= Protein chemistry
\b[Protein chemistry] studies the composition, connectivity, chemical transformations and physical behavior of <proteins> and <peptides>, together with methods for isolating and measuring them. <Polypeptide> sequence, <post-translational modification>, <protein folding> and <protein denaturation> answer different questions about the same material. Chemical modification changes connectivity or chemical groups; changing conformation need not change the encoded sequence.
Measurement itself requires a chain of chemical and physical operations. In a <Western blot>, separated <proteins> are transferred from a gel to a receiving sheet before detection. The 1979 study by <Harry Towbin>, <Theophil Staehelin> and <Julian Gordon> used electrophoretic transfer to <nitrocellulose> and then immunological detection. Transfer of the tested ribosomal proteins from urea-containing gels was quantitative, but the SDS-gel procedure preserved the band pattern without quantitative transfer. A recognizable pattern therefore did not prove complete recovery of every protein.<Electrophoretic transfer of proteins from polyacrylamide gels to nitrocellulose sheets: procedure and some applications by Harry Towbin and others>[Abstract; pp.4350–4352, transfer procedures, Figure1 and Table1]{ref}
An original measurement model separates abundance from recovery. In a linear detection range, write the observed signal as
$$
S=k\,r\,Q,
$$
where $Q$ is the amount loaded, $r$ the recovered fraction and $k$ the detection response per recovered amount. If two samples each contain100 units of protein but their recovered fractions are0.8 and0.4, the signals are $80k$ and $40k$. A twofold signal difference occurs without a twofold abundance difference. This simplified model is not a calibration curve for a particular assay.
<Western blot controls> help test such alternatives, while structural and chemical methods ask other questions. Finding a <protein> band does not by itself determine its <protein tertiary structure> or identify every covalent modification. Protein chemistry connects the molecular definition of a sample to the transformations and recovery steps used to observe it.
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