50. Detailed NMR spectra of a 20-residue peptide were recorded using a 600 MHz instrument. If the peptide adopts an α-helical conformation, which one of the following statements is correct?
(A) Prominent NHᵢ − NHᵢ₊₁ NOE peaks would be observed along with ³JNH–Hα coupling constants 8.5 Hz.
(B) Prominent CαHᵢ − NHᵢ₊₁ NOE peaks would be observed along with ³JNH–Hα coupling constants 4.8 Hz.
(C) Prominent CαHᵢ − NHᵢ₊₁ NOE peaks with ³JNH–Hα coupling constants 8.5 Hz.
(D) Prominent NHᵢ − NHᵢ₊₁ NOE peaks along with ³JNH–Hα coupling constants 4.8 Hz.
Identifying an Alpha-Helical Peptide Using NOE Patterns and J Coupling Constants in NMR Spectroscopy
Correct Answer
Option (4): Prominent NHi–NHi+1 NOE peaks along with 3JHN-Hα coupling constants of 4.8 Hz.
Explanation
Nuclear Magnetic Resonance (NMR) spectroscopy is one of the most informative techniques for determining the secondary structure of peptides in solution. Two important parameters used for structural assignment are the vicinal scalar coupling constant (3JHN-Hα) and Nuclear Overhauser Effect (NOE) connectivities observed in NOESY spectra. These two measurements provide complementary information about backbone geometry and spatial proximity of atoms, allowing reliable identification of secondary structural elements.
The 3JHN-Hα coupling constant depends on the backbone dihedral angle (φ) according to the Karplus relationship. In an α-helix, the backbone adopts dihedral angles of approximately φ = −57° and ψ = −47°. These angles produce relatively small vicinal coupling constants, generally ranging from 3 to 5 Hz. A coupling constant of approximately 4.8 Hz therefore strongly supports an α-helical backbone. In contrast, β-sheet conformations possess more extended backbone geometries and typically exhibit coupling constants between 8 and 10 Hz.
The NOESY spectrum provides independent structural evidence. In an α-helix, adjacent backbone amide protons are positioned close together in three-dimensional space because of the regular helical arrangement. Consequently, strong sequential NHi–NHi+1 NOE cross-peaks are observed throughout the helix. Helical peptides also exhibit characteristic medium-range NOEs such as NHi–NHi+2, NHi–NHi+3, NHi–NHi+4, and CαHi–NHi+3, but the most consistent sequential feature is the strong NHi–NHi+1 connectivity.
Since both the small coupling constant and the sequential NH-NH NOE pattern are characteristic of an α-helix, the correct combination is the one described in Option (4).
Why Option (1) is Incorrect
The presence of strong NHi–NHi+1 NOEs is consistent with an α-helix, but the reported coupling constant of 8.5 Hz is not. Such a large vicinal coupling constant is characteristic of extended backbone conformations such as β-sheets. Therefore, this option combines two mutually inconsistent structural indicators.
Why Option (2) is Incorrect
The coupling constant of 4.8 Hz is appropriate for an α-helical structure. However, the characteristic sequential NOE in an α-helix is NHi–NHi+1, whereas CαHi–NHi+1 NOEs are generally weaker in helices and are more commonly associated with extended conformations. Therefore, this option does not describe the most characteristic NMR signature of an α-helix.
Why Option (3) is Incorrect
This option combines a CαHi–NHi+1 NOE with a large coupling constant of 8.5 Hz. The large coupling constant corresponds to an extended β-sheet-like backbone rather than an α-helix. Consequently, this option is inconsistent with the helical conformation described in the question.
Why Option (4) is Correct
Strong sequential NHi–NHi+1 NOE connectivities indicate that adjacent amide protons are close together in space, a defining feature of α-helices. The coupling constant of approximately 4.8 Hz is also characteristic of the backbone dihedral angles present in α-helical structures. Since both experimental observations independently support the same secondary structure, this option correctly describes an α-helical peptide.
Relationship Between J Coupling and Backbone Geometry
The vicinal scalar coupling constant between the amide proton and the α-proton varies systematically with the backbone dihedral angle according to the Karplus equation. Smaller coupling constants are observed when the peptide backbone adopts the compact geometry of an α-helix, whereas larger values occur in the extended backbone conformations typical of β-sheets. Measurement of 3JHN-Hα therefore provides valuable information about peptide secondary structure.
Importance of NOESY in Secondary Structure Analysis
NOESY spectroscopy detects through-space interactions between protons separated by less than approximately 5 Å. Because different secondary structures position backbone atoms differently, each structural motif generates a characteristic pattern of NOE cross-peaks. α-Helices display strong sequential NH-NH and medium-range NOEs, whereas β-sheets are characterized by numerous long-range inter-strand NOEs between residues located on adjacent strands. These characteristic interaction patterns are widely used for solution structure determination.
Combining Independent NMR Parameters
Reliable assignment of protein secondary structure is achieved by integrating multiple experimental observations rather than relying on a single measurement. Coupling constants reveal backbone dihedral angles, NOE connectivities identify spatial relationships between atoms, and chemical shift dispersion provides information about structural order. Agreement among these independent parameters significantly increases confidence in the final structural interpretation.
Conclusion
An α-helical peptide is characterized by relatively small 3JHN-Hα coupling constants, typically around 4–5 Hz, together with strong sequential NHi–NHi+1 NOE cross-peaks in the NOESY spectrum. Therefore, the correct answer is Option (4).
