DFT and surface-enhanced Raman scattering study of tryptophan-silver complex

Area of Science:

• Analytical Chemistry
• Biochemistry
• Materials Science

Background:

• Tryptophan is an essential amino acid vital for human growth and nitrogen balance.
• Understanding tryptophan's surface interactions is crucial due to its biological significance.
• Surface-enhanced Raman scattering (SERS) is a powerful technique for studying molecular adsorption.

Purpose of the Study:

• To investigate the surface adsorption properties of tryptophan on a silver hydrosol using SERS.
• To elucidate the binding mechanism and orientation of tryptophan on the silver surface.
• To support experimental findings with theoretical density functional theory (DFT) calculations.

Main Methods:

• Surface-enhanced Raman scattering (SERS) spectroscopy was employed.
• Silver hydrosol was used as the SERS substrate.
• Density Functional Theory (DFT) calculations with B3LYP/LANL2DZ were performed.

Main Results:

• A significant enhancement of the 1343 cm(-1) band (CO(2) sym. stretching) was observed in SERS spectra, with a 63 cm(-1) red shift.
• Experimental and theoretical data indicate chemisorption of tryptophan onto the silver surface.
• Tryptophan adsorbs via oxygen and nitrogen atoms of carboxylate and amino groups, with an edge-on orientation.

Conclusions:

• The observed spectral shifts and enhancements suggest a significant contribution of the chemical mechanism to SERS activity.
• Tryptophan chemisorbs to silver, adopting an orientation with the indole ring nearly perpendicular to the surface.
• SERS enhancement factors for tryptophan vibrations range from 10(5) to 10(6), highlighting the technique's sensitivity.