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Chapter 27: Problem 30

For lysine and arginine, the isoelectric point, pI, occurs at a pH where the net charge on the nitrogen-containing groups is \(+1\) and balances the charge of \(-1\) on the \(\alpha\)-carboxyl group. Calculate pI for these amino acids.

Short Answer

Expert verified
Question: Calculate the isoelectric point (pI) for the amino acids lysine and arginine. Answer: The isoelectric point (pI) values for lysine and arginine are approximately 5.24 and 6.95, respectively.

Step by step solution

01

Identify the Ka values for lysine and arginine

For solving this problem, you will need the acid dissociation constant (Ka) values for the amino acids lysine and arginine. You can find these values in a biochemistry or chemistry reference book, or online. The Ka values represent the strength of the acidic groups present in the amino acids. Lysine: \(Ka_1 = 2.18 \times 10^{-2}\) \(Ka_2 = 1.50 \times 10^{-9}\) Arginine: \(Ka_1 = 1.80 \times 10^{-2}\) \(Ka_2 = 7.00 \times 10^{-13}\)
02

Calculate the pKa values

Once you have the Ka values for lysine and arginine, you need to convert them to pKa values. The pKa is the negative logarithm of the Ka and can be calculated using the following formula: \(pKa = -\log_{10}(Ka)\) Lysine: \(pKa_1 = -\log_{10}(2.18 \times 10^{-2}) = 1.66\) \(pKa_2 = -\log_{10}(1.50 \times 10^{-9}) = 8.82\) Arginine: \(pKa_1 = -\log_{10}(1.80 \times 10^{-2}) = 1.74\) \(pKa_2 = -\log_{10}(7.00 \times 10^{-13}) = 12.15\)
03

Calculate the isoelectric point (pI)

The isoelectric point (pI) can be calculated as the average of the two pKa values which flank the desired overall charge (in this case, zero). In the case of lysine and arginine, the pKa values flanking the isoelectric point are those derived from the α-carboxyl and amino groups. To calculate the pI value, use the following formula: \(pI = \frac{pKa_1 + pKa_2}{2}\) Lysine: \(pI = \frac{1.66 + 8.82}{2} = \frac{10.48}{2} = 5.24\) Arginine: \(pI = \frac{1.74 + 12.15}{2} = \frac{13.89}{2} = 6.95\) So, the pI values for lysine and arginine are approximately 5.24 and 6.95, respectively.

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Most popular questions from this chapter

Draw a structural formula for the form of each amino acid most prevalent at pH \(1.0\). (a) Threonine (b) Arginine (c) Methionine (d) Tyrosine

Although only L-amino acids occur in proteins, D-amino acids are often a part of the metabolism of lower organisms. The antibiotic actinomycin D, for example, contains a unit of D-valine, and the antibiotic bacitracin A contains units of D-asparagine and D-glutamic acid. Draw Fischer projections and three- dimensional representations for these three D-amino acids.

The side-chain carboxyl groups of aspartic acid and glutamic acid are often protected as benzyl esters. (a) Show how to convert the side-chain carboxyl group to a benzyl ester using benzyl chloride as a source of the benzyl group. (b) How do you deprotect the side-chain carboxyl under mild conditions without removing the BOC-protecting group at the same time?

Several \(\beta\)-amino acids exist. There is a unit of \(\beta\)-alanine, for example, contained within the structure of coenzyme A (Problem 25.35). Write the structural formula of \(\beta\)-alanine.

At pH 7.4, the pH of blood plasma, do the majority of protein-derived amino acids bear a net negative charge or a net positive charge? Explain.
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