Mass Spectrometry for Peptide Analysis

Mass spectrometry (MS) is indispensable for peptide characterization, providing molecular weight confirmation, sequence verification, and purity assessment. Modern peptide analysis employs multiple MS techniques, each optimized for specific applications.

Mass Spectrometry Techniques

MALDI-TOF MS

Matrix-Assisted Laser Desorption/Ionization Time-of-Flight is the most common technique for peptide molecular weight confirmation.

Principle:

1. Peptide co-crystallized with matrix (e.g., α-cyano-4-hydroxycinnamic acid, CHCA)
2. Laser desorbs and ionizes peptide-matrix crystals
3. Ions accelerate through electric field
4. Time-of-flight measured (m/z separation)
5. Spectrum recorded

Operating Parameters:

Parameter	Value
Matrix	CHCA (0.5–1 mg/mL in 50% ACN/0.1% TFA)
Laser	337 nm nitrogen laser
Accelerating voltage	15–25 kV
Polarity	Positive (most common)
Mass range	500–50,000 Da
Resolution	2,000–5,000 FWHM
Accuracy	±0.1–0.5% (external calibration)

Sample Preparation:

1. Dissolve peptide in water or dilute ACN (1 pmol/µL)
2. Mix 1:1 with matrix solution
3. Spot 1 µL on MALDI target
4. Air dry (crystallization)
5. Acquire spectrum (100–500 laser shots)

ESI-MS

Electrospray Ionization MS produces multiply charged ions from solution, enabling molecular weight determination of larger peptides.

Principle:

1. Peptide solution sprayed through charged capillary
2. Electric field produces charged droplets
3. Solvent evaporates, droplets shrink
4. Coulombic explosions produce gas-phase ions
5. Ions analyzed by mass analyzer (quadrupole, TOF, or Orbitrap)

Operating Parameters:

Parameter	Value
Solvent	50% ACN/0.1% formic acid
Flow rate	0.2–1.0 µL/min (nanospray)
Capillary voltage	2–4 kV
Polarity	Positive or negative
Mass range	100–100,000 Da
Resolution	5,000–100,000
Accuracy	±0.01–0.1% (internal calibration)

Charge State Distribution:

• [M+2H]²⁺ for peptides 1–2 kDa
• [M+3H]³⁺ for peptides 2–4 kDa
• [M+4H]⁴⁺ or higher for peptides >4 kDa

LC-MS/MS

Liquid Chromatography coupled with tandem MS provides sequence information through fragmentation.

Workflow:

1. Peptide separated by RP-HPLC (C18, 75 µm × 150 mm)
2. Eluate electrosprayed into mass spectrometer
3. Survey scan (MS1) identifies peptide ions
4. Selected ions fragmented (MS2)
5. Fragment spectrum interpreted for sequence

Fragmentation Methods:

Method	Mechanism	Best For
CID	Collision-induced dissociation	Standard sequencing
HCD	Higher-energy CID	Improved y-ion series
ETD	Electron-transfer dissociation	PTMs, intact proteins
ECD	Electron-capture dissociation	Labile modifications

Fragmentation Patterns

Peptide Bond Cleavage

Fragmentation produces N-terminal (a, b, c) and C-terminal (x, y, z) ions:

     a₁   b₁   c₁          x₁   y₁   z₁
      |    |    |            |    |    |
H₂N—AA₁—AA₂—AA₃—AA₄—COOH
      |    |    |            |    |    |
     a₂   b₂   c₂          x₂   y₂   z₂

b/y ions (most common in CID):

• b-ions: N-terminal fragments (acylium ions)
• y-ions: C-terminal fragments (protonated)
• Sequence read from b-ions (N→C) or y-ions (C→N)

Common Fragment Ions

Ion Type	Formation	Mass Shift
b-ion	Amide bond cleavage, charge retained on N-term	Residue MW
y-ion	Amide bond cleavage, charge retained on C-term	Residue MW + 18
a-ion	b-ion - CO (28 Da)	Residue MW - 28
immonium	Side chain loss	Characteristic per AA

Characteristic Fragment Ions

Amino Acid	Immonium Ion (m/z)
Gly	30
Ala	44
Val	72
Leu/Ile	86
Pro	70
Phe	120
Trp	159
Tyr	136
Met	104
Cys	76
Ser	60
Thr	74
Asp	88
Glu	102
Asn	87
Gln	101
Lys	101
Arg	100

De Novo Sequencing

Manual Sequencing

1. Identify b-ion or y-ion series
2. Calculate mass differences between consecutive ions
3. Match differences to amino acid residue masses
4. Verify with immonium ions and neutral losses

