Practice question

Among organic electronic transitions, excitation energy correlates inversely with wavelength. While σ–σ* demands vacuum UV, n–σ* requires far UV, π–π* and n–π* lie in near UV region accessible to conventional spectrophotometers. In biological chromophores, intense, symmetry-allowed π→π* transitions of conjugated aromatic rings, double bonds and heterocyclic bases possess relatively small energy gaps and large extinction coefficients, appearing at longer wavelengths around 260-280 nm with strong intensity. Although n→π* is even lower energy, it is forbidden and weak, so prominent lowest-energy strong absorption routinely observed for proteins and nucleic acids is π–π*.