Week 10 Lab: Mass Spectrometry

This laboratory module introduces Liquid Chromatography-Mass Spectrometry (LC-MS) as a premier tool for protein characterization, focusing on enhanced Green Fluorescent Protein (eGFP). The experiment follows a logical progression from “minimally perturbative” measurements to increasingly disruptive techniques, allowing students to determine three critical pieces of information: molecular weight, protein folding/structure, and primary amino acid sequence. By moving from intact protein analysis under both native and denaturing conditions to bottom-up peptide mapping, the lab demonstrates how a protein’s physical state directly influences its behavior in a mass spectrometer.Beyond standard eGFP analysis, the curriculum highlights the biochemical nuances of the protein, such as the autocatalytic self-cyclization of its fluorophore which involves a specific $20\text{ Da}$ mass loss. As a specialized bonus, the lab introduces Charge Detection Mass Spectrometry (CDMS) to analyze macromolecular structures that are too large for conventional MS. Students will use this technology to investigate the quaternary structure of Keyhole Limpet Hemocyanin (KLH), a massive complex that exists in multiple oligomeric states within the megadalton range.

Station 1:

The first stage involves a baseline analysis of the eGFP standard using LC-MS (Liquid Chromatography-Mass Spectrometry). During this process, the LC conditions convert the protein into its denatured form. The goal is to determine the protein’s overall molecular weight by measuring its mass-to-charge (m/z) ratio and charge (z) using the Waters Xevo G3 QTof system.

In this step, we didn’t really get to do much hands-on activity, or really get to enter things into the system. There was one step where someone in my group got to pipette a sample.

Station 2:

This was the second station for us, but technically the last stage. The final stage utilizes a “bottom-up” proteomics approach. The eGFP is enzymatically digested by trypsin, which cleaves the protein chain at specific Lysine and Arginine residues. The resulting peptides are fragmented in a collision cell using nitrogen gas. By measuring these fragments, the researchers can determine the precise amino acid sequence (primary structure) of the protein, a process known as peptide mapping.

Station 3:

Unfortunately, I had to leave by this time. In this step, chromatography is bypassed for direct infusion via a syringe pump to compare the protein’s physical states. Students compare “native” eGFP (folded and compact) with eGFP denatured by formic acid (unfolded and elongated). Because folded proteins have fewer solvent-accessible sites, they exhibit lower charge states, while unfolded proteins produce higher charge states, allowing the mass spectrometer to act as a tool for measuring protein folding and structural integrity.