Practice question

Purine and pyrimidine nitrogenous bases contain heteroaromatic rings with extensive π electron delocalization and participation of lone pairs, creating strong allowed π→π* and weaker n→π* transitions. Conjugation significantly lowers energy gap, shifting absorption from vacuum UV into middle UV. Maximum absorption occurs near 260 nm with high molar extinction around 10,000 L mol-1 cm-1 per base. This spectral property underpins quantification of nucleic acids by A260, assessment of purity by A260/A280 and A260/A230 ratios, and detection of denaturation via hyperchromic effect as stacked bases unstack and absorb more intensely.