Isoelectric Point Of Amino Acids Table

[Piedad Coughlin]

NoAmino acids are acids. They are also bases containing an amino group. The term amphoteric is often used to describe amino acids, meaning that they are capable of acting as both acids and bases.

If the pH is decreased to a low enough value (e.g. pH 1) then the carboxylate salt will be protonated to give the neutral carboxylic acid, and the molecule will have a net charge of +1.

For a typical amino acid, there will be a range of pH values where the positively charged form dominates, another where the neutral form dominates, and finally one where the negatively charged one dominates.

If only there were some formula we could use for figuring out the pH of points A and B on the graph above, where the acid and its conjugate base are present in equal concentration.

There is more to isoelectric point than just the calculation of pI values for individual amino acids! The same concepts also apply to peptides and proteins, each of which will have a pI value that is influenced by the characteristics of its side chains.

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An amino acid is a compound that contains both an amine group \(\left( \ce-NH_2 \right)\) and a carboxyl group \(\left( \ce-COOH \right)\) in the same molecule. While any number of amino acids can possibly be imagined, biochemists generally reserve the term for a group of 20 amino acids which are formed and used by living organisms. The figure below shows the general structure of an amino acid. Either structure is considered correct for an amino acid.

The amine and carboxyl groups of an amino acid are both covalently bonded to a central carbon atom. That carbon atom is also bonded to a hydrogen atom and an \(\ceR\) group. It is this \(\ceR\) group which varies from one amino acid to another and is called the amino acid side chain.

The nature of the side chains accounts for the variability in physical and chemical properties of the different amino acids. Each amino acid is grouped based on the properties of the side chain. The groups are designated as polar (hydroxylic, sulfur-containing, amidic) , nonpolar (aliphatic and aromatic), acidic, or basic.

In addition to the full name of the amino acid, there are also one-letter and three-letter abbreviations for each. These abbreviations are especially helpful when listing the amino acids in a protein (a chain of many amino acids that will be discussed later).

The following rules (along with two exceptions) can help you classify amino acids as nonpolar, polar acidic (sometimes called acidic), polar basic (sometimes called basic), or polar neutral. We will look at two exceptions but note that the transition from nonpolar to polar neutral is a gradual transition (like the colors of a rainbow) so you may see variations in how amino acids are classified if you look at other sources.

Amino acids are typically drawn either with no charges or with a plus and minus charge (see figure 13.1.1). When an amino acid contains both a plus and a minus charge in the "backbone", it is called a zwitterion and has an overall neutral charge. The zwitterion of an amino acid exists at a pH equal to the isoelectric point. Each amino acid has its own pI value based on the properties of the amino acid. At pH values above or below the isoelectric point, the molecule will have a net charge which depends on its pI value as well as the pH of the solution in which the amino acid is found.

When pH is less than pI, there is an excess amount of \(\ceH^+\) in solution. The excess \(\ceH^+\) is attracted to the negatively charged carboxylate ion resulting in its protonation. The carbohydrate ion is protonated, making it neutral, leaving only a positive charge on the amine group. Overall, the amino acid will have a charge of \(+1\).

When pH is greater than pI, there is an excess amount of \(\ceOH^-\) in solution. The excess \(\ceOH^-\) is attracted to the positively charged amine group resulting in the removal of an \(\ceH^+\) ion to form (\ceH_2O\). The amine group has a neutral charge leaving only a negative charge on the carboxylate group. Overall, the amino acid will have a charge of \(-1\).

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The isoelectric point, also referred to as the zwitterion's pH, signifies the pivotal moment when a molecule attains a neutral charge. Picture this: no positive or negative charges prevail, as an equal count of these charged entities coexist harmoniously. When we focus specifically on amino acids, this moment occurs precisely when the molecule's positive and negative charges are perfectly balanced, a delicate equilibrium dictated by the pKa values assigned to the amino and carboxyl groups.

To fully grasp the intricacies of this concept, it is paramount to acknowledge that amino acids form the fundamental building blocks of proteins. These amino acids possess a dual nature, with an amino group (-NH2) on one end and a carboxyl group (-COOH) on the other. These groups exhibit characteristics of weak acids and bases, respectively, and their protonation or deprotonation hinges on the pH of their surroundings. At lower pH levels, the amino group happily embraces protonation (NH3+), while the carboxyl group resists such a transformation (COO-), resulting in an overall positive charge for the molecule. In contrast, at higher pH levels, the amino group stands proud as it sheds its proton (NH2), while the carboxyl group welcomes protonation (COOH), engendering a negative charge for the molecule. However, at the magical isoelectric point, the amino acid stands in its glory as a zwitterion, gracefully adorned with both positive and negative charges.

The isoelectric point serves a significant purpose, serving as a metric to gauge a molecule's acidity or basicity, elements that profoundly influence its solubility, stability, and activity. This value also wields the power to prognosticate the behavior of proteins under diverse conditions, envisioning scenarios involving chromatography or electrophoresis with remarkable accuracy.

The isoelectric point of an amino acid is influenced by its chemical structure, specifically the side chain (R group) that determines its properties. There are 20 common amino acids, each with a unique side chain, and their isoelectric points range from 5.5 to 12.0. The table below lists the isoelectric points of some of the most common amino acids:

Each amino acid has a different isoelectric point, which is influenced by the number and type of charged groups present in the molecule. Amino acids with acidic side chains, such as aspartic acid and glutamic acid, have low isoelectric points, while those with basic side chains, such as arginine and histidine, have high isoelectric points.

At the isoelectric point, the molecule has no net charge, which means it is not attracted or repelled by charged particles in the environment. This may affect its solubility, as charged molecules tend to dissolve more readily in polar solvents. If the pH of the solution is below the isoelectric point of the amino acid, the molecule will have a net positive charge and will be attracted to negatively charged surfaces, making it less soluble. Conversely, if the pH is above the isoelectric point, the molecule will have a net negative charge and be repelled by a negatively charged surface, also decreasing its solubility.

In addition to solubility, the isoelectric point can affect the behavior of molecules during chromatography and electrophoresis. These techniques rely on the separation of molecules based on their charge and size, and the isoelectric point can be used to predict where molecules will move in these systems. For example, during isoelectric focusing, proteins are separated based on their isoelectric point in a pH gradient. As proteins move through the gradient, they will reach a region where the pH is equal to their isoelectric point, causing them to stop moving because they have no net charge and thus can be separated based on the protein's isoelectric point.

Detecting the isoelectric point accurately is crucial in various biological and analytical applications. One widely used technique for this purpose is Capillary Isoelectric Focusing (cIEF) technology. cIEF utilizes the principle of electrophoresis to separate and analyze proteins based on their isoelectric points. In this method, a capillary tube is filled with a pH gradient, typically created using a mixture of carrier ampholytes or immobilized pH gradient (IPG) strips. The protein sample is then introduced into the capillary tube and subjected to an electric field. As the electric current is applied, proteins migrate through the pH gradient until they reach a pH value matching their isoelectric point. At this point, the proteins cease to migrate further due to their neutrality, resulting in their sharp focusing at specific positions along the capillary. The separated protein bands can be visualized using various detection methods such as UV absorbance, fluorescence, or mass spectrometry. cIEF technology offers high resolution, sensitivity, and automation, making it a valuable tool for precise determination and characterization of isoelectric points in complex protein mixtures.

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