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

Abstract

We describe a new optical parametric amplifier (OPA) that employs lithium thioindate, LiInS₂ (LIS), to create tunable infrared light between 1500 cm⁻¹ and 2000 cm⁻¹. 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, AgGaS₂ (AGS), or silver gallium selenide, AgGaSe₂ (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 (d₈-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-NH₂. The amide I mode at the d₈-PS-buffer interface was found to be centered around 1655 cm⁻¹. This can be interpreted as the peptide having maintained its α-helical structure when adsorbed on the hydrophobic surface, although other interpretations are discussed.

Keywords

Sum frequency generation SFG Lithium thioindate LIS Biointerface LK peptide Solid–liquid interface Silver gallium sulfide

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