Mass Spectrometry in Biochemistry

MS Fundamentals - What's the Charge?

• Core principle: Measures mass-to-charge ratio ($m/z$) of ions. Neutral molecules are invisible.
• Sample molecules are ionized (charged) in the gas phase.
  • Common charges: $\mathbf{+1}, \mathbf{+2}, \mathbf{-1}$.
• Ions are accelerated and separated by electric/magnetic fields based on their $m/z$.
• The $m/z$ value is key: for positive ions, $m/z = (M_{molecule} + n \cdot M_{H^+})/n$.
• Output: Mass spectrum (ion abundance vs. $m/z$).

⭐ Multiply charged ions (e.g., proteins, $z > \mathbf{1}$) appear at lower $m/z$ values, extending the instrument's effective mass range.

The MS Toolkit - Parts & Players

• Ion Source: Generates gas-phase ions from analyte.
  • MALDI: Sample + matrix + laser. For peptides, proteins. Soft ionization.
  • ESI: Charged capillary, liquid sample. For polar, labile molecules. Forms multiply charged ions.
• Mass Analyzer: Separates ions by $m/z$ ratio.
  • TOF: Measures ion flight time. Lighter ions are faster.
  • Quadrupole: Electric fields filter ions by $m/z$.
  • Orbitrap: Traps ions; offers high resolution & mass accuracy.
• Detector: Counts ions; generates mass spectrum (intensity vs. $m/z$).

⭐ ESI is a "soft" ionization method, vital for analyzing intact large biomolecules (e.g., proteins) with minimal fragmentation, often producing multiply charged ions.

Key Techniques - Ionize & Analyze!

• Ionization Methods:
  • MALDI (Matrix-Assisted Laser Desorption/Ionization):
    • Large biomolecules (proteins, peptides); solid sample + matrix.
    • Laser desorption/ionization; primarily singly charged ions $[M+H]^+$.
    • 📌 MALDI: "Mighty Molecules, Laser Driven Ionization".
  • ESI (Electrospray Ionization):
    • Peptides, proteins from liquid solutions.
    • High voltage → charged droplets → multiply charged ions $[M+nH]^{n+}$.
    • 📌 ESI: "Existing Solutions/Samples, Electrically Sprayed Ions".

    ⭐ ESI's multiple charging allows analysis of very large molecules (e.g., proteins > 100 kDa) on analyzers with limited $m/z$ ranges.

  • EI (Electron Ionization):
    • Small, volatile molecules; extensive fragmentation (structural info).
• Mass Analyzers:
  • TOF (Time-of-Flight):
    • Ions drift; lighter ions faster. $t \propto \sqrt{m/z}$.
    • High resolution; common with MALDI.
  • Quadrupole (Q):
    • Mass filter using DC/RF fields; stable ion paths for selected $m/z$.
  • Ion Trap (IT):
    • Traps ions; sequential ejection by $m/z$. Good for MSn.
  • Orbitrap:
    • High resolution/accuracy; ions orbit central electrode.

Biochem Applications - MS Solves It!

Mass Spectrometry (MS) is a powerhouse in biochemical analysis, offering diverse applications:

• Protein Characterization:
  • Identification: Peptide Mass Fingerprinting (PMF) matches peptide masses to database proteins.
  • Sequencing: Tandem MS (MS/MS) fragments peptides for de novo sequencing.
  • PTM Analysis: Detects phosphorylation, glycosylation, ubiquitination.
• Proteomics:
  • Quantitative analysis (e.g., SILAC, iTRAQ) to compare protein levels.
  • Mapping protein interaction networks.
• Metabolomics:
  • Profiling endogenous small molecules (metabolites).
  • Identifying disease biomarkers.
• Drug Discovery:
  • Pharmacokinetics (ADME: Absorption, Distribution, Metabolism, Excretion).
  • Metabolite identification of drugs.
• Clinical Diagnostics:
  • Newborn screening for metabolic disorders (e.g., PKU, MCADD).
  • Rapid microbial identification.
  • Cancer biomarker detection.

⭐ MS is pivotal in newborn screening, detecting inborn errors of metabolism like Phenylketonuria (PKU) by analyzing amino acid and acylcarnitine profiles from dried blood spots.

High‑Yield Points - ⚡ Biggest Takeaways

• MS Principle: Measures mass-to-charge ratio (m/z) of ions for identification/quantification.
• Core Components: Ion Source (MALDI, ESI), Mass Analyzer (TOF, Quadrupole), Detector.
• MALDI: For large biomolecules (proteins, peptides); ESI: For polar molecules, often with LC.
• Tandem MS (MS/MS): Enables structural elucidation and peptide sequencing via ion fragmentation.
• Key Applications: Proteomics (protein ID), metabolomics, drug analysis, newborn screening.
• Clinical Utility: Microorganism ID, biomarker discovery, therapeutic drug monitoring.