Question

Write the structural formula for species that exist predominantly for the given amino acids at pH \(1.0\) and 13 respectively. The isoelectric points are: Amino acid Lysine Glutamic acid Isoleucine Alanine Isoelectric point \(9.7\) \(3.2\) \(6.0\) \(6.0\)

Short answer

At pH 1.0, all amino acids are protonated; at pH 13, they are deprotonated.

Step-by-step solution

Understand the Question

We are asked to determine the predominant forms of certain amino acids at pH levels 1.0 and 13, based on their isoelectric points (pI). The isoelectric point is the pH at which the amino acid carries no net charge.

Analyze the Isoelectric Points and pH Levels

For pH 1.0, which is acidic, amino acids will exist in protonated forms. For pH 13, which is basic, amino acids lose protons, existing in deprotonated forms. The isoelectric points (pI) provide key reference points for how the amino acids behave relative to the given pH.

Determine Predominant Form at pH 1.0

At pH 1.0, all amino acids are likely to be protonated because pH 1 is much lower than all given pI values. This means the amino groups will have an extra proton (NH extsubscript{3} extsuperscript{+}) and the carboxyl groups will remain protonated (COOH). - **Lysine**: NH extsubscript{3} extsuperscript{+} - CH extsubscript{2} - CH extsubscript{2} - CH extsubscript{2} - CH extsubscript{2} - NH extsubscript{3} extsuperscript{+} - COOH - **Glutamic acid**: NH extsubscript{3} extsuperscript{+} - CH extsubscript{2} - CH extsubscript{2} - COOH - CH extsubscript{2} - COOH - **Isoleucine**: NH extsubscript{3} extsuperscript{+} - CH extsubscript{3} - CH - CH extsubscript{2} - CH extsubscript{3} - COOH - **Alanine**: NH extsubscript{3} extsuperscript{+} - CH extsubscript{2} - CH extsubscript{3} - COOH

Determine Predominant Form at pH 13

At pH 13, which is highly basic, all amino acids will be deprotonated. The amino groups will lose a proton (NH extsubscript{2}), and the carboxyl groups will become COO extsuperscript{-}. - **Lysine**: NH extsubscript{2} - CH extsubscript{2} - CH extsubscript{2} - CH extsubscript{2} - CH extsubscript{2} - NH extsubscript{2} - COO extsuperscript{-} - **Glutamic acid**: NH extsubscript{2} - CH extsubscript{2} - CH extsubscript{2} - COO extsuperscript{-} - CH extsubscript{2} - COO extsuperscript{-} - **Isoleucine**: NH extsubscript{2} - CH extsubscript{3} - CH - CH extsubscript{2} - CH extsubscript{3} - COO extsuperscript{-} - **Alanine**: NH extsubscript{2} - CH extsubscript{2} - CH extsubscript{3} - COO extsuperscript{-}

Key concepts

Isoelectric Point

The isoelectric point (pI) is a crucial concept in understanding amino acids. It is the specific pH at which an amino acid exists without a net electric charge. This balance occurs because the positive and negative charges on the amino acid cancel each other out. The significance of the isoelectric point becomes clear when considering the behavior of amino acids in different pH environments.

For instance, if the pH is below the pI, the amino acid will be predominantly in a protonated form, carrying an overall positive charge. Conversely, if the pH is above the pI, the deprotonated form prevails, giving the molecule a negative charge. This property is essential for applications in biochemistry and molecular biology, such as protein purification and electrophoresis.

Protonation and Deprotonation

Protonation and deprotonation are fundamental processes that describe how amino acids respond to changes in pH.

In acidic solutions, which have a low pH, protonation occurs. This means amino groups (NH extsubscript{2}) are likely to gain a proton and become NH extsubscript{3} extsuperscript{+}, while carboxyl groups (COO extsuperscript{-}) remain as COOH. As a result, amino acids adopt a positively charged form.

In contrast, deprotonation takes place in basic solutions, where the pH is high. Amino groups lose a proton, reverting to NH extsubscript{2}, and carboxyl groups lose their proton as well, forming COO extsuperscript{-}. This results in a negatively charged molecule.

Understanding these processes can help predict the structure of amino acids in different environments, which is essential for their proper function and interaction in biological systems.

Acidic and Basic pH

The terms "acidic" and "basic" refer to the pH scale, which measures the acidity or alkalinity of a solution. This scale ranges from 0 to 14, with 7 considered neutral, like pure water.

In an acidic environment (pH less than 7), the concentration of hydrogen ions (H extsuperscript{+}) is high. This causes amino acids to be protonated, as they gain extra hydrogen ions. An example of an acidic pH is pH 1.0, which was mentioned in the exercise. Here, amino acids like lysine, glutamic acid, isoleucine, and alanine predominantly exist in their protonated forms.

Conversely, a basic environment (pH greater than 7) features a lower concentration of hydrogen ions and a higher concentration of hydroxide ions (OH extsuperscript{-}). This causes deprotonation of the amino acids, leading them to lose protons. pH 13, noted in the exercise, depicts a basic condition where amino acids become deprotonated.

Recognizing how acidic and basic pH affects amino acids is crucial for understanding their role in biological systems.

Structural Formula of Amino Acids

The structural formula of an amino acid reveals the specific arrangement of atoms and bonds in the molecule. Each amino acid comprises a basic amino group (NH extsubscript{2}), a carboxyl group (COOH), a hydrogen atom, and a distinctive side chain (R group) attached to a central carbon atom.

The variability in the R group defines the unique characteristics and functions of each amino acid. For example, lysine has a long side chain ending in an additional amino group, whereas glutamic acid features an extra carboxyl group in its side chain.

Drawing the structural formula can help visualize how amino acids will behave at different pH levels. Under acidic conditions, the amino groups accept additional protons, becoming NH extsubscript{3} extsuperscript{+}, while the carboxyl groups remain unchanged as COOH. In basic conditions, both the amino and carboxyl groups lose protons, becoming NH extsubscript{2} and COO extsuperscript{-} respectively.

Mastering these structural nuances aids in predicting the chemical behavior and reactivity of amino acids in varying environmental settings.