The following mass spectra of an IgG-antigen complex were obtained by ESI at low, denaturing pH (top spectrum) and moderate pH 6.9 (bottom spectrum).
a. A and B which have m/z ratios of 2605.5 and 2652.0, and C and D have m/z ratios of 5721.3 and 5950.2. What are the Mr’s of the IgG and of the IgG-antigen complex?
b. What is the Mr of the antigen?
c. How many + charges would you expect the IgG to carry at pH 2, assuming average amino acid
composition? How does this compare to the + charge carried by the major species in the low pH
spectrum and to the maximum charge seen in the spectrum?
d. For the IgG-antibody complex at pH 6.9 (lower spectrum), how many + charges does it carry in
the major peak? Is the pI of the complex above or below 6.9? Justify your answer.
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