Question

Which amino acids have positively charged \(R\) groups at \(\mathrm{pH} 7 ?\)

Short answer

Arginine, lysine, and histidine have positively charged R groups at pH 7.

Step-by-step solution

Identify Amino Acids with Charged Side Chains

Amino acids can have side chains (R groups) that possess charges. To solve this problem, we need to identify which amino acids have side chains that are positively charged at a neutral pH (around 7).

Recall the Specific Amino Acids

At physiological pH (around 7), only a few amino acids have positively charged side chains. These are arginine, lysine, and histidine. Each of these has a side chain that can accept a proton and become positively charged.

Consider the Properties of Each Amino Acid

Arginine contains a guanidinium group, lysine has an amino group, and histidine contains an imidazole group. At pH 7, these groups can be protonated and carry a positive charge.

Verify Amino Acids' Charge in Physiological Conditions

Check that at pH 7, these amino acids indeed possess a positive charge: - Arginine's pKa for the guanidinium group is about 12.5, making it positively charged. - Lysine's pKa for its amino group is around 10.5, also positively charged. - Histidine has a pKa of about 6 for its imidazole group, so it can act as a buffer but is often depicted as neutral with a slight positively-charged tendency.

Key concepts

Arginine

Arginine is an essential amino acid and is known for its positively charged side chain at physiological pH. This property is a result of its unique side chain that contains a guanidinium group. The guanidinium group of arginine is particularly good at accepting protons. This makes it positively charged at a pH of around 7.
At a technical level, this is because the pKa of arginine's guanidinium group is about 12.5. The pKa value indicates the pH at which half of the available molecules are deprotonated. Therefore, at pH 7, the majority of arginine molecules remain protonated and positively charged.

• Guanidinium group in arginine is stable when protonated.
• Helps in binding negatively charged molecules like DNA.

Arginine's role is not only structural but functional too. It's often involved in the formation of proteins and plays a critical role in cell division, healing of wounds, and immune function. Understanding its protonation at physiologic conditions helps elucidate how proteins fold and interact.

Lysine

Lysine is another amino acid that carries a positive charge at neutral pH, and this charge originates from its unique side chain. Lysine has an aliphatic chain capped with an amino group. This amino group is the portion that is particularly adept at gaining protons, resulting in its positive charge.
The amino group of lysine has a pKa of about 10.5, which is significantly higher than the physiological pH of 7. This implies that under such conditions, lysine remains predominantly in its protonated and positively charged form.

• Contains an aliphatic side chain, ending in an amino group.
• Highly involved in the cross-linking of proteins, enhancing structural strength.
• Essential for binding negatively charged molecules and compounds in biological systems.

Lysine’s positive charge makes it vital for protein interactions, especially in DNA-associated proteins like histones. It adds to the solubility of proteins in water, thus playing a critical role in maintaining cellular structures and functions.

Histidine

Histidine is a slightly unique player when it comes to charge properties at physiological pH. It has an imidazole side chain that can either carry a positive charge or remain neutral depending on subtle pH shifts. This gives histidine the ability to act as a buffer around physiological pH.
The pKa of histidine's imidazole group is approximately 6.0. While this is close to the physiological pH of 7, it means histidine can exist in both its neutral and protonated forms around this pH. Thus, it often acts as a regulator of protonation and charge in enzymatic reactions.

• Possesses an imidazole group which helps in proton transfer.
• European electrophiles can target the partially positive imidazole.
• Vital in enzyme active sites for catalysis.

Histidine’s flexible protonation state explains its importance in pH-sensitive environments in the body, such as different cellular compartments or enzyme active sites. It’s an amino acid crucial for enzymatic function and plays a significant role in various biochemical pathways.