Chemical Effects in Protein Analysis: A Systematic Investigation of Amino Acid Spontaneous Raman and SERS Responses
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Date
2023-05
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The Ohio State University
Abstract
Spontaneous Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS) are powerful non-destructive tools for analysis of biomolecules such as proteins, peptides, and the amino acids comprising them. However, spectral differences observed within literature between spontaneous Raman and SERS spectra of amino acids such as tryptophan have led to issues in effectively analyzing protein and peptide structures via these methods. The cause of these spectral differences is not yet well understood; However, it's believed these differences are due to chemical mechanisms such as charge transfer and pH effects. These mechanisms appear to occur more abundantly within amino acids containing chemical features such as side-chain nitrogen and aromatic rings, providing better SERS enhancement of Raman signal relative to other amino acids. In this work, each amino acid's limit of detection (LOD) was determined via spontaneous Raman spectroscopy to ensure SERS experiments could be performed without spontaneous Raman spectral contributions. SERS experiments were then performed on each amino acid while varying experimental conditions, such as solution pH, to elucidate the chemical mechanism behind each amino acid's SERS enhancement. It was observed experimentally that amino acids containing side-chain nitrogen, such as arginine, are likely enhanced through a pH-dependent effect, and amino acids with aromatic rings, such as tyrosine, are more likely to be enhanced via a charge-transfer mechanism. By better understanding the relationship leading to these differences in SERS enhancement, the chemical mechanisms behind the SERS spectral differences within amino acids may be better understood, leading to improved application of SERS as a bioanalytical tool.
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Keywords
SERS, Raman, Amino Acids, Chemical Effects, Spectroscopy, Hot-electron transfer