NMR for Peptide Structure Determination

Nuclear magnetic resonance (NMR) spectroscopy provides atomic-resolution structural information for peptides in solution. Unlike X-ray crystallography, NMR determines structure under near-physiological conditions, revealing dynamic properties and solvent interactions.

Principles of Peptide NMR

Nuclear Spin and Resonance

NMR measures the magnetic properties of nuclei with non-zero spin quantum numbers:

Nucleus	Spin	Natural Abundance	Sensitivity	Application
¹H	1/2	99.98%	1.00	Primary detection
¹³C	1/2	1.11%	0.016	Backbone/side chain
¹⁵N	1/2	0.37%	0.001	Backbone assignment
²H	1	0.015%	0.010	Solvent suppression

Chemical Shift

The resonance frequency of a nucleus depends on its electronic environment:

• Shielded: Higher electron density → lower frequency (upfield)
• Deshielded: Lower electron density → higher frequency (downfield)
• Chemical shift (δ): Referenced to TMS (0 ppm) or DSS (0 ppm)

1D NMR

¹H NMR Spectrum

A typical peptide ¹H NMR spectrum shows:

Region (ppm)	Assignment
10–12	Trp indole NH, Arg guanidinium
8.0–8.5	Amide NH (α-helix: ~8.0; β-sheet: ~8.5)
7.0–7.5	His C2-H, C4-H
6.5–7.5	Tyr aromatic, Phe aromatic
5.5–6.0	Trp aromatic
4.5–5.0	α-H (α-helix: ~4.0; β-sheet: ~4.5)
3.5–4.5	β-H, side chain CH₂
2.0–3.0	Lys ε-CH₂, Arg δ-CH₂, Met CH₃
0.8–1.5	Val, Leu, Ile methyl groups

Amide Region Analysis

The amide region (8.0–8.5 ppm) provides secondary structure information:

• α-Helix: Narrow dispersion, NH at ~8.0 ppm
• β-Sheet: Wide dispersion, NH at ~8.5 ppm
• Random coil: Narrow dispersion, NH at ~8.3 ppm

2D NMR

TOCSY (Total Correlation Spectroscopy)

TOCSY identifies amino acid spin systems through scalar (J) coupling:

Cross-peaks: Connect protons within the same amino acid residue

Amino Acid	Characteristic TOCSY Pattern
Gly	Single cross-peak (α-H₂)
Ala	Doublet at 1.4 ppm (β-CH₃)
Val	Two doublets at 0.9–1.0 ppm
Leu/Ile	Two doublets at 0.8–0.9 ppm
Lys	Three cross-peaks (β, γ, δ, ε)
Arg	Three cross-peaks (β, γ, δ)
Pro	Characteristic α-H at 4.5 ppm

Experiment Parameters:

• Mixing time: 60–80 ms (spin lock)
• Sweep width: 12–14 ppm (¹H)
• Data points: 2048 × 512
• Solvent: D₂O or 90% H₂O/10% D₂O

NOESY (Nuclear Overhauser Effect Spectroscopy)

NOESY identifies spatially close protons (<5 Å) through dipolar coupling:

Cross-peaks: Connect protons close in space (not necessarily bonded)

Sequential Assignment Strategy:

1. Identify dαN(i, i+1) connectivities: α-H(i) → NH(i+1)
2. Identify dNN(i, i+1) connectivities: NH(i) → NH(i+1)
3. Build chain from N-terminus to C-terminus

Characteristic NOE Patterns:

Structure	NOE Pattern
α-helix	Strong dNN(i, i+1), weak dαN(i, i+1)
β-sheet	Strong dαN(i, i+1), weak dNN(i, i+1)
Turn	dNN(i, i+1), dαN(i+2, i+3)

COSY (Correlation Spectroscopy)

COSY identifies scalar-coupled protons (2–3 bonds):

• α-H → β-H: Within each residue
• β-H → γ-H: Side chain assignment
• Less useful than TOCSY for peptides (limited to J-coupled networks)

HSQC (Heteronuclear Single Quantum Coherence)

HSQC correlates ¹H with directly bonded ¹³C or ¹⁵N:

¹H-¹⁵N HSQC:

• Each amide N-H shows one cross-peak
• Fingerprint of the peptide
• Used for backbone assignment and monitoring folding

¹H-¹³C HSQC:

• Correlates ¹H with directly bonded ¹³C
• Side chain assignment
• Methyl groups appear in aliphatic region

Chemical Shift Assignment

Strategy Overview

1. Identify amino acid types: TOCSY
2. Sequential assignment: NOESY (dαN, dNN connectivities)
3. Side chain assignment: TOCSY + COSY + HSQC
4. Stereospecific assignment: NOE patterns, J-coupling

