NMR spectroscopy stands out as the key technique for resolving the atomic-level structure of small globular proteins like those around 15 kDa, making option (d) the correct choice in this multiple-choice question. This method excels in solution-state analysis, providing precise internuclear distances and angles essential for high-resolution models.​

Correct Answer

The correct option is (d) NMR spectroscopy.​
Nuclear Magnetic Resonance (NMR) spectroscopy delivers atomic resolution structures for proteins up to about 30-40 kDa, particularly globular ones around 15 kDa (~130 amino acids). Studies confirm its use for 10-15 kDa proteins with heavy-atom RMSD values below 1.5 Å at optimal resolution.​

Option Explanations

Raman Spectroscopy

Raman spectroscopy probes vibrational modes to assess secondary structures like alpha-helices or beta-sheets in proteins. It offers conformational insights but lacks atomic resolution for full 3D coordinates.​

IR Spectroscopy

Infrared (IR) spectroscopy identifies functional groups and secondary structure content via amide bond vibrations. Like Raman, it provides ensemble-averaged data without precise atomic positions.​

UV Spectroscopy

UV spectroscopy measures aromatic amino acid absorbance to estimate protein concentration or folding states. It reveals tertiary changes grossly but cannot resolve atomic structures.​

NMR Spectroscopy

NMR uses nuclear Overhauser effects (NOEs), coupling constants, and chemical shifts to calculate atomic coordinates in solution. For 15 kDa proteins, high digital resolution minimizes overlap, yielding structures with ~1.1 Å accuracy.​

Comparison Table