Surface-Enhanced Raman Spectra of Eye Lens Pigments

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Abstract
Surface-enhanced Raman spectroscopy (SERS) has been applied to study lenticular pigments that are present in the eyes of certain diurnally active animals. Using Ag hydrosols pre-aggregated with NaClO4, we have obtained SERS spectra from dilute solutions of various model pigment compounds, including kynurenine, N-formylkynurenine, 3-hydroxykynurenine, β-carboline, bityrosine, anthranilic acid, 3-hydroxyanthranilic acid, and oxindole. The results obtained from these model compounds show that SERS is a particularly sensitive technique for the identification of lens pigments. We also report a procedure that enables high-quality SERS data to be obtained for the yellow pigments in the lens homogenates of grey squirrels, ground squirrels, and chipmunks. The surface Raman results confirm the identity of the low-molecular-weight, water-soluble pigment in the grey squirrel lens as a derivative of 3-hydroxykynurenine, but reveal that lens pigmentation in ground squirrels and chipmunks involves new chromophores. The significance of this finding in relation to the metabolic/photochemical generation of lens pigments is discussed.