Step-by-Step Assignment

Step 1: Amino Acid Identification (TOCSY)

Residue	TOCSY Signature
Gly	α-H₂ at ~3.9 ppm
Ala	β-CH₃ doublet at 1.4 ppm
Val	β-H doublet at 2.1 ppm; γ-CH₃ doublets
Leu/Ile	β-H multiplet; δ-CH₃ doublets
Pro	α-H at 4.5 ppm; no amide NH
Lys	ε-CH₂ at 3.0 ppm
Arg	δ-CH₂ at 3.2 ppm

Step 2: Sequential Assignment (NOESY)

1. Start at N-terminus (dαN(1,2))
2. Walk through sequence using dαN(i, i+1)
3. Verify with dNN(i, i+1) and dβN(i, i+1)
4. Use prolines as break points (no amide NH)

Step 3: Side Chain Assignment

1. From α-H assignment, use TOCSY to find side chain protons
2. Use COSY to connect within spin systems
3. Use ¹³C HSQC for carbon chemical shifts

Chemical Shift Index (CSI)

Compare observed α-H shifts to random coil values:

Deviation	Interpretation
Δδ > 0.1 ppm upfield	α-helix
Δδ < −0.1 ppm downfield	β-sheet
	Δδ

Structure Calculation

Distance Restraints

From NOESY cross-peak intensities:

NOE Intensity	Distance Range (Å)
Strong	1.8–2.5
Medium	1.8–3.5
Weak	1.8–5.0

Calculation Methods

Distance Geometry:

• Convert NOE distances to 3D coordinates
• Algorithms: DGSA, DGS, AMBER

Simulated Annealing:

• Minimize energy with NOE restraints
• Programs: X-PLOR, CNS, CYANA, ARIA

Molecular Dynamics:

• Refine structures with explicit solvent
• Programs: GROMACS, NAMD, AMBER

Structure Quality Metrics

Metric	Good	Acceptable
RMSD (backbone)	<0.5 Å	<1.0 Å
RMSD (all heavy)	<1.0 Å	<1.5 Å
NOE violations	<0.1 Å	<0.3 Å
Ramachandran favored	>90%	>80%
Ramachandran allowed	>98%	>95%
PROCHECK score	>−0.5	>−1.0

Practical Considerations

Sample Requirements

Parameter	Requirement
Concentration	0.5–2 mM (optimal)
Volume	300–600 µL
Solvent	D₂O or 90% H₂O/10% D₂O
pH	4–7 (exchange-dependent)
Temperature	25°C (standard)
Salt	50–100 mM NaCl
Isotopic labeling	¹⁵N, ¹³C for large peptides (>15 aa)

Exchange Considerations

• Amide exchange: Broadens NH peaks at high pH
• Optimal pH: 4–5 for most peptides
• D₂O exchange: Progressive loss of NH signals
• H₂O/D₂O: Use 90% H₂O/10% D₂O to preserve NH signals

Temperature Effects

Temperature	Effect
5°C	Slower exchange, sharper peaks
25°C	Standard
37°C	Physiological, faster exchange
50°C	Reduced viscosity, broader lines

Applications

Secondary Structure Determination

• Chemical shift analysis: CSI method
• J-coupling: ³J_{HNα} values (helix: ~4 Hz; sheet: ~9 Hz)
• NOE patterns: dαN vs dNN connectivity patterns
• Hydrogen bonding: H/D exchange experiments

Dynamics Studies

• Relaxation (T₁, T₂): Backbone dynamics
• Exchange rates: Conformational flexibility
• μs-ms dynamics: Conformational exchange (CPMG, R₁ρ)
• ps-ns dynamics: Internal motions (Model-free analysis)

Binding Studies

• Chemical shift perturbation: Map binding interface
• Saturation transfer difference (STD): Ligand binding
• WaterLOGSY: Ligand screening
• Relaxation dispersion: Binding kinetics

Limitations

• Size limit: >50 aa requires isotopic labeling and TROSY
• Conformational exchange: Broadens lines, obscures peaks
• Overlap: Sequence-specific assignment challenging
• Time: Full structure determination requires weeks
• Cost: NMR spectrometer time is expensive

References

1. Wüthrich K. “NMR of proteins and nucleic acids.” Wiley 1986.
2. Cavanagh J, et al. “Protein NMR Spectroscopy.” Academic Press 2007.
3. Williamson MP. “Using chemical shift perturbation to characterise ligand binding.” Prog Nucl Magn Reson Spectrosc 2013;73:1-16.
4. Marion D. “Introduction to biological NMR spectroscopy.” J Biomol NMR 2013;55:303-310.
5. Pervushin K, et al. “TROSY: a new approach for high-resolution protein NMR.” J Biomol NMR 1998;12:345-353.