Example (b-ion series):

• b₁ = 175 → Gly (75)
• b₂ = 246 → +71 = Ala (89) → wait, need to recalculate

Correct approach:

• b₁ = 75 → Gly
• b₂ = 164 → +89 = Ala
• b₃ = 309 → +145 = Gln? Check residue masses

Software-Assisted Sequencing

Software	Platform	Features
PEAKS	Commercial	De novo + database search
Mascot	Commercial	Database search
X!Tandem	Open source	Database search
Novor	Free	Real-time de novo
Andromeda	Free (MaxQuant)	Database search

Sequencing Challenges

Challenge	Cause	Solution
Ambiguous Leu/Ile	Same mass (113.08 Da)	MS³ or chemical modification
Gln/Lys	Near-identical mass (128.06 vs 128.09)	High-resolution MS (>50,000)
Asn/Gln deamidation	Mass +1 Da	Check for +1 Da peaks
Methionine oxidation	Mass +16 Da	Check for +16 Da peaks
Pyroglutamate formation	N-terminal Gln cyclization	Mass -17 Da

Purity Assessment by MS

Single-Component Analysis

• Confirm [M+H]⁺ matches expected mass
• Check for ±1 Da variants (deamidation, oxidation)
• Verify isotopic envelope matches theoretical distribution

Multi-Component Mixtures

• LC-MS separates components before MS analysis
• Deconvolution software resolves overlapping charge states
• Extracted ion chromatograms quantify individual components

Sample Preparation

For MALDI-TOF

1. Dissolve peptide: 1 pmol/µL in water or 0.1% TFA
2. Prepare matrix: 10 mg/mL CHCA in 50% ACN/0.1% TFA
3. Mix 1:1 (v/v) sample:matrix
4. Spot 1 µL on target, air dry
5. Optional: Wash crystals with cold water

For ESI-MS

1. Dissolve peptide: 1–10 pmol/µL in 50% ACN/0.1% formic acid
2. Filter (0.22 µm syringe filter)
3. Infuse directly or inject via LC
4. Optimize spray parameters

For LC-MS/MS

1. Dissolve peptide: 100 fmol/µL in water
2. Inject 1–5 µL
3. Gradient: 5–50% ACN over 30 minutes
4. Column: C18, 75 µm × 150 mm, 3 µm
5. Flow rate: 300 nL/min

Common Mass Shifts

Modification	Mass Shift (Da)	Cause
Oxidation (Met)	+16	Air exposure
Deamidation (Asn/Gln)	+1	Aging, basic pH
Pyroglutamate	−17	N-terminal Gln
Trt incomplete	+234	Incomplete deprotection
Acetylation	+42	N-terminal acetylation
TFA adduct	+118	Residual TFA
Sodium adduct	+22	Na⁺ replacement of H⁺

Quantitation

Amino Acid Analysis (AAA)

• Hydrolyze peptide (6M HCl, 110°C, 24 h)
• Derivatize with OPA/FMOC
• HPLC with fluorescence detection
• Compare to amino acid standard curve

UV Spectroscopy

• Use extinction coefficient (ε₂₈₀) calculated from Trp/Tyr/Cys content
• Beer-Lambert law: A = ε × c × l
• Accurate for pure peptides with aromatic residues

MS-Based Quantitation

• SIR/MRM: Selected ion monitoring for targeted quantitation
• Label-free: Normalized spectral counting
• Isotope-labeled: SILAC or AQUA internal standards

Safety Considerations

• High voltage: MALDI and ESI use kilovolt potentials
• Solvents: ACN, methanol are flammable and toxic
• Laser: MALDI laser is Class 3B — eye protection required
• Vacuum: MS instruments operate under high vacuum
• Matrix: CHCA is an irritant — handle in fume hood

References

1. Fenn JB, et al. “Electrospray ionization for mass spectrometry of large biomolecules.” Science 1989;246:64-71.
2. Hillenkamp F, et al. “Matrix-assisted laser desorption/ionization mass spectrometry of biopolymers.” Anal Chem 1991;63:1193A-1203A.
3. Domon B, Costello CE. “A systematic nomenclature for carbohydrate fragmentations in FAB-MS/MS spectra.” Glycoconj J 1988;5:397-409.
4. Aebersold R, Mann M. “Mass spectrometry-based proteomics.” Nature 2003;422:198-207.
5. Paizs B, Suhai S. “Fragmentation pathways of protonated peptides.” Mass Spectrom Rev 2005;24:508-548.