Which transition occurs
Amide chromophore exhibits two major electronic transitions relevant to protein CD. Pi to pi-star around 190 nanometers is intense, electrically allowed, occurring below 210 nanometers and sensitive to exciton coupling in helices. n to pi-star near 220 nanometers is weaker. Sigma to sigma-star requires high vacuum ultraviolet energy. Pi to n is not meaningful for amides. Both pi to pi-star and n to pi-star are optically active in chiral polypeptide arrangement, but pi to pi-star below 210 nanometers provides dominant band for secondary structure quantification.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
