As a domain expert in the field of biochemistry, I can provide you with a detailed explanation of what absorbs light at 280 nm. The absorbance at 280 nm is a characteristic wavelength often used to determine the concentration of proteins in solution. This is because certain components within proteins are particularly good at absorbing ultraviolet (UV) light at this wavelength.

Proteins are large biomolecules made up of amino acids, which are the building blocks of proteins. Among these amino acids, those with aromatic rings are particularly noteworthy. Aromatic amino acids include tryptophan, tyrosine, and phenylalanine. The presence of these aromatic rings allows these amino acids to absorb UV light quite effectively, especially at wavelengths around 280 nm. This absorption is due to the π-electron systems within the aromatic rings, which can interact with the photons of light at these wavelengths.

The absorbance maxima at 280 nm is primarily attributed to the presence of these aromatic amino acids. However, it's important to note that while tryptophan has a strong absorbance at 280 nm, it is not the only contributor. Tyrosine and phenylalanine also contribute to the absorbance, albeit to a lesser extent.

In addition to the aromatic amino acids, the peptide bonds that link the amino acids together in a protein also absorb UV light, but they do so at a different wavelength, around 200 nm. This is due to the electronic transitions that can occur within the peptide bond structure.

When measuring the absorbance of a protein solution at 280 nm, the absorbance is directly proportional to the concentration of the protein, assuming that the light path and the instrument used are calibrated correctly. This relationship is described by the Beer-Lambert Law, which states that the absorbance (A) is equal to the product of the molar absorptivity (ε), the path length (l), and the concentration (c) of the absorbing species:

\[ A = ε \cdot l \cdot c \]

This law is fundamental in spectrophotometry and allows for the quantification of protein concentrations in a sample.

It's also worth mentioning that while the absorbance at 280 nm is a common method for protein quantification, it is not the only method. Other wavelengths, such as 260 nm, can also be used, particularly to quantify nucleic acids, which absorb strongly at this wavelength due to the presence of their aromatic bases.

In summary, proteins in solution absorb light at 280 nm primarily due to the presence of aromatic amino acids with their π-electron systems. Peptide bonds also contribute to the absorbance at this wavelength, but to a lesser extent, with their main absorbance peak occurring at 200 nm. This property of proteins is exploited in various biochemical assays and techniques for protein quantification and analysis.

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Proteins in solution absorb ultraviolet light with absorbance maxima at 280 and 200 nm. Amino acids with aromatic rings are the primary reason for the absorbance peak at 280 nm. Peptide bonds are primarily responsible for the peak at 200 nm.read more >>