Principles of UV Absorption in Proteins and Nucleic Acids

Principles of UV Absorption

Both proteins and nucleic acids can absorb ultraviolet (UV) light. This absorption is due to the transition of certain π-electrons and unpaired n-electrons in their molecules to higher energy states after absorbing light of a certain energy. The aromatic amino acids in proteins (such as tyrosine, phenylalanine, and histidine) are mainly responsible for UV absorption. The aromatic rings in these amino acids contain π-electrons, which can transition to a higher energy state upon absorbing UV light. The absorption in nucleic acids mainly comes from the bases they contain (such as adenine, thymine, guanine, and cytosine). The π-structures in these bases also contain π-electrons that can transition upon UV absorption.

Differences

Although both proteins and nucleic acids can absorb UV light, their absorption peaks occur at different wavelengths. Nucleic acids typically have a maximum absorption at 260 nm, while proteins typically have a maximum absorption at 280 nm.

Charge Mechanism

Both protein and nucleic acid molecules carry charges in solution, mainly due to the ionization of their functional groups such as carboxyl groups (COOH) and amino groups (NH2) at specific pH levels. For example, proteins are electrically neutral at their isoelectric point, but above or below this point, their amino or carboxyl groups ionize, making the protein positively or negatively charged. Nucleic acids' phosphate groups are always negatively charged in solution.

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