Interpretation The Isoelectric point (pI) of tyrosine is to be calculated. In an electrophoresis experiment, will tyrosine migrate at a pH = 7 , and the terminal to which it will migrate is to be determined. Concept introduction: Electrophoresis is a common way of separating a mixture of amino acid. In electrophoresis, any ion that has a net positive charge, at that pH, will migrate toward the negatively charged terminal whereas any ion having a net negative charge will migrate toward the positively charged terminal. An amino acid’s pI is the average of its two pKa values associated with the zwitterion- the pKa value corresponding to the proton transfer reaction in which the zwitterion is a product, and the pKa value corresponding to the proton transfer reaction in which the zwitterion is a reactant.

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Chapter 6, Problem 6.74P

Interpretation Introduction

Interpretation

The Isoelectric point (pI) of tyrosine is to be calculated. In an electrophoresis experiment, will tyrosine migrate at a pH = 7, and the terminal to which it will migrate is to be determined.

Concept introduction:

Electrophoresis is a common way of separating a mixture of amino acid. In electrophoresis, any ion that has a net positive charge, at that pH, will migrate toward the negatively charged terminal whereas any ion having a net negative charge will migrate toward the positively charged terminal.

An amino acid’s pI is the average of its two pKa values associated with the zwitterion- the pKa value corresponding to the proton transfer reaction in which the zwitterion is a product, and the pKa value corresponding to the proton transfer reaction in which the zwitterion is a reactant.

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(b) Describe how the charge of some amino groups in a protein might differ at pH 9.0 and pH 5.0. the charge on the amino group will differ at pH 5 and pH 9 which will depend on the pKa of the amino acid. (c) Describe how the charge of some carboxyl groups in a protein might differ at pH 9.0 and pH 5.0. charge on carboxyl group will differ at pH 5 and pH 9 which will depend on the pKa of the amino acid. (d) Given your answers to parts (b) and (c), what kind of intramolecular interactions in beta-galactosidase are most likely to be affected by a change in pH from 9.0 to 5.0? (e) Could the interactions you mention in part (d) affect the catalytic activity of beta-galactosidase?

c) An essential amino acid with the systematic name 2-amino-3-phenylpropanoic acid. It has pka1 = 1.83 and pka2 = 9.13. i. iii. Draw the deprotonated form of this amino acid. Calculate the p/ of this amino acid. Show the form of this amino acid at pH=5. d) A polyester is prepared by reaction of pentanedioic acid with ethane-1,2-diol. Show the molecular structure of this polyester.

Answer the following questions on the basis of the free energies of formation at 25°C shown in the table at the top of the next page: (a) Does the peptide bond between alanine and glycine form spontaneously under standard conditions?