As a molecular biologist with a focus on genetic material and its interactions with light, I can provide an insightful explanation for why DNA absorbs at 280 nm. It's important to understand the basic structure of DNA and the components that contribute to its absorption spectrum.
DNA, or deoxyribonucleic acid, is a molecule that carries the genetic instructions used in the growth, development, and reproduction of all living organisms. It is composed of two strands that coil around each other to form a double helix. Each strand is made up of nucleotides, which are the building blocks of DNA. Nucleotides consist of three components: a sugar molecule (deoxyribose), a phosphate group, and a nitrogenous base. The four nitrogenous bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T).
The absorption of light by DNA is primarily due to the presence of these nitrogenous bases. Each base has a unique structure that allows it to absorb light at specific wavelengths. When it comes to the 280 nm wavelength, the absorption is particularly influenced by the presence of adenine and guanine. Here's why:

1. Adenine and Guanine: These two bases are purines, which means they have a double-ring structure. The structure of these rings allows them to absorb ultraviolet (UV) light effectively. The absorption at 280 nm is attributed to the presence of the purine bases because they have a conjugated system of double bonds that can absorb light in the UV range.

2. Conjugated Systems: In chemistry, a conjugated system is a series of alternating double and single bonds. The presence of such a system in the purine bases of adenine and guanine allows for the delocalization of electrons, which in turn enables the absorption of light at longer wavelengths, including 280 nm.

3. Pi-Electron Delocalization: The double bonds in the purine bases facilitate pi-electron delocalization, which is the spreading out of the electrons across the conjugated system. This delocalization lowers the energy gap between the ground state and the excited state, allowing the molecule to absorb light at lower energies, which corresponds to longer wavelengths.

4. Quantum Mechanics: On a quantum mechanical level, the absorption of light occurs when a molecule absorbs a photon, causing an electron to be excited from a lower energy level to a higher one. The energy of the photon must match the energy difference between these two levels for absorption to take place. In the case of DNA, the energy levels associated with the purine bases correspond to the energy of photons at around 280 nm.

5. Comparison with Proteins: It's worth noting the difference in absorption between DNA and proteins. Proteins primarily absorb at 280 nm due to the presence of aromatic amino acids like tyrosine and tryptophan, which have phenyl rings that can absorb light in the UV range. However, the phenol ring of phenylalanine absorbs weakly at 260 nm, which is why the dominant UV features of proteins are associated with the tyrosyl and indole rings. In contrast, the nucleobases in DNA absorb well at 260 nm, which is a characteristic absorption peak for DNA.
In summary, the absorption of DNA at 280 nm is a result of the electronic properties of the purine bases, adenine and guanine, which have conjugated systems that allow for the absorption of light in the UV range. This understanding is crucial for various applications, including the study of DNA's interactions with light, which can have implications for DNA damage and repair mechanisms, as well as the development of new techniques in molecular biology and genetic engineering.
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For proteins, the absorbance is related to the Tyrosine, Pheynylalanine, and Tryptophan rings. ... The Phenol ring of Phenylalanine actually absorbs weakly at 260 so the dominate UV-vis features of proteins are the tyrosyl and the indole which absorb at 280 nm. Conversely, the nucleobases absorb well at 260 nm.read more >>