I have some questions about the absorption of proteins. First a general question, why have most proteins a yellow color at high concentrations, even though they do not bind a cofactor? What causes this absorbtion?
I work with a protein which is slightly yellow at low concentrations (1mg/mL). I run a UV-vis scan which showed a weak absorption shoulder at 420-30nm. FeS cluster binding proteins show the same weak shoulder at 420nm. But a sequence analysis gave no hint of the presence of a typical Fe or FeS binding motif.
Could this high-concentration-yellow give rise to a shoulder at 420? There are no homologs, maybe there are some similarities to NADH binding proteins, but with bad score. Could the absorption of NADH shift to 420nm when bound to a protein?
Could other cofactors or metals be present? I don´t think that PLP or FAD are present because the absorption spectra look different.
I would be glad for some advise,
Fixed your size problem... Bob.