Electrophoresis and Protein Separation (5C) - Chem & Physics

1. A lab separated two proteins (P and Q) using native agarose gel electrophoresis at constant voltage (120 V) with the same gel concentration. The buffer pH was varied. Observed migration toward the anode (+) from the cathode (−): - At pH 5.5: P migrated 0.4 cm; Q migrated 3.6 cm - At pH 8.5: P migrated 4.2 cm; Q migrated 0.6 cm Which conclusion about their isoelectric points (pI) is most consistent with the data?
   Answer: P has a pI between 5.5 and 8.5, while Q has a pI below 5.5. This question tests the inference of isoelectric points (pI) from migration patterns in native agarose gel electrophoresis across different pH values. Electrophoresis separates proteins by net charge, where proteins are positive below their pI, negative above it, and neutral at pI, with size affecting migration speed through the gel. In this setup with varying buffer pH and migration toward the anode, changes in distance indicate shifts in net charge relative to pI. The correct answer A is consistent because P migrated little at pH 5.5 (likely positive) but far at pH 8.5 (negative), suggesting pI between 5.5 and 8.5, while Q showed the opposite, indicating pI below 5.5. Distractor D is wrong as it assumes pI near 7.0 for both due to swapped distances, ignoring that migration reversal reflects charge sign changes around distinct pIs. For verification, test at intermediate pH like 7.0 to pinpoint where each protein has zero migration, confirming pI ranges. Comparing with known pI standards can ensure migration patterns align with expected charge behaviors at different pH.
2. A lab used SDS-PAGE (non-reducing vs reducing) to assess whether a secreted protein exists as a disulfide-linked dimer. Samples were run at pH 8.3, 160 V, on the same gel percentage. Under non-reducing conditions, a single band appeared at ~100 kDa. After adding 50 mM DTT and heating, the dominant band shifted. Which result is most consistent with a disulfide-linked homodimer? Data (apparent band position): - Non-reducing: 100 kDa - Reducing: ?
   Answer: A dominant band at ~50 kDa under reducing conditions This question tests understanding of how reducing agents affect disulfide-linked protein complexes in SDS-PAGE. Under non-reducing conditions, the protein migrated as a 100 kDa band, suggesting either a single 100 kDa protein or a disulfide-linked complex. DTT (dithiothreitol) is a reducing agent that breaks disulfide bonds, separating any covalently linked subunits. If the 100 kDa band represents a homodimer of two 50 kDa subunits linked by disulfide bonds, reduction would produce a dominant band at 50 kDa. The correct answer (B) identifies this expected result for a disulfide-linked homodimer. Choice A incorrectly suggests the mass would double under reducing conditions, when reduction actually separates complexes into smaller subunits. To verify disulfide-linked complexes, compare non-reducing and reducing SDS-PAGE: a shift to half the apparent mass indicates a homodimer.
3. A researcher ran SDS-PAGE (reducing conditions) for four proteins at pH 8.3, constant 160 V, loading at the cathode (−). Each protein binds SDS similarly. Migration distances were: Protein | MW (kDa) | Distance (cm) W | 20 | 6.1 X | 35 | 4.8 Y | 55 | 3.3 Z | 90 | 2.0 Which protein is most likely to migrate fastest if the gel percentage is decreased (e.g., from 12% to 8%) while keeping voltage and run time constant?
   Answer: W, because smaller proteins experience less sieving and will still migrate farthest. This question probes the effect of gel percentage on protein migration in SDS-PAGE, emphasizing size-based separation. Electrophoresis in SDS-PAGE separates by molecular weight (MW) as SDS equalizes charge-to-mass, with smaller proteins migrating faster through gel pores. In this reducing SDS-PAGE setup, decreasing gel percentage enlarges pores, reducing sieving and allowing all proteins to migrate farther, but smallest ones benefit most. The correct answer B is supported because W (20 kDa), the smallest, will experience the least resistance in a lower-percentage gel, migrating fastest relative to others. Distractor D fails by stating SDS equalizes migration distances, misunderstanding that SDS equalizes charge but size still determines speed via sieving. A transferable check is to compare migration in gradient gels, where small proteins resolve better in high-percentage zones. Verifying with MW ladders can confirm smaller proteins gain more mobility as gel percentage decreases.
