Why isn't biuret reagent required for determining the absorption spectrum of hemoglobin?

Biuret test is useful for knowing the arrangement of peptide bonds in a compound. The copper ion of biuret forms violet colored complexes with peptide which indicates the number of peptide bonds present.

The carbonyl group of biuret is hindered by 3 amino which creates hindered interaction with nitro group of haemogobin and thus the spectra will not be observed.

Azo reagents and dithizone are used for detecting the absorption spectrum of haemoglobin

Thus biuret reagent not useful for detecting absorption spectrum of haemoglobin