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Chapter 2: Problem 60

The amino acid glycine \(\left(\mathrm{H}_{2} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{COOH}\right)\) has \(\mathrm{p} K\) values of \(2.35\) and \(9.78\). Indicate the structure and net charge of the molecular species that predominate at \(\mathrm{pH} 2,7\), and 10 .

Short Answer

Expert verified
At pH 2, glycine has a net charge of +1; at pH 7, it is neutral and zwitterionic; at pH 10, the net charge is -1.

Step by step solution

01

Understanding Glycine Ionization

Glycine has two ionizable groups: the carboxyl group (鈭扖OOH) with a pK value of 2.35, and the amino group (鈭扤H2) with a pK value of 9.78. At different pH levels, these groups will either gain or lose protons, affecting the charge of the molecule.
02

Analyzing pH 2

At pH 2, the environment is more acidic than the pK of the carboxyl group (2.35), meaning the carboxyl group is mostly in its protonated form (鈭扖OOH) and carries no charge. The amino group is also protonated (鈭扤H3鈦�), contributing a positive charge. Therefore, the net charge of glycine at pH 2 is +1.
03

Analyzing pH 7

At pH 7, which is higher than the pK of the carboxyl group (2.35) but lower than the pK of the amino group (9.78), the carboxyl group is deprotonated (鈭扖OO鈦�), carrying a negative charge, while the amino group remains protonated (鈭扤H3鈦�). The charges cancel each other out, resulting in a net charge of 0. The predominant structure is as a zwitterion, with both positive and negative charges present.
04

Analyzing pH 10

At pH 10, the environment is more basic than both the carboxyl group (pK = 2.35) and the amino group (pK = 9.78), so both groups are deprotonated. The carboxyl group remains as 鈭扖OO鈦�, and the amino group becomes 鈭扤H2, losing its proton and charge. Consequently, the net charge at pH 10 is -1.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Glycine
Glycine is the smallest and simplest amino acid found in proteins. It is non-polar and often used as a building block for peptides. The structure of glycine consists of two primary functional groups:
  • A Carboxyl group (-COOH)
  • An Amino group (-NH2)
These functional groups determine the properties of glycine, and their behavior in different pH environments significantly impacts its ionization state and net charge.

In biochemistry, glycine plays an essential role due to its small size and flexibility. It is involved in the synthesis of proteins and other amino acids. It also participates as a precursor in various metabolic reactions.
Zwitterion
A zwitterion is a molecule that contains both positive and negative charges but is overall electrically neutral. Many amino acids, including glycine, form zwitterions in certain pH ranges. This occurs when the molecule has both a negatively charged carboxyl group and a positively charged amino group.

At its zwitterionic form, glycine typically has:
  • A deprotonated carboxyl group (-COO鈦�)
  • A protonated amino group (-NH3鈦�)
The charges from these groups balance each other out, resulting in a net charge of zero.

The zwitterionic form is most commonly found at a pH level between the two pKa values of glycine's functional groups. For glycine, this is typically around physiological pH, roughly pH 7, where it plays a crucial role in proteins as it contributes to overall structure and function without adding a net charge to the molecule.
pKa Values
The pKa value is a critical term in understanding amino acid ionization and behavior at various pH levels. It indicates the pH at which a particular group in a molecule is half ionized. In glycine, there are two pKa values:
  • The carboxyl group with a pKa of 2.35
  • The amino group with a pKa of 9.78


Each pKa value guides the protonation-deprotonation behavior of functional groups based on the surrounding pH. When the pH is below the pKa, the group is likely to remain protonated, while above the pKa, the group prefers to remain deprotonated. Knowing these values allows us to predict which ionic form the amino acid will predominate under different conditions.

Understanding pKa provides insights into glycine's behavior under various environmental conditions, aiding in predicting its biochemical interactions and functions.

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Most popular questions from this chapter

What is the \(\mathrm{pH}\) of \(1.0 \mathrm{~L}\) of water to which a. \(1.5 \mathrm{~mL}\) of \(3.0 \mathrm{M} \mathrm{HCl}\) or b. \(1.5 \mathrm{~mL}\) of \(3.0 \mathrm{M} \mathrm{NaOH}\) has been added?

Ammonium sulfate, \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\), is a water-soluble salt. Draw the structures of the hydrated ions that form when ammonium sulfate dissolves in water.

Drug delivery is a challenging problem because the drug molecules must be sufficiently water-soluble to dissolve in the blood, but also sufficiently nonpolar to pass through the cell membrane for delivery. A medicinal chemist proposes to encapsulate a water-soluble drug into a vesicle (see Section 2.2). How does this strategy facilitate the delivery of the drug to its target cell?

Metabolic acidosis is a general term that describes a number of disorders in metabolism in the body that result in a lowering of the blood pH from \(7.4\) to \(7.35\) or below. The kidney plays a vital role in regulating blood \(\mathrm{pH}\). The kidney can either excrete or reabsorb various ions, including phosphate, \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\); ammonium, \(\mathrm{NH}_{4}^{+}\); or bicarbonate, \(\mathrm{HCO}_{3}^{-}\). Which ions are excreted and which ions are reabsorbed in metabolic acidosis? Explain, using relevant chemical equations.

The bacterium \(E\). coli can adapt to changes in the solute concentration of its growth medium. The cell consists of a cytoplasmic compartment bounded by a cell membrane surrounded by a porous cell wall; both the membrane and the cell wall allow the passage of water and ions. Under nongrowing conditions, only cytoplasmic water content is regulated. What happens to the cytoplasmic volume if \(E\). coli is grown in a growth medium with a a. high salt concentration or b. low salt concentration?
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