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Calculate the isoelectric point (pI) of an amino acid

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Calculate the isoelectric point (pI) of each of the following amino acids
(a) glycine
(b) glutamic acid
(c) arginine

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View the attachment below for the chemical structures.

The isoelectric point (or pI) of an amino acid is the pH at which the amino acid has a net charge of zero.

Step one: Consider the ionization of the amino acid. Draw the structures of the +1, 0, and -1 charged states of each amino acid.

Step ...

Solution Summary

This type of question is common on introductory biochemistry exams and laboratory assignments, but students often find this calculation to be challenging. In this solution, a generalized "how to" step by step procedure is outlined so that students will be prepared to apply this method to future calculations. Then, the step by step solution (including structures) is shown for parts (a) and (b) of the problem. This step by step solution includes definitions of important concepts/terms. The step by step solution for part (c) is deliberately not shown, in order to encourage the student to work through the entire method on his/her own. Instead, the numerical answer is provided. After studying the solutions for parts (a) and (b), you will use the same approach for part (c) and be able to confirm your correct answer using the numerical answer.

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Calculating the isoelectric point of a protein

I'm trying to calculate the isoelectric point of a protein. I understand that you need to substitute variations of pH until you find a pI that is close to zero. What I need is a detailed example of the process for a simple protein (say 4 amino acids, preferably charged). It would be helpful if you could also provide a similar protein and answer only so I can check my understanding. It would also be helpful if you could declare any constants (i.e. pH=5) in advance of both problems. Thanks very much.

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