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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

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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 (d 8-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 d 8-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: BIOINTERFACE; LIS; LITHIUM THIOINDATE; LK PEPTIDE; SFG; SILVER GALLIUM SULFIDE; SOLID-LIQUID INTERFACE; SUM FREQUENCY GENERATION

Document Type: Research Article

DOI: http://dx.doi.org/10.1366/000370208785793227

Affiliations: 1: Department of Chemistry, University of California, Berkeley, Berkeley, California 94720; Materials Science Division, Lawrence Berkeley National Laboratories, Berkeley, California 94720; Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138 2: Department of Chemistry, University of California, Berkeley, Berkeley, California 94720; Materials Science Division, Lawrence Berkeley National Laboratories, Berkeley, California 94720 3: LaserVision, Bellevue, Washington 98004 4: The Polymer Technology Group, Berkeley, California 94710

Publication date: September 1, 2008

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