What is the principle of UV absorption of proteins and nucleic acids? What are the differences? What is the mechanism of charge?
1. Principle of Ultraviolet Absorption:
Proteins and nucleic acids can both absorb ultraviolet light. This absorption is due to certain parts of their molecules where π electrons and unpaired n electrons transition to a higher energy state after absorbing light of a certain energy. The aromatic amino acids in proteins (such as tyrosine, phenylalanine, and histidine) are primarily responsible for ultraviolet absorption. The aromatic rings in these amino acids contain π electrons that can transition to a higher energy state upon absorbing ultraviolet light. The absorption of nucleic acids mainly comes from their bases (such as adenine, thymine, guanine, and cytosine). The π structure in these bases also contains π electrons that can transition upon absorbing ultraviolet light.
2. Differences:
Although both proteins and nucleic acids can absorb ultraviolet light, their absorption peaks are at different wavelengths. Nucleic acids typically have maximum absorption at 260 nm, whereas proteins usually have maximum absorption at 280 nm.
3. Charge Mechanism:
Protein and nucleic acid molecules carry charges in solution, primarily due to the ionization of their functional groups, such as carboxyl (COOH) and amino (NH2) groups, at specific pH levels. For instance, proteins are uncharged at their isoelectric point, but above or below this point, their amino or carboxyl groups ionize, resulting in a positive or negative charge on the protein. The phosphate groups of nucleic acids are always negatively charged in solution.
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