Which component exhibits an electronic excited state closer in energy to the ground state, a peptide bond or an aromatic side chain?

Which component exhibits an electronic excited state closer in energy to the ground state, a peptide bond or an aromatic side chain?

• A. An aromatic side chain; the absorbed photon energy is higher
• B. An aromatic side chain; the absorbed photon energy is lower
• C. A peptide bond; the absorbed photon energy is higher
• D. A peptide bond; the absorbed photon energy is lower

Answer: (B)

The correct answer indicates that an aromatic side chain exhibits an electronic excited state closer in energy to its ground state, with the energy of the absorbed photon being lower. Aromatic side chains, such as phenylalanine, tyrosine, and tryptophan, have delocalized π electrons due to their conjugated systems. This delocalization results in electronic transitions that typically require lower energy (longer wavelength) photons to promote an electron from the ground state to an excited state. Since the energy gap between the ground and the excited state is smaller compared to other bonds in different molecular structures, the energies of the absorbed photons are lower. In contrast, peptide bonds, which are formed between amino acids, primarily involve less delocalization of electrons and have higher energy transitions. The electronic transitions associated with peptide bonds usually require higher energy photons because they involve σ to σ* transitions, which are generally higher in energy than the π to π* transitions found in aromatic systems. Therefore, since the aromatic side chains can be excited with lower energy photons and have excited states closer to the ground state, the choice indicating that the energy of absorbed photons for an aromatic side chain is lower is accurate.

The correct answer indicates that an aromatic side chain exhibits an electronic excited state closer in energy to its ground state, with the energy of the absorbed photon being lower.

Aromatic side chains, such as phenylalanine, tyrosine, and tryptophan, have delocalized π electrons due to their conjugated systems. This delocalization results in electronic transitions that typically require lower energy (longer wavelength) photons to promote an electron from the ground state to an excited state. Since the energy gap between the ground and the excited state is smaller compared to other bonds in different molecular structures, the energies of the absorbed photons are lower.

In contrast, peptide bonds, which are formed between amino acids, primarily involve less delocalization of electrons and have higher energy transitions. The electronic transitions associated with peptide bonds usually require higher energy photons because they involve σ to σ* transitions, which are generally higher in energy than the π to π* transitions found in aromatic systems.

Therefore, since the aromatic side chains can be excited with lower energy photons and have excited states closer to the ground state, the choice indicating that the energy of absorbed photons for an aromatic side chain is lower is accurate.