In chemistry, the molar absorption coefficient or molar attenuation coefficient (ε)[1] is a measurement of how strongly a chemical species absorbs, and thereby attenuates, light at a given wavelength.
The set of the following simultaneous equations can be solved to find the concentrations of each absorbing species: The molar absorption coefficient (in units of M−1cm−1) is directly related to the attenuation cross section (in units of cm2) via the Avogadro constant NA:[5] The mass absorption coefficient is equal to the molar absorption coefficient divided by the molar mass of the absorbing species.
where In biochemistry, the molar absorption coefficient of a protein at 280 nm depends almost exclusively on the number of aromatic residues, particularly tryptophan, and can be predicted from the sequence of amino acids.
[6] Similarly, the molar absorption coefficient of nucleic acids at 260 nm can be predicted given the nucleotide sequence.
If the molar absorption coefficient is known, it can be used to determine the concentration of a protein in solution.