Practice question

Circular dichroism distinguishes secondary structures through differential absorption of circularly polarized light by chiral peptide bonds. Alpha helix shows exciton coupling of pi-pi* transitions producing intense positive band near 193 nm and two negative bands at 208 nm for parallel polarized component and 222 nm for n-pi* transition. Beta sheet exhibits single minimum near 218 nm. Observing double minima at 208 and 222 nm strongly indicates helical content, though NMR chemical shifts, J-couplings, and NOEs provide residue-specific confirmation. Combined CD and NMR offers robust structural validation for recombinant proteins and peptides.