4. A researcher attempted to separate two similarly sized proteins (A and B, each ~60 kDa) by native PAGE at pH 7.0. The buffer was then changed to pH 9.0, keeping voltage (140 V), gel percentage, and run time constant. Migration was measured toward the anode (+). Which conclusion about relative net charge is most supported by the observed change in migration? Data: At pH 7.0: A = 14 mm, B = 16 mm At pH 9.0: A = 34 mm, B = 18 mm
   Answer: Protein A becomes more negatively charged than B at pH 9.0 This question tests understanding of pH effects on protein charge and relative migration in native PAGE. At pH 7.0, proteins A and B migrated similarly (14 mm vs 16 mm), but at pH 9.0, protein A migrated much farther (34 mm) while B showed minimal change (18 mm). Increasing pH from 7.0 to 9.0 deprotonates acidic residues, making proteins more negatively charged. Protein A's dramatic increase in migration indicates it gained significantly more negative charge, likely due to having more acidic residues or a pI between 7-9. The correct answer (A) correctly identifies that protein A becomes more negatively charged than B at pH 9.0. Choice B incorrectly suggests A becomes more positive at higher pH, contradicting basic pH-charge relationships. To predict pH effects on migration, remember that increasing pH makes proteins more negative (faster anode migration) while decreasing pH makes them more positive.
5. A membrane protein complex was analyzed by native PAGE at pH 7.4 (no SDS). Two conditions were tested: 100 V and 200 V, each for 15 min, same gel and buffer. Migration distances toward the anode were measured. Condition (V) vs migration (mm): - 100 V: Protein M = 9, Protein N = 18 - 200 V: Protein M = 17, Protein N = 34 Based on the data, which factor most directly explains the change in migration distance when voltage is increased?
   Answer: Higher voltage increases the electric field, increasing electrophoretic force on charged proteins. This question tests understanding of how voltage affects protein migration in electrophoresis. Electrophoresis separates proteins by applying an electric field that exerts force on charged molecules. Doubling the voltage from 100V to 200V doubles the electric field strength, which doubles the electrophoretic force on each charged protein. The data shows both proteins approximately doubled their migration distances (M: 9→17 mm, N: 18→34 mm), confirming that migration distance is proportional to applied voltage. Choice B is incorrect because voltage doesn't change protein charge - the charge depends on amino acid composition and pH. The key principle is that electrophoretic velocity equals (charge × field strength) / friction, so doubling field strength doubles velocity.
6. A researcher ran native PAGE at pH 7.4 to compare two proteins with similar size: U (pI 7.2, 48 kDa) and V (pI 5.4, 50 kDa). At 160 V for 25 min (loaded at cathode), U migrated 0.9 cm toward the anode, while V migrated 4.6 cm. Which conclusion about net charge at pH 7.4 is most supported?
   Answer: V is more net negative than U at pH 7.4, leading to greater migration toward the anode. This question tests deduction of net charge from migration in native PAGE, based on pI relative to pH. In electrophoresis, proteins with pI below pH are net negative and migrate toward the anode, with greater charge difference increasing distance. In this setup at pH 7.4 with similar sizes, farther migration indicates more negative charge. The correct answer B is supported as V (pI 5.4 << 7.4) migrated farther (4.6 cm) than U (pI 7.2 ≈ 7.4, 0.9 cm), showing V is more negative. Distractor D fails by stating V is net positive (pI < pH means negative), misunderstanding pI-pH relationship. For verification, adjust pH above both pIs to equalize charges and check size effects. Comparing migration ratios can quantify charge differences in similar setups.
7. A lab compared native PAGE migration of two proteins at pH 7.8, 150 V: J (pI 7.7, 20 kDa) and K (pI 6.0, 80 kDa). J migrated 0.7 cm; K migrated 3.2 cm toward the anode (+). Which interpretation is most consistent with the data?
