Question

Even at relatively high resolution, most of the peaks in a HNMR spectrum of a protein are broad, which makes it difficult to extract information about biological samples using NMR. This is because:

• A. Presence of paramagnetic ions in biological molecules
• B. Single monomer chain in biopolymers
• C. Absence of paramgnetic ions in biopolymers
• D. Absence of Chemical exchange of protons between different close-by sites

Hint:

In NMR spectroscopy, remember the main causes of line broadening: slow molecular tumbling (large molecules), presence of paramagnetic species, and chemical exchange at an intermediate rate. The absence of these factors generally leads to sharper peaks.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful technique for determining the structure of molecules. The width of an NMR peak is related to the spin-spin relaxation time (\(T_2\)). Broader peaks correspond to shorter \(T_2\) times. For large molecules like proteins, peaks are inherently broad due to their slow tumbling in solution, which leads to efficient relaxation and thus a short \(T_2\). The question asks for a reason for this broadening.
Step 2: Detailed Explanation:
Let's analyze the given options in the context of NMR line broadening:
1. Presence of paramagnetic ions in biological molecules: Paramagnetic species (molecules with unpaired electrons), such as certain metal ions (e.g., Fe\(^{3+}\), Mn\(^{2+}\), Cu\(^{2+}\)) that can be present in metalloproteins, create strong local magnetic fields. These fields fluctuate as the molecule tumbles, providing a very efficient mechanism for nuclear spin relaxation. This drastically shortens \(T_1\) and \(T_2\) times, leading to significant line broadening. This is a valid and strong cause for broadening.
2. Single monomer chain in biopolymers: The fact that a protein is a polymer (a chain of monomers) is related to its large size, but the term "single monomer chain" itself does not explain the broadening. The broadening is due to the properties of the polymer as a whole (large size, slow tumbling), not the fact it's made of monomers.
3. Absence of paramagnetic ions in biopolymers: The absence of paramagnetic ions would generally lead to sharper lines, not broader ones.
4. Absence of Chemical exchange of protons...: Chemical exchange (e.g., a proton moving between different chemical environments) at an intermediate rate on the NMR timescale is a well-known cause of line broadening. The absence of such exchange would lead to sharper signals.
Step 3: Final Answer:
While the primary reason for the general broadness of protein NMR peaks is slow molecular tumbling (a consequence of large size), this is not an option. Among the choices provided, the presence of paramagnetic ions is a potent and correct physical reason for severe line broadening in NMR spectra of biological molecules. The other options are either irrelevant or describe conditions that would lead to sharper, not broader, peaks. Therefore, this is the most plausible answer.