I have found graphs for the light absorption of water, haemoglobin, lipids, melanin, etc., but I am having a hard time finding graphs about enzymes.
Are there any websites or other reference sources for this type of information?
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It is not typical to investigate spectral absorption of compounds that are not useful for light absorption qualities, such as the compounds you've listed. Enzymes in particular will have less information because of the changes in binding partners our physical conformation to be enzymatically active. If you think there should be known info then I would try Google Scholar, Pubmed and a google search, in that order.
Edit: The intended meaning of the question is related to spectroscopy and quantification of protein concentration. There are two common ways to determine protein concentration using optical methods (spectroscopy). The methods use colorimetric assays (colour intensity is proportional to protein concentration) or light absorbance at a reference wavelength. Both methods rely on light absorbance at a specific wavelength, but the former relies on an reagent that changes colour in response to protein quantity, and the latter depends on direct UV light absorbance. Here's a quick breakdown of each.
For both of these assays, to determine the quantity of a specific protein, they require a source of purified protein.
These are traditional biochemical assays, of which the most widely used are the: Bradford, BCA and Lowry protein assays. These assays rely on reduction-oxidation reactions between the key assay reagent (Bradford's reagent, bicinchonic acid, or Lowry's reagent) and protein. For example, the BCA reagent is reduced in the presence of protein, causing a chelated copper ion, Cu(II), to be reduced to Cu(I), and effecting a purple colour. This purple colour intensity, which read at a specific wavelength (~520 nm) will indicate the amount of protein.
This method relies on 280 nm (near-UV) absorbance. Usually absorbance at 260 nm is also used for a correction factor. Specifically, amino acids with aromatic rings (eg, tryptophan) cause absorption at 280 nm. Peptide bonds absorb at 200 nm, and various other bonds, pH, ionic conditions, all influence and absorb at various wavelengths.
These methods cannot be used with a protein sample alone, because there are a number of conditions that can affect readings. To correct for this, a protein standard is also used. A protein standard uses a series of known concentrations of a known protein. The common protein concentrations can range from 20-2000 µg/mL; the common protein standards are bovine serum albumin, chicken ovalbumin or purified IgG. The protein standard acts as a reference for protein concentrations that are valid for that experiment, under those specific conditions. A curve is fit to the readings of the protein standard, and the sample concentrations can be read off the standard curve by using the absorbance readings.