   Answer: K likely has a larger net negative charge magnitude at pH 7.8, overcoming its larger size and enabling greater migration. This question tests the understanding of protein separation by electrophoresis, specifically how native polyacrylamide gel electrophoresis (PAGE) separates proteins based on their charge and size. In electrophoresis, proteins migrate through a gel matrix under an electric field, with the direction and speed influenced by their net charge—negatively charged proteins move toward the anode—and their size, where smaller proteins typically encounter less resistance. In this native PAGE setup at pH 7.8, proteins retain their native structure, so separation depends on both the net charge determined by the pH relative to each protein's isoelectric point (pI) and their molecular weight (MW). The data show that protein K (pI 6.0, 80 kDa) migrates farther (3.2 cm) than protein J (pI 7.7, 20 kDa), which is consistent with K having a larger net negative charge at pH 7.8 (since 7.8 is much greater than its pI) that overcomes its larger size, enabling greater migration toward the anode. A common distractor, such as choice D, fails because it incorrectly assumes native PAGE separates solely by MW, ignoring the role of charge in native conditions, which is a misconception confusing it with SDS-PAGE. To verify charge effects, consider how altering the buffer pH closer to a protein's pI would reduce its migration distance due to decreased net charge. Additionally, in scenarios with similar charges, smaller proteins like J would migrate farther, highlighting the need to evaluate both charge magnitude and size in electrophoresis outcomes.
8. A lab compared migration of a 70 kDa enzyme in native PAGE under two buffer conditions at the same voltage/time: Buffer 1 at pH 7.0 and Buffer 2 at pH 10.0. The enzyme migrated 8 mm toward the anode at pH 7.0 and 26 mm toward the anode at pH 10.0. Which conclusion about the enzyme's $pI$ is most consistent with the data?
   Answer: The enzyme's $pI$ is likely below 7.0. This question tests understanding of how protein migration changes with pH relative to pI. In native PAGE, proteins are positively charged below their pI and negatively charged above their pI. The enzyme migrated toward the anode at both pH values, indicating negative charge, but migrated much farther at pH 10.0 (26 mm) than at pH 7.0 (8 mm). This shows the enzyme becomes more negative as pH increases from 7.0 to 10.0, which only occurs when the pI is below the pH range tested. If the pI were above 7.0, the protein would be positive at pH 7.0 and migrate toward the cathode. Choice A incorrectly suggests pI > 10.0, which would make the protein positive at both pH values tested.
9. A lab used native PAGE to compare two hemoglobin variants in a physiological buffer at pH 8.6, 180 V. Variant H1 (pI 7.1) migrated 3.9 cm toward the anode, while H2 (pI 8.9) migrated 0.6 cm. Which conclusion about their relative net charges at pH 8.6 is most supported by the results?
   Answer: H1 is more net negative than H2 at pH 8.6. This question assesses relative charge comparison in native PAGE for hemoglobin variants, based on pI and migration. In electrophoresis, at pH above pI, lower pI means more negative charge, leading to farther anode migration. In this pH 8.6 setup, H1 (pI 7.1 < 8.6) migrated farther (3.9 cm) than H2 (pI 8.9 > 8.6, 0.6 cm). The correct answer A follows as H1 is more net negative than H2. Distractor C fails by claiming H1 net positive drives anode migration, confusing positive charge's cathode direction. To verify, test at pH 10.0 to enhance both negativities and check distances. Comparing with pI standards can confirm lower pI correlates with greater negative charge at high pH.
10. A clinical lab analyzed a plasma protein mixture (albumin plus three globulins) using native agarose gel electrophoresis in 50 mM Tris buffer at pH 8.6. Samples were loaded near the cathode and run at 120 V for 25 min. Migration distance toward the anode was recorded. Data (migration toward anode): Albumin: 42 mm Protein X: 28 mm Protein Y: 12 mm Protein Z: 3 mm Which conclusion about protein charge at pH 8.6 is most supported by these electrophoresis results?
