Question

In quite alkaline solution, an amino acid contains two basic groups, \(-\mathrm{NH}_{2}\) and \(-\mathrm{COO}^{-}\). Which is the more basic? To which group will a proton preferentially go as acid is added to the solution? What will the product be?

Short answer

The -NH2 group is more basic than the -COO- group in an amino acid, as it has a higher affinity for protons. When acid is added to the solution, the proton (H+) will preferentially bind to the -NH2 group, converting it to -NH3+. The resulting product is a zwitterionic form of the amino acid with functional groups -NH3+ and -COO-.

Step-by-step solution

Understand basic concept of basicity#acitivity

Basicity is the ability of a chemical species to accept a proton (H+) and form a bond with it. The more basic a group is, the higher its affinity for protons. So, to determine which group is more basic between -NH2 and -COO-, we need to compare their abilities to accept a proton.

Compare basicity of -NH2 and -COO- groups

The -NH2 group can accept a proton to form -NH3+ and the -COO- group can accept a proton to form -COOH. We can compare their basicity by comparing their respective pKa values. The pKa value represents the strength of an acid when it loses a proton; however, the lower the pKa, the stronger the acid, and conversely, the stronger the base it can form upon accepting a proton. The pKa values of -NH2 and -COO- groups in amino acids are as follows: - pKa of -NH2 group: 9-10 - pKa of -COO- group: around 2 Since the pKa value of the -COO- group is lower than that of the -NH2 group, the -COO- group will form a stronger acid upon losing a proton and therefore, the -NH2 group is more basic.

Determine which group the proton will preferentially bind to

As mentioned earlier, the -NH2 group has a higher propensity to accept a proton because it's more basic. So, when acid is added to the solution, the proton (H+) will preferentially bind to the -NH2 group.

Determine the product formed

When a proton is added to the -NH2 group, the nitrogen atom in -NH2 forms a bond with the proton, resulting in a -NH3+ group. The product formed will be the amino acid with -NH3+ and -COO- as its functional groups, also known as a zwitterion. In summary: 1. The -NH2 group is more basic than the -COO- group. 2. When acid is added to the solution, a proton will preferentially bind to the -NH2 group. 3. The product formed will be the zwitterionic form of the amino acid, with -NH3+ and -COO- groups.

Key concepts

Proton Acceptance in Amino Acids

Amino acids are fascinating molecules that play a critical role in various biological processes. One of their notable characteristics is their ability to interact with protons (H⁺ ions), in a process commonly referred to as proton acceptance.

Proton acceptance is essentially a contest of how eagerly different parts of a molecule will bond with an incoming proton. In the context of amino acids, this involves the amino group (-NH₂) and the carboxylate ion (-COO^-). The winner of this contest is usually determined by the basicity of each group – how readily it can accept a proton.

To make this easier to understand, picture a game where the proton is a prize, and the groups are players trying to grab it. The amino group (-NH₂), with its pair of lonely electrons, is like a player with open arms ready to hug the proton, making it a strong contender. This behavior is a crucial aspect of many biochemical reactions, such as the formation of proteins from amino acids.

pKa Values in Amino Acids

To get deeper into the chemistry of amino acids, it's important to master the role of pKa values. These values are like the stats on a player's card, giving you insight into how well a group can play the game of proton acceptance.

pKa is a scale used to measure the strength of an acid which, inversely, indicates the basicity of the associated conjugate base. In amino acids, we pay particular attention to the pKa values of the amino (-NH₂) and carboxylate (-COO^-) groups. Remember, a lower pKa means a stronger acid and thus a weaker base, while a higher pKa implies a weaker acid and a stronger base.

For instance, the amino group usually has a pKa between 9 and 10, whereas the carboxylate group hovers around 2. This tells us that the amino group is less acidic, and therefore, more basic. In simpler terms, the amino group is a better player at snagging protons than the carboxylate group. This knowledge is not just trivia; it's fundamental in predicting the behavior of amino acids in different environments, such as in our bodies.

Zwitterion Formation

Imagine an amino acid as someone who can dress according to the weather – this is analogous to zwitterion formation in amino acids. A zwitterion is an amino acid that has adapted to its surroundings by taking on both a positive and a negative charge.

Under specific conditions, usually around the amino acid's isoelectric point (the pH at which it has no net charge), the amino group (-NH₂) will grab a proton to become -NH₃⁺, while the carboxylate group (-COO^-) holds onto its extra electron. This creates a dual-personality molecule with both a positive and a negative charge.

The magic of the zwitterion lies in its neutrality; it's balanced and stable. This is crucial in the world of chemistry and biology because it influences how amino acids interact with each other and with other molecules, playing a pivotal role in the structure and function of proteins. Zwitterions are the epitome of adaptability in the molecular universe, showcasing the stunning versatility of amino acids.