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Wavelength-Dependent Conformational Changes in Collagen after Mid-Infrared Laser Ablation of Cornea

Yaowu Xiao ^*, Mingsheng Guo , Peng Zhang , Ganesh Shanmugam , Prasad L. Polavarapu  and M. Shane Hutson ^*

^* Department of Physics and Astronomy and Vanderbilt Institute for Integrative Biosystem Research and Education, Vanderbilt University, Nashville, Tennessee 37235; Department of Physics, Fisk University, Nashville, Tennessee 37208; and Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235

Correspondence: Address reprint requests to M. Shane Hutson, Vanderbilt University, Station B No. 351807, Nashville, TN 37235. Tel.: 615-343-9980; Fax: 615-343-7263; E-mail: shane.hutson{at}vanderbilt.edu.

We ablated porcine corneas with a free electron laser tuned to either 2.77 or 6.45 µm, two matched wavelengths that predominantly target water and protein, respectively. The ejected nonvolatile debris and the crater left behind were examined by circular dichroism, Raman spectroscopy, and scanning electron microscopy to characterize the postablation conformation of collagen proteins. We found near-complete unfolding of collagen secondary and tertiary structure at either ablating wavelength. On the other hand, we found excess fibril swelling and evidence for excess cis-hydroxyproline in the 6.45-µm debris. These results support the hypothesis that the favorable ablative properties of protein-targeting wavelengths rest on selective heating of tissue proteins.