Q.28 Which one of the following methods CANNOT be used to determine the
secondary structure content of a protein?
(A) Circular dichroism spectroscopy
(B) Fourier transform infrared spectroscopy
(C) Mass spectrometry
(D) X–ray crystallography

Mass spectrometry cannot determine the secondary structure content of a protein, making it the correct answer (C). This multiple-choice question tests knowledge of biophysical techniques for analyzing protein folding, crucial for exams like IIT JAM or GATE Biotechnology. Other options effectively probe secondary structures like alpha-helices and beta-sheets through distinct spectroscopic or diffraction principles.

Option Analysis

Circular dichroism (CD) spectroscopy measures differential absorption of circularly polarized light by chiral protein structures, producing characteristic spectra for alpha-helices (strong negative bands at 222 nm and 208 nm) and beta-sheets. Tools like DichroWeb analyze these spectra to quantify secondary structure percentages accurately, even for low sample amounts under physiological conditions. CD excels for rapid, solution-based assessments without crystallization.

Fourier transform infrared (FTIR) spectroscopy detects amide I and II vibrations sensitive to hydrogen bonding in secondary structures, with alpha-helices showing peaks around 1650 cm⁻¹ and beta-sheets at 1630 cm⁻¹. Deconvolution of FTIR spectra estimates helix, sheet, turn, and coil contents reliably, often complementing CD in hydrated or film samples. It handles complex mixtures effectively.

X-ray crystallography provides atomic-resolution electron density maps from diffracted X-rays off protein crystals, directly revealing secondary structure elements like hydrogen-bonded helices and sheets. Refinement yields precise percentages, though it requires crystallizable samples and does not suit disordered regions well. This gold-standard method confirms structures CD and FTIR predict.

Mass spectrometry primarily ionizes proteins, fragments them (e.g., via collision-induced dissociation), and measures mass-to-charge ratios to identify sequences, modifications, or topologies like hydrogen-deuterium exchange patterns. It infers some folding dynamics indirectly but cannot quantify secondary structure content like helices or sheets directly, as fragmentation disrupts local conformations.

Comparison Table