DAT OChem 2016 Question#227

[Ashish]

DAT Destroyer 2016 Orgo #227. Without knowing the pka value of the Amino acid, how can we figure out the physiological state of amino acid Serine? (And for all of the amino acids for that matter. ) Thanks

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[mouse rat]

Amino acids don't really have pKas, but their substituents do. All you need to know is that carboxylic acids have a pKa of around 5 (deprotonated at physiological pH ~ 7), and amino groups have a pKa around 10 (protonated at physiological ~ 7).

You should also probably know that the 3 basic amino acids are Histidine, Arginine, and Lysine and that the 2 acidic ones are Aspartic acid and Glutamic acid.

 

[DAT Destroyer]

DAT Destroyer 2016 Orgo #227. Without knowing the pka value of the Amino acid, how can we figure out the physiological state of amino acid Serine? (And for all of the amino acids for that matter. ) Thanks

If a problem says Physiological pH......we are at about 7.4 Essentially all the COOH groups are in the COO- form....and the NH2 groups are in the NH3+ form. I will discuss it further if you are still struggling, just let me know.

Hope this helps
Dr. Romano

 

[rak173]

Are the acidic AA always negative at physiological pH and basic AA always positive at physiological pH?

 

[DAT Destroyer]

Are the acidic AA always negative at physiological pH and basic AA always positive at physiological pH?

That is correct!

 

[Ashish]

If a problem says Physiological pH......we are at about 7.4 Essentially all the COOH groups are in the COO- form....and the NH2 groups are in the NH3+ form. I will discuss it further if you are still struggling, just let me know.

Hope this helps
Dr. Romano

Great. Thank you Dr. Romano. For the DAT, do we need to know what form the COOH and NH2 group are present other than the physiological state? Also, so from the explanation above, if the pH was 4, the COOH would not be deprotonated since pH < pH of COOH?


Thank you for your time.

 

[DAT Destroyer]

Great. Thank you Dr. Romano. For the DAT, do we need to know what form the COOH and NH2 group are present other than the physiological state? Also, so from the explanation above, if the pH was 4, the COOH would not be deprotonated since pH < pH of COOH?


Thank you for your time.

Indeed so. Let us do an example.....In the stomach where the pH is 1.5 The carboxy group will be fully protonated in the COOH form, and the amino group will be in the NH3+ form. If the pH was very high, let us say 11......the carboxy group will be in the COO- or deprotonated form, and the amino group in the NH2 form. At other pH values, things get more complicated , but reserve that until BioChemistry. For the DAT, this should be fine. To answer your other question, at pH of 4,,,,,most of the carboxy group is in the COO- form because it is beyond the pKa of 2.6. Under 2.6 we are in the COOH form....above this value, deprotonation occurs heavily. Also above 9.6 the pKa of the amino group ,we lose the bulk of protons off the amino group, and see NH2 form predominate.

Hope this helps.

Dr. Romano