    Answer: Albumin has the greatest net negative charge at pH 8.6. This question tests understanding of protein charge and migration in native agarose gel electrophoresis. In electrophoresis, proteins migrate based on their net charge - negatively charged proteins move toward the positive anode, with more negative proteins migrating farther. At pH 8.6, which is above most protein pI values, proteins typically carry net negative charges. Since albumin migrated the farthest (42 mm) toward the anode, it must have the greatest net negative charge among the tested proteins. The migration pattern (albumin > Protein X > Protein Y > Protein Z) indicates decreasing net negative charge in that order. Choice C is incorrect because proteins migrating toward the anode are negatively charged, not positive.
11. A researcher ran native PAGE at pH 8.0 on two proteins with identical MW (40 kDa): B1 (pI 4.0) and B2 (pI 10.0). The gel was loaded at the cathode (−) and run toward the anode (+) at 150 V. Which protein is most likely to migrate faster toward the anode under these conditions?
    Answer: B1, because at pH 8.0 it is strongly net negative and experiences greater electrophoretic force toward the anode. This question tests prediction of migration speed in native PAGE based on pI and charge at specific pH. In electrophoresis, at pH above pI, proteins are net negative, with lower pI indicating stronger negative charge and faster anode migration for same size. In this pH 8.0 setup with identical MW, charge difference drives speed toward anode. The correct answer B is supported as B1 (pI 4.0 << 8.0) is strongly negative, migrating faster than B2 (pI 10.0 > 8.0, less negative or positive). Distractor A fails by claiming higher pI (B2) makes more negative, reversing the pI-pH charge rule. A transferable check is to run at pH below both pIs, expecting B2 (higher pI) to be less positive and migrate slower to cathode. Comparing with neutral markers can isolate charge effects on speed.
12. A lab separated proteins from cerebrospinal fluid using native agarose electrophoresis at pH 8.6. The anode was placed opposite the wells. A faint band migrated toward the cathode, while most bands migrated toward the anode. Which statement is most consistent with electrophoresis principles for the cathode-migrating band?
    Answer: It likely had a net positive charge at pH 8.6. This question tests understanding of protein charge and migration direction in electrophoresis. In native agarose electrophoresis, proteins migrate based on their net charge - negative proteins toward the anode, positive proteins toward the cathode. Since this band migrated toward the cathode (opposite most proteins), it must have a net positive charge at pH 8.6. This is unusual because pH 8.6 is above most protein pI values, typically rendering proteins negative. However, some proteins with very high pI values (>8.6) remain positively charged at this pH. Choice C incorrectly assigns negative charge to a cathode-migrating protein, contradicting fundamental electrophoresis principles. The key verification is migration direction: cathode = positive charge.
13. A researcher separated a protein mixture by native PAGE at pH 9.0, 140 V for 16 min. The anode was at the bottom. A strongly stained band remained at the loading well, while other bands migrated toward the anode. Which explanation is most consistent with electrophoresis principles for the band that remained at the well?
    Answer: The protein likely had near-zero net charge at pH 9.0, resulting in minimal electrophoretic mobility. This question tests understanding of protein behavior at or near its isoelectric point in native PAGE. In electrophoresis, proteins migrate based on their net charge - but proteins at their pI have zero net charge and therefore minimal electrophoretic mobility. At pH 9.0, the protein that remained at the well likely has a pI near 9.0, resulting in near-zero net charge and no driving force for migration. Other proteins migrated toward the anode because they have pI values below 9.0, making them negatively charged at this pH. Choice B incorrectly suggests high negative charge causes minimal mobility - actually, high charge causes maximal mobility. The key principle is that proteins don't migrate when pH equals pI.
14. A researcher separated three proteins by native PAGE at pH 8.0, 150 V, same gel. Proteins had similar MW (~40 kDa) but different pI. Protein | pI | Distance toward anode (+) (cm) M | 4.5 | 5.0 N | 7.9 | 1.2 O | 9.3 | 0.4 Which conclusion about their relative net charges at pH 8.0 is most supported by the data?
