a. Which amino acid has the lowest pI value? b. Which amino acid has the highest pI value? c. Wh

A. Which amino acid has the lowest pI value? b. Which amino...

Key Concepts

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH, pKa, and the ratio of the concentrations of the deprotonated and protonated forms of an ionizable group. It is a fundamental tool in biochemistry for predicting the degree of ionization of each group in an amino acid at a given pH, thereby helping to understand its net charge under different conditions.

Ionization States of Amino Acids

Amino acids can exist in different ionization states (protonated, deprotonated, or zwitterionic) depending on the pH of the environment. The balance between these states, dictated by the pKa values of the ionizable groups, affects the overall charge of the molecule. This concept is key in understanding which amino acid might carry more negative or positive charge at a specific pH.

pKa Values

pKa values indicate the pH at which half of a given ionizable group is deprotonated. Each ionizable group within an amino acid (such as the carboxyl group, amino group, or side chain) has its own pKa. These values are essential for calculating the net charge of the amino acid at different pH levels, which in turn helps determine its pI and behavior in solution.

Isoelectric Point (pI)

The isoelectric point is the pH at which an amino acid has no net electrical charge. It is determined by the pKa values of the ionizable groups (carboxyl, amino, and any side chain groups) present in the molecule. Understanding pI is crucial for separating amino acids and predicting their behavior in different pH environments.

Transcript

00:01 Okay, with this question, again, it's important to know what pi or the isoelectric point is.

00:08 So the isoelectric point is the average value of two pkks.

00:30 And again, this is when, well, let me rephrase this.

00:43 So the isoelectric point occurs where, or when the ph is at a point where there is no net charge.

01:10 Okay.

01:11 So for the pi, i selective point, there is two things when you keep in mind.

01:17 So if the amino acid has an ionizable side chain, so with ionizable side chain, which i'm going to abbreviate as sc, the pi is the average of two similarly ionizable or right.

02:07 Rather ionizing now.

02:19 If the amino acid doesn't have a side chain, so without an ionizable side chain, the pi is the average of the two p .a.

02:57 Values.

02:59 Okay.

03:01 So with that, let's move on to the question.

03:04 So the first question asks us to find the amino acid with the lowest isoelectric point.

03:14 Now we can sit here and go through all of the calculations for every single amino acid, but instead, all we need to do is look for the amino acid with the lowest p -k -a values.

03:47 Okay.

03:50 Because since the p -i or the is the average of the two values, we know that it's going to be between the two points.

04:00 So if those two points themselves are really low, then the average of the two are also going to be really low.

04:07 So if we scan through it all, we'll see that as bartate has a carboxyl group pca of 2 .09, an amino group pca of 9 .82, and a side chain pca of 3 .86.

04:36 And you'll find that 2 .09 and 3 .86 are the two lowest values on that table.

04:44 So because of that, aspartate is our answer.

04:53 Okay.

04:56 Moving on to part b, we can use the same method that we use for part a.

05:05 So we want to look for the amino acid with the biggest p -k -a values.

05:29 And again, if we go through it and compare everything, we'll find that arginine has a carboxyl pca of 2 .17, an amino pka of 9 .04, and a side chain pca of 12 .42...

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