We describe a new optical parametric amplifier (OPA) that employs lithium thioindate, LiInS<sub>2</sub> (LIS), to create tunable infrared light between 1500 cm<sup>−1</sup> and 2000 cm<sup>−1</sup>. The OPA based on LIS described within provides intense infrared light with a good beam profile relative to similar OPAs built on silver gallium sulfide, AgGaS<sub>2</sub> (AGS), or silver gallium selenide, AgGaSe<sub>2</sub> (AGSe). We have used the new LIS OPA to perform surface-specific sum frequency generation (SFG) vibrational spectroscopy of the amide I vibrational mode of a model peptide at the hydrophobic deuterated polystyrene (<i>d</i><sub>8</sub>-PS)–phosphate buffered saline interface. This model polypeptide (which is known to be an α-helix in the bulk solution under the high ionic strength conditions employed here) contains hydrophobic leucyl (L) residues and hydrophilic lysyl (K) residues, with sequence Ac-LKKLLKLLKKLLKL-NH<sub>2</sub>. The amide I mode at the <i>d</i><sub>8</sub>-PS–buffer interface was found to be centered around 1655 cm<sup>−1</sup>. This can be interpreted as the peptide having maintained its α-helical structure when adsorbed on the hydrophobic surface, although other interpretations are discussed.
Vol. 3, Iss. 11 Virtual Journal for Biomedical Optics
Roger L. York, George J. Holinga, Dean R. Guyer, Keith R. McCrea, Robert S. Ward, and Gabor A. Somorjai, "A New Optical Parametric Amplifier Based on Lithium Thioindate Used for Sum Frequency Generation Vibrational Spectroscopic Studies of the Amide I Mode of an Interfacial Model Peptide," Appl. Spectrosc. 62, 937-940 (2008)