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30 original questions, modeled on the official MCAT format and covering Chem/Phys, CARS, Bio/Biochem and Psych/Soc. Every question comes with why the answer is right, why the other choices are wrong, a key takeaway and a tip. Practice mode explains each question as you go; Timed mode runs one clock and saves the explanations for the end.
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In native gel electrophoresis, no detergent is added, so each peptide keeps its own net charge as it moves through the gel. A research team synthesized four peptides, P1–P4, to study how charge and size affect migration. In every peptide, the N-terminal amine was acetylated and the C-terminal carboxyl group was converted to an amide, so neither end of the chain can gain or lose a proton. Only the ionizable side chains listed in Table 2 contribute to net charge; their pKa values are given in Table 1. Each peptide's molar mass was confirmed by mass spectrometry.
The gel and the running buffer both contained 50 mM sodium phosphate adjusted to pH 8.2. Near this pH, the buffering pair is H₂PO₄⁻/HPO₄²⁻, whose pKa is 7.2. The samples were loaded into wells cast across the middle of the gel, halfway between the two electrodes, so that a peptide could migrate toward either one. A power supply held a constant 150 V across electrodes 10.0 cm apart; assume that the electric field between them is uniform. After 60 min, the gel was stained, and the distance of each band from its well was measured (Table 2).
| Residue | Side-chain pKa |
|---|---|
| Aspartic acid (Asp) | 3.65 |
| Glutamic acid (Glu) | 4.25 |
| Histidine (His) | 6.00 |
| Lysine (Lys) | 10.53 |
| Arginine (Arg) | 12.48 |
| Peptide | Ionizable side chains | Molar mass (kDa) | Distance from well (mm) |
|---|---|---|---|
| P1 | 2 Asp, 1 Glu, 1 Lys | 1.2 | 24 |
| P2 | 1 Asp, 1 Glu, 1 Lys | 1.2 | 12 |
| P3 | 2 Asp, 2 Glu, 1 Lys, 1 Arg | 2.4 | 16 |
| P4 | 1 Asp, 1 Glu, 2 His, 1 Lys | 1.3 | 11 |
Distances are measured from the well, whichever way the band moved.
MCAT, Chem/Phys: question 1 of 8
What is the net charge of P4 at the pH of the run, and toward which electrode does P4 migrate?
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