    Answer: M is most net negative, N is slightly net negative, and O is net positive or near neutral. This question assesses relative net charge inference from native PAGE migration, using pI and distance data. In electrophoresis, at pH above pI, proteins are net negative with magnitude increasing as pI decreases, leading to farther anode migration for similar sizes. In this pH 8.0 setup with comparable MW, migration distance reflects negative charge strength. The correct answer A follows as M (pI 4.5) migrated farthest (5.0 cm, most negative), N (pI 7.9) moderately (1.2 cm), and O (pI 9.3) least (0.4 cm, least negative or positive). Distractor B is wrong, claiming O (highest pI) most negative, inverting the pI-charge relationship at high pH. A check is to run at pH below all pIs, expecting reversed migration orders. Verifying with charge calculations can confirm distance correlates with net negative charge.
15. A researcher performed SDS-PAGE (pH 8.3) on two proteins D1 and D2 that differ in MW (D1: 15 kDa; D2: 150 kDa). At 120 V for 50 min, D1 migrated 6.5 cm and D2 migrated 1.8 cm. The researcher then doubled the voltage but kept run time the same; both proteins migrated farther, but the distance between their bands (in cm) decreased. Which factor most likely contributed to the decreased band spacing at higher voltage?
    Answer: Increased heating and diffusion at higher voltage broadened bands and compressed apparent spacing. This question evaluates voltage's effect on band spacing in SDS-PAGE, considering heating and diffusion. In electrophoresis, higher voltage increases speed but causes heating, broadening bands and compressing apparent spacing, especially for disparate MW. In this pH 8.3 setup, doubled voltage decreased spacing between 15 and 150 kDa despite farther migration. The correct answer A is consistent as heating broadened bands, reducing effective separation. Distractor D is incorrect, claiming voltage enlarges pores and eliminates sieving, a misconception since voltage doesn't alter gel structure. A check is to run with constant current to minimize heating and preserve spacing. Monitoring band widths at varying voltages can verify diffusion's role in resolution loss.
16. A researcher ran SDS-PAGE (reducing conditions) to assess a recombinant protein mixture. The gel (10% acrylamide) was run in Tris-glycine-SDS at pH 8.3. Samples were loaded at the cathode and run toward the anode at 90 V for 10 min. Observed migration from the well (mm): Protein A (20 kDa): 32 Protein B (45 kDa): 21 Protein C (90 kDa): 12 Protein D (150 kDa): 7 Which protein is most likely to migrate faster if the voltage is increased to 180 V while keeping run time and gel composition constant?
    Answer: Protein A, because smaller SDS-coated proteins experience less sieving and move faster. This question tests understanding of protein separation by size in SDS-PAGE. In SDS-PAGE, proteins are coated with SDS detergent, giving them uniform negative charge density, so separation occurs primarily by size through the gel matrix. Smaller proteins experience less resistance from the gel pores and migrate faster toward the anode. The data shows inverse correlation between size and migration: 20 kDa (32 mm) > 45 kDa (21 mm) > 90 kDa (12 mm) > 150 kDa (7 mm). When voltage doubles, the electric field doubles, proportionally increasing migration speed for all proteins, but smaller proteins will still migrate fastest. Choice B incorrectly suggests larger proteins move faster, contradicting the fundamental principle of size-based separation in SDS-PAGE.
17. A lab separated a physiological mixture of muscle proteins by native electrophoresis at pH 8.8, applying 130 V for 30 min. Wells were near the cathode (−). Two bands were excised and identified by mass spectrometry: Band 1 contained protein R (20 kDa, pI 4.9) and Band 2 contained protein S (20 kDa, pI 9.1). The recorded migration distances toward the anode were R = 5.0 cm and S = 0.1 cm. Which statement is most consistent with electrophoresis principles under these conditions?
