The relationship between molar extinction coefficient (ε molar) and percent extinction coefficient (ε percent) is as follows: (A / ε percent) 10 = concentration in mg/ml If one wishes to report concentration in terms of mg/ml, then an adjustment factor of 10 must be made when using these percent solution extinction coefficients (i.e., one must convert from 10 mg/ml units to 1 mg/ml concentration units). Solving the expression of Beer’s law for concentration, one can easily see what values are needed to determine the concentration of a peptide or protein solution: Where n is the number of each residue and the stated values are the amino acid molar absorptivities at 280nm and ε molar is molar extinction coefficient. At 280nm, this value is approximated by the weighted sum of the 280nm molar absorption coefficients of these three constituent amino acids, as described in the following equation: The molar absorption coefficient of a peptide or protein is related to its tryptophan (W), tyrosine (Y) and cysteine (C) amino acid composition. Standard laboratory spectrophotometers are fitted for use with 1cm-width sample cuvettes hence, the path length is generally assumed to be equal to one and the term is dropped altogether in most calculations.Ī λ = εcL = εc, when L = 1cm and where c is the protein concentration. For this reason, molar absorptivities are called molar absorption coefficients or molar extinction coefficients. Beer’s Law states that molar absorptivity is constant (and the absorbance is proportional to concentration) for a given substance dissolved in a given solute and measured at a given wavelength.
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