Soccerboi's questions thread

[soccerboi]

Thanks

[soccerboi]

How are the secondary and tertiary structures of an enzyme crucial to its functioning?

[pi]

The active site is usually of a highly specific shape, altering this shape (ie. sec+tert structures) may deem the enzyme to lose it's primary function of binding to a substrate.

[soccerboi]

If we alter the primary structure, will this have the same effect?

[charmanderp]

^The shape of the active site would indeed change, and the enzyme would be complementary to a different substrate.

[pi]

If we alter the primary structure, will this have the same effect?

Not all the time, only if that particular amino acid is close to the active site or has a fundamental role in the tertiary structure.

[stephanieteddy]

Okay, this question is killing me, and I am not quite understanding VCAA's solution either.

This is the question.

Salicylic acid reacts with aqueous FeCl3 to form a purple compound. Aspirin does not react with aqueous
FeCl3.
c. Describe how the purity of the product could be quantitatively determined using UV-visible
spectroscopy.

This is the answer

identify the appropriate wavelength at which a solution of the purple compound (salicylic acid/FeCl3) absorbs
strongly
• make up a set of standards of salicylic acid/FeCl3(aq) and measure absorbances to construct a calibration curve
• make up a solution of a measured amount of the product/FeCl3(aq) and measure its absorbance
• use the absorbance to calculate the amount of salicylic acid (hence aspirin) in the compound

What I am not understanding is the significance of the FeCl3 and the point of measuring the amount of product/FeCl3 when it says that aspirin doesn't react with it. I pretty much don't understand the question.

Thank you in advance, sorry it's a long one.

[thushan]

Okay, this question is killing me, and I am not quite understanding VCAA's solution either.

This is the question.

Salicylic acid reacts with aqueous FeCl3 to form a purple compound. Aspirin does not react with aqueous
FeCl3.
c. Describe how the purity of the product could be quantitatively determined using UV-visible
spectroscopy.

This is the answer

identify the appropriate wavelength at which a solution of the purple compound (salicylic acid/FeCl3) absorbs
strongly
• make up a set of standards of salicylic acid/FeCl3(aq) and measure absorbances to construct a calibration curve
• make up a solution of a measured amount of the product/FeCl3(aq) and measure its absorbance
• use the absorbance to calculate the amount of salicylic acid (hence aspirin) in the compound

What I am not understanding is the significance of the FeCl3 and the point of measuring the amount of product/FeCl3 when it says that aspirin doesn't react with it. I pretty much don't understand the question.

Thank you in advance, sorry it's a long one.

Salicylic acid reacts with FeCl3 to produce a COLOURED compound. We can use UV-vis to detect absorbance of this coloured compound.

The more aspirin that forms, the less remaining salicylic acid, the less intense the purple colour. Hence, we can determine the purity of aspirin by drawing a calibration line denoting the purity of aspirin against absorbance.

[soccerboi]

Can someone confirm if these are correct to say:
1. the Z group of an amino  acid can be protonated and deprotonated depending on the pH of the solution it is in.
2. the carboxyl group on the z group of an amino acid cannot react with the amine group on the main chain of another amino acid to form a peptide linkage and vice versa. Can the carboxyl of the z group react with the amine of an amino acid (amine that's not on the z group) at all?

[sin0001]

You need a coloured solution (this can only be done by reacting salicylic with FeCl3), otherwise it can't be analysed using UV-spec. And when your analysing the concentration of salicylic acid in the aspirin/FeCl3 solution, only the salicylic acid will react with the FeCl3 to form a purple solution and only this purple solution absorbs UV light, so when you get the absorbance, you can use the cal. curve to find the concentration of salicylic acid in a particular mass of aspirin. Remember, you are quantitatively analysing the aspirin, so you need its mass so that you can find the percentage composition of salicylic acid in it, and hence the % impurity

[stephanieteddy]

Okay thank you both they were really good answers! I am so close to getting it. I can feel the click moment coming. I just need clarification on this point:

• make up a solution of a measured amount of the product/FeCl3(aq) and measure its absorbance

Why do I do this?? ^^^

[pi]

Can someone confirm if these are correct to say:
1. the Z group of an amino  acid can be protonated and deprotonated depending on the pH of the solution it is in.
2. the carboxyl group on the z group of an amino acid cannot react with the amine group on the main chain of another amino acid to form a peptide linkage and vice versa. Can the carboxyl of the z group react with the amine of an amino acid (amine that's not on the z group) at all?

1. Yes, but you don't have to know when if the pH is between 2-14 or something
2. That usually doesn't occur afaik. That carboxylic group (when there is one) will be involved in (most likely) hydrogen bonding in the secondary/tertiary structures.

[stephanieteddy]

CLICK! I got it!

Because the product/fecl3 solution will have both salicylic acid and aspirin in it! So by finding its absorbance we are finding the amount of salicylic acid!! WOOOHOOO

Okay thank you so much guys.

I was a bit stressed because it has been a few years since it was on the exam, and the last two exams were quite easy so I thought they might spring a couble of these babies on us this year.

[soccerboi]

Can someone confirm if these are correct to say:
1. the Z group of an amino  acid can be protonated and deprotonated depending on the pH of the solution it is in.
2. the carboxyl group on the z group of an amino acid cannot react with the amine group on the main chain of another amino acid to form a peptide linkage and vice versa. Can the carboxyl of the z group react with the amine of an amino acid (amine that's not on the z group) at all?

1. Yes, but you don't have to know when if the pH is between 2-14 or something
2. That usually doesn't occur afaik. That carboxylic group (when there is one) will be involved in (most likely) hydrogen bonding in the secondary/tertiary structures.

Thanks

[soccerboi]

The molecule HOCH2CH2OH will have a H NMR spectrum consisting of:
A) two singlets
B) a singlet and a triplet

I chose option B but answer is A, why? I know it's symmetrical but there's a CH2 next to the CH2 so wouldn't it be 2+1=3 peaks?