### **Official Solution & Step-by-Step Derivation (GATE BT 2017 Q.09)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Biophysics & Structural Biology (High-Resolution Protein Structure Determination)
**2. Detailed Step-by-Step Analysis**:
- To determine the three-dimensional tertiary structure of a protein at **atomic resolution** (
lt; 2.5\text{ \AA}$), the scientific community relies on two primary experimental techniques: **X-ray crystallography** and **Nuclear Magnetic Resonance (NMR) spectroscopy** (and more recently, high-resolution cryo-EM for large complexes).
- Multi-dimensional heteronuclear NMR spectroscopy ($^1\text{H}$, $^{13}\text{C}$, $^{15}\text{N}$) measures nuclear Overhauser effects (NOEs) between neighboring proton pairs (
lt; 5\text{ \AA}$ apart) in solution, providing distance restraints that enable full 3D atomic structure determination.
- NMR is ideally suited for small to moderate-sized globular proteins up to approximately $25 - 30\text{ kDa}$ (such as this 15 kDa protein), because smaller proteins tumble rapidly in solution, yielding narrow, well-resolved spectral resonance lines without excessive relaxation broadening.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Raman spectroscopy provides information about vibrational modes and secondary structure content, but cannot determine 3D atomic coordinates.
- **Option (B) [INCORRECT]**: Infrared (IR) spectroscopy reports on amide vibrational transitions, giving only broad secondary structural estimates.
- **Option (C) [INCORRECT]**: UV-Vis spectroscopy measures electronic transitions of aromatic residues and prosthetic groups, giving no atomic coordinates.
- **Option (D) [CORRECT]**: NMR spectroscopy provides complete 3D atomic-resolution structures for a 15 kDa globular protein in solution.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **D**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).