1       2
ASSNTFECTVGPITWCVKSASNA

The rest of the sequence contains all thinkable amino acids but no cysteine. It was observed that nexistepas changes its weight a little bit if it stands for many hours at pH 7.2 at room temperature. And the weight change is compatible with the oxidation of one cysteine. Can you help the group that works on nexistepas by predicting which of the two cysteines it is that gets oxidized? Justify your answer in no more than 20 words.

Oxidation is, oversimplified, a reaction with oxygen. But whatever, oxidation makes the peptide heavier because something binds to it. And that something binds at the outside. The peptide is a hairpin (the beginning and end are a bit unclear, but the bit around the turn is very clear). The first C is inside and the second points outside. (Be aware that in the strands the residues alternatingly point in and out, so a residue that sits between an Arg and a Glu, for example, will itself point inwards; while a C that sits between a Trp and a Phe or some other very hydrophobic, inward-pointing residues will itself point outwards.

                 1
   A-S-S-N-T-F-E-C-T-V-G
                       |
 A-N-S-A-S-K-V-C-W-T-I-P
               2

In exactly 20 words: Oxidation happens at the surface. Cysteine-2 is at the hydrophilic side of the β-strand, and thus oxidized at the surface.