What is the point of calculating extinction coefficients of a protein without Cys residues?

Question

ProtParam computes various physico-chemical properties that can be deduced from a protein sequence. One of these parameters are "Extinction coefficients". They provide two values. One value considers that all pairs of Cys residues form cystines. The other considers they are reduced.
What is the point of giving two values? Is it to provide the upper and lower bounds?

canadianer · Accepted Answer

Cystine absorbs 280 nm light due to the disulfide bond (ε = 125 M-1cm-1) whereas cysteine does not (appreciably). Thus the extinction coefficient of a protein as a whole depends on the redox state of its cysteine residues. Cysteine residues in a protein may or may not form a disulfide bond and the sequence alone does not contain that information.
You can read about this in the ProtParam documentation.

What is the point of calculating extinction coefficients of a protein without Cys residues?

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