:YAG 1064 nm radiation and laser-induced fluorescence by 337 nm, pulsed nanosecond nitrogen laser excitation, at 405, 410 and 415 nm emission wavelengths were registered at eight temperatures (40°, 50°, 55°, 60°, 65°, 70°, 75° and 80°C) and normalised with respect to the corresponding values at the ambient temperature of 30°C. The heating protocol used in this work, was selected to monitor only permanent changes reflecting in the optical properties of the samples under investigation. In this context, the SHG, directly related to the collagen fibril population in triple helix conformation, indicated on irreversible phase transition around 64°C. On the other hand fluorescence related to the destruction of cross-linked chromophores in collagen, some of which are related to the triple helix tertiary structure, also indicated a permanent phase transition around 63°C. These results are in agreement with previous results from studies with differential scanning calorimetry. However SHG and fluorescence, being non-invasive optical methods are expected to have a significant impact in the fields of laser ablative surgery and laser tissue welding.