Abstract

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.

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