Molar attenuation coefficient
Molar attenuation coefficient, is a measurement of how strongly a chemical species attenuates light at a given wavelength. It is an intrinsic property of the species. The SI unit of molar attenuation coefficient is the square metre per mole (m2/mol), but in practice, it is usually taken as the M−1⋅cm−1 or the L⋅mol−1⋅cm−1. In older literature, the cm2/mol is sometimes used with corresponding values 1,000 times larger. These units may look different, but it is just a matter of expressing volume in cm3 or in L. The use of the terms "molar extinction coefficient" and "molar absorptivity" for molar attenuation coefficient is discouraged.
- ε is the molar attenuation coefficient of that material;
- c is the amount concentration of those species;
- ℓ is the pathlength.
Different disciplines have different conventions as to whether absorbance is decadic (10-based) or Napierian (e-based), i.e., defined with respect to the transmission via common logarithm (log10) or a natural logarithm (ln). The molar attenuation coefficient is usually decadic. When ambiguity exists, it is best to indicate which one applies.
When there are N attenuation species in a solution, the overall absorbance is the sum of the absorbances for each individual specie i:
The composition of a mixture of N attenuating species can be found by measuring the absorbance at N wavelengths (the values of the molar coefficient of attenuation for each specie at these wavelengths must also be known). The wavelengths chosen are usually the wavelengths of maximum absorption (absorbance maxima) for the individual species. None of the wavelengths must be an isosbestic point for a pair of species. The set of the following simultaneous equations can be solved to find the concentrations of each attenuating specie:
In biochemistry, the molar attenuation 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.
If the molar attenuation coefficient is known, it can be used to determine the concentration of a protein in solution.
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Extinction".
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Absorptivity".
- Lakowicz, J. R. (2006). Principles of Fluorescence Spectroscopy (3rd ed.). New York: Springer. p. 59. ISBN 9780387312781.
- Gill, S. C.; von Hippel, P. H. (1989). "Calculation of protein extinction coefficients from amino acid sequence data". Analytical Biochemistry 182 (2): 319–326. PMID 2610349.
- Nikon MicroscopyU: Introduction to Fluorescent Proteins includes a table of molar attenuation coefficient of fluorescent proteins.