    Answer: Protein S migrated less because at pH 8.8 it is near its pI and has reduced net charge. This question tests understanding of how protein pI values determine net charge and migration in native electrophoresis at a specific pH. In native gel electrophoresis, proteins migrate based on their net charge, which depends on the relationship between the running pH and the protein's pI - proteins far from their pI have larger net charges and migrate more. At pH 8.8, Protein R (pI 4.9) is 3.9 pH units above its pI, giving it a large negative charge and explaining its substantial migration toward the anode (5.0 cm). Protein S (pI 9.1) is only 0.3 pH units below its pI at pH 8.8, making it nearly neutral with minimal net charge, which explains its negligible migration (0.1 cm) - proteins near their pI have very small net charges regardless of their size. The correct answer properly identifies that S migrated less because it's near its pI and has reduced net charge at the running pH. Choice C incorrectly attributes the difference to size effects on electric force, when both proteins are the same size (20 kDa) and the difference is purely due to charge. For native electrophoresis, the key principle is that migration distance correlates with net charge magnitude - proteins near their pI (within ~0.5 pH units) show minimal migration regardless of size.
18. A researcher compared separation efficiency of a physiological protein mixture on SDS-PAGE at two voltages using the same gel (12% resolving) and buffer (pH 8.3). All samples were reduced and treated with SDS. At 100 V for 60 min, bands for 45 kDa and 50 kDa proteins were clearly resolved (distinct bands). At 220 V for 27 min, these bands partially overlapped, although total run time was shorter. Which factor most likely explains the reduced separation efficiency at higher voltage in this experiment?
    Answer: Higher voltage increases diffusion and heating, broadening bands and reducing resolution. This question evaluates the impact of voltage on separation efficiency in SDS-PAGE, highlighting factors like heating and diffusion in protein separation. In electrophoresis, proteins migrate based on charge and size, but high voltage can cause Joule heating and increased diffusion, affecting band sharpness and resolution. In this SDS-PAGE setup at pH 8.3, higher voltage shortens run time but can compromise separation by broadening bands, especially for closely sized proteins. The correct answer A is supported as higher voltage (220 V) led to overlapping 45 and 50 kDa bands due to heating-induced diffusion, while lower voltage (100 V) allowed clear resolution. Distractor B fails by suggesting voltage reverses SDS-coated protein direction, which is a misconception since SDS imparts consistent negative charge regardless of voltage. To check voltage effects, run gels at intermediate voltages and monitor temperature to correlate heating with resolution loss. Additionally, using cooling systems can verify if mitigating heat preserves band separation in high-voltage runs.
19. A lab ran native PAGE at pH 7.5 to separate proteins based on both size and charge. Two proteins, A1 and A2, had identical net charge at pH 7.5 (estimated from titration) but different hydrodynamic radii. At 150 V for 20 min, A1 migrated 10 mm and A2 migrated 22 mm toward the anode. Which statement is most consistent with these observations?
    Answer: A2 likely has a smaller hydrodynamic radius than A1, reducing friction and increasing mobility. This question tests understanding of how both charge and size affect migration in native PAGE. In native electrophoresis, migration depends on the balance of electrophoretic force (proportional to charge) and frictional resistance (proportional to size). Since A1 and A2 have identical net charge but different migration (A2 migrates farther), the difference must be due to size. A2's greater migration distance indicates it experiences less friction, meaning it has a smaller hydrodynamic radius than A1. The smaller size allows A2 to move more easily through the gel matrix despite having the same driving force (charge). Choice B incorrectly suggests larger proteins migrate faster - in reality, larger proteins experience more friction and migrate slower when charge is equal.
20. A lab used SDS-PAGE to estimate sizes of four secreted proteins at pH 8.3. All samples were fully denatured and SDS-coated. At 110 V for 30 min, the migration distances were: Protein E: 38 mm Protein F: 26 mm Protein G: 17 mm Protein H: 10 mm Which protein is most likely the smallest by molecular weight?
    Answer: Protein E This question tests understanding of size-based separation in SDS-PAGE. In SDS-PAGE, proteins are denatured and coated with SDS, giving them uniform negative charge density, so separation occurs by size through gel sieving. Smaller proteins encounter less resistance from gel pores and migrate farther toward the anode. The migration pattern shows: E (38 mm) > F (26 mm) > G (17 mm) > H (10 mm), indicating size order of E < F < G < H. Therefore, Protein E, which migrated farthest, is the smallest. Choice B incorrectly suggests larger proteins migrate faster - in SDS-PAGE, the opposite is true due to increased friction in the gel matrix.

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