Probing the vibrational modes of proteins diffusing on a charged plasmonic surface

Proteins are essential biomolecules that play vital roles in almost all biological processes. Changes in their structure, even at extremely low concentrations, have been long associated with diseases like Alzheimer’s or Parkinson’s. Studying and understanding the protein folding process and factors that can influence it are therefore important to be studied. Raman spectroscopy of proteins allow researchers to identify protein structure on the basis of their vibrational modes. Raman signatures are weak and thus Surface-Enhanced Raman Spectroscopy (SERS) is used widely by researchers today. SERS uses metallic nanostructures to greatly enhance the Raman signal. To increase stability of this enhanced Raman signal self-assembled monolayers are used to act as a barrier between the gold nanoparticles and the protein. This research aims to understand the influence of the (net-)charge of proteins and their environment on their structural conformation, while in solution. In this report the Surface-Enhanced Raman Spectra of two protein solutions, containing Bovine Serum Albumin (BSA) or Lysozyme, on top of bare gold nanoparticles or a negatively charged self-assembled-monolayer are analyzed. Two gold nanoparticle aggregation protocol and a second linking molecule are also evaluated for SERS measurements in solution. The research shows that the gold nanoparticle aggregation protocol using acetone is unsuitable for SERS measurements in solution as the acetone on top of the aggregation interferes with the signal coming from the protein, even when little acetone is used. Due to inconsistent signal appearing in dry conditions, cysteamine hydrochloride was found to be unsuitable as a self-assembled monolayer for protein solution measurements. The research does show succesful measurements with both proteins on top op an MPA self-assembled monolayer. From these measurements it was found that BSA undergoes a structural change from mostly α-helix to a more β-sheet conformation. The negative charge of MPA was found to induce α-helix formation in lysozyme by interacting with its positively charged side-chains.