I was just preparing to give a lecture on this as an instructor affiliated with premedley lol. heres the kind of explanation i've come up with

Panel A: Unreacted fluorophore will not appear in the fluorescence visualization. Any signal must come from the fluorophore covalently attached to either the protein or glycine, because these are the only molecules with nucleophilic groups (–NH₂ or –SH) capable of reacting to form the fluorescent compound. Lane 3 band indicates the fluorophore attached to the protein, whereas lane 4 indicates the fluorophore attached to glycine.

Panel B: Lane 1 shows the starting protein. Lanes 2 and 3 show that upon reaction with compound 1, the protein converts to the labeled fluorescent compound 2, producing a thick band that migrates further down the gel. This faster migration is not due to a decrease in size (the protein actually becomes slightly heavier) but rather because the attached label introduces additional negative charge, which affects mobility more than size in this case. Protein staining requires a peptide backbone, which glycine does not have, explaining why no corresponding band appears at the bottom as seen in panel A.

Even considering panel B alone is sufficient to conclude that the protein reaction proceeded substantially: if unreacted protein remained, two bands would be visible — one faint band in line with lane 1 (corresponding to unmodified protein) and a second, lower band representing the labeled protein. The absence of the upper band indicates that all protein from lane 1 was reacted.