A naturally occurring amino acid such as alanine has a group...

A naturally occurring amino acid such as alanine has a group that is a carboxylic acid and a group that is a protonated amine. The $\mathrm{p} K_{\mathrm{a}}$ values of the two groups are shown. a. If the $\mathrm{p} K_{\mathrm{a}}$ value of carboxylic acid such as acetic acid is about 5 (see Table 2.1), then why is the $\mathrm{p} K_{\mathrm{a}}$ value of the carboxylic acid group of alanine so much lower? b. Draw the structure of alanine in a solution at $\mathrm{pH}=0$. c. Draw the structure of alanine in a solution at physiological $\mathrm{pH}(\mathrm{pH}$. 4). d. Draw the structure of alanine in a solution at $\mathrm{pH}=12$. e. Is there a pH at which alanine will be uncharged (that is, neither group will have a charge)?

A naturally occurring amino acid such as alanine has a group that is a carboxylic acid and a group that is a protonated amine. The $\mathrm{p} K_{\mathrm{a}}$ values of the two groups are shown.
a. If the $\mathrm{p} K_{\mathrm{a}}$ value of carboxylic acid such as acetic acid is about 5 (see Table 2.1), then why is the $\mathrm{p} K_{\mathrm{a}}$ value of the carboxylic acid group of alanine so much lower?
b. Draw the structure of alanine in a solution at $\mathrm{pH}=0$.
c. Draw the structure of alanine in a solution at physiological $\mathrm{pH}(\mathrm{pH}$. 4).
d. Draw the structure of alanine in a solution at $\mathrm{pH}=12$.
e. Is there a pH at which alanine will be uncharged (that is, neither group will have a charge)?

Key Concepts

pH-Dependent Protonation States

The protonation state of functional groups in molecules like amino acids changes with the pH of the environment. At very low pH, acidic groups are typically protonated and basic groups are protonated, while at high pH, acidic groups are deprotonated and basic groups lose their protons. This shift in protonation states affects the molecule’s structure, reactivity, and overall charge distribution.

Isoelectric Point

The isoelectric point is the pH at which a molecule, such as an amino acid, carries no net electrical charge because the positive and negative charges are balanced. This concept is important for predicting the solubility, reactivity, and separation of amino acids in various biochemical processes and analytical techniques.

Acid Dissociation Constant (pKa)

The pKa is a numerical value that indicates the strength of an acid by measuring its tendency to donate a proton. It is a crucial concept in understanding how functional groups in a molecule will behave under varying pH conditions. Lower pKa values correspond to stronger acids that release protons more readily, impacting the molecule's overall charge state and reactivity.

Inductive Effects on Acidity

Inductive effects refer to the electron-withdrawing or electron-donating influence exerted by nearby atoms or groups, which can stabilize or destabilize charged species. In amino acids, nearby charged groups (such as a protonated amine) can stabilize the conjugate base formed upon deprotonation of a carboxylic acid, thereby lowering its pKa compared to similar carboxylic acids that lack such stabilizing interactions.

Zwitterion Formation

A zwitterion is a molecule that contains both positive and negative charges, but is overall electrically neutral. In the context of amino acids, this typically involves a protonated amine group and a deprotonated carboxyl group. The formation of zwitterions is central to understanding the behavior of amino acids in aqueous solutions at certain pH levels.

Transcript

00:01 In this question we have been given the structure of protonated alanine and the structure given is ch3 ch then we have this nh3 positive attached to it and further one carboxylic acid group is attached to this ch group and the pca values for these groups are given as well so pca is 2 .34 for the carboxylac acid group and for the nh3 positive group the pka is equal to 9 .69 so using this we need to just answer a few questions depending on this structure now the first question is we need to identify we need to explain why the pca of this carboxylic acid is much lower than the usual pca for a carboxylac acid that is equal to around 4 .76 or we can say around 5 so the thing is that if there is this carboxylic acid group if there is this carboxylic acid group and if it dissociates as an acid we will be getting this carboxlate radical and also this proton so if this conjugate base or this carboxylet radical can be stabilized we can say that the overall acidity of this carboxeric acid would improve and the pca value would go down.

01:04 Here we can say that if this particular hydrogen is lost and this oxygen gets a negative charge, this negative charge density can be reduced by this electronegative nitrogen present here, which can just attract the electron density towards itself, thereby making it a stable conjugate base and making this carboxic acid a better acid and also the pca value goes down to 2 .3 .4.

01:26 So this is the answer to the first.

01:28 Part now moving on to the next part which says that we need to draw the structure of alanin in a solution which is having a ph equals to zero so if the ph equals to zero we can clearly see that this ph is less than the pca values for both of this carboxylac acid as well as this ns3 positive groups which means that both of them would be there in the their acidic form now acidic form is nothing but we can say that a form with the proton undissociated the proton should be there only.

01:58 Thus we can write ch3, ch, c double bond o and oh because the proton has not been dissociated and also nh3 positive.

02:09 Here also the proton is intact.

02:11 So this is the structure of elanin at a ph equals to 0...

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