Question

What is an extinction coefficient? Why is it different for each protein? Show how you would convert an extinction coefficient in M-1 cm-1 units to mg-1 mL cm-1 and how you would convert an extinction coefficient in %-1 cm-1 to mg-1 mL cm-1

Answer 1

an extinction coefficient define how strongly a substance absorbs light at a given wavelength, per mass density or per molar concentration, respectively.

It will vary depending on the functional group present on a molecule because of the diferent transitions of each group.  proteins and peptides absorb UV-light in proportion to their aromatic amino acid content and total concentration, That is why it is different to each protein.

in the general formula, the units for concentration, c, are percent solution (i.e., 1% = 1 g/100 ml = 10 mg/ml).

A / εpercent = percent concentration

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).

(A / εpercent) 10 = concentration in mg/ml

The relationship between Molar Extinction Coefficient (εmolar) and Percent Extinction Coefficient (εpercent) is as follows

(εmolar) 10 = (εpercent) × (molecular weight of protein)