An Error or not an Error that is the Question.

[BloodySurgeon]

The passage is talking about Ion-Exchange Filtration and the beads are made up of negatively charged anions.

Q: It can be inferred from the passage that an amino acid will be eluted most efficiently if its pI is:

A: lower than the pH of a sodium acetate solution.

My Answer: (C) Opposite. When the pI is greater than the pH, the amino acid is negative and will be repelled by the negatively charged beads.


Is it just me or isn't the amino acid positively charged when the pI is greater than the pH? Therefore it will will bind to the negatively charge beads and elute most efficiently. Or does efficiently mean quicker? I hate the wordings of 90% of this test, it is the downfall of my score.

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[Vihsadas]

I'm pretty rusty on pI but let's assume that your last statement is true;

If the amino acid is positively charged with a pI greater than pH and the beads are negative, wouldn't that mean that the amino acid would STICK to the beads? Wouldn't that imply that they would elute less efficiently? This is assuming that the beads are there to collect the impurities, and not the amino acid, which may or may not be true because I didn't look it up. (Opposites attract, and similars push away from each other)

Moving to Q&A.

 

[RPedigo]

Vish is correct. Elute efficiently means to come out of the other end of the column efficiently. You have your idea of pH and pI correct, but the positive charge allows the amino acids to pass through the beads and come out the other end. Presumably any impurity would 'stick' to the beads and have an inefficient and long elution time.

Were the amino acids negatively charged, they'd be chillin' with the beads for hours, which isn't something a researcher wants because elution then isn't efficient. You have all the concepts right, just some oddball terminology (elution) that threw you off.

 

[tncekm]

My Answer: (C) Opposite. When the pI is greater than the pH, the amino acid is negative and will be repelled by the negatively charged beads.

Just to clarify something that wasn't in Vish or R. Pedigo's wonderful explanations, it just came down to you had the perfect thought process but mixed up the pI pH stuff.

When pI > pH there are excess protons and the amino acid is protonated, or positively charged, not negatively charged like you assumed when you chose your answer.

pI < pH => negatively charged due to lack of hydrogen ions allowing for the acidic proton from the AA to dissociate
pI = pH => zwitterion
pI > pH => positively charged due to excess hydrogen ions protonating the amine group

 

[BloodySurgeon]

ok thats what I wanted to know... efficient elution means quicker elution. I self taught myself molecular bio so I'm a little rough on the edges. So in an experiment that is trying to extract basic amino acids... you are going to put positive charged beads and elute a protein down the filtration so that you have the basic amino acid come down first, quickest, and most efficient?

Also why does efficient mean fastest?

 

[Vihsadas]

ok thats what I wanted to know... efficient elution means quicker elution. I self taught myself molecular bio so I'm a little rough on the edges. So in an experiment that is trying to extract basic amino acids... you are going to put positive charged beads and elute a protein down the filtration so that you have the basic amino acid come down first, quickest, and most efficient?

Also why does efficient mean fastest?

Efficiency implies both quickness and completeness. It depends on the context of the question.

 

[Gaggle83]

My humble opinion is that efficiency does not refer to "quickness/speed" but rather to [eventual] "completeness". e.g. the efficiency of diamond's conversion into graphite is 100%. But I also believe that enough washes will elute all the amino acids eventually; as water may break the ionic bonds. Therefore, I am led to believe that the question improperly used the word efficiency.

 

[sleepy425]

it's asking you what will elute (which means come off the column) most efficiently. that means, it wants to know, which will come off the column most efficiently. so in this case the only possible meaning is the speed at which it comes off the column, because it is only referring to elution.