The tripeptide is T-A-K
1. What is the overall charge of the tripeptide below pH 1.0?
2. What is the overall charge of the tripeptide at pH 12?
3. What type of intermolecular attractions could the side chains of the amino acids make with the solvent (water) at pH 7.4? (Note that these are attractions with water, not with other amino acids.)
4. If the tripeptide was dissolved in a buffer that is pH 8.5, what would be the net charge on the most abundant chemical species?
To answer these questions, we need to consider the ionization states of the amino acids in the tripeptide at different pH levels. The charge of an amino acid depends on the protonation or deprotonation of its functional groups.
1. To determine the overall charge of the tripeptide at pH 1.0, we need to consider the pKa values of the functional groups in the amino acids. At pH 1.0, the environment is highly acidic. Since the pKa values for the carboxyl group (COOH) and the amino group (NH2) are around 2.0 and 9.0, respectively, they will both be protonated at pH 1.0.
In T-A-K, we have a terminal carboxyl group (T-COOH) and an amino group (A-NH2), both of which will be protonated. The neutral side chain group of lysine (K) will also be protonated at pH 1.0. So, all three amino acids will have a positive charge.
Therefore, the overall charge of the tripeptide at pH 1.0 is +3.
2. At pH 12, the environment is highly basic/alkaline. At this pH, the carboxyl group (COOH) will be deprotonated, becoming a carboxylate group (COO-), while the amino group (NH2) will remain uncharged.
In T-A-K, the terminal carboxyl group (T-COOH) will lose its proton and become negatively charged. However, the amino group (A-NH2) will remain uncharged, and the side chain amino group of lysine (K) will also remain uncharged at pH 12.
Therefore, the overall charge of the tripeptide at pH 12 is -1.
3. The type of intermolecular attractions of the side chains with water at pH 7.4 depends on the ionization states of their functional groups. At pH 7.4, which is close to physiological pH, certain functional groups can form hydrogen bonds with water molecules.
In the tripeptide T-A-K, the side chain of threonine (T) contains a hydroxyl group (OH) which can form hydrogen bonds with water. The side chain of alanine (A) is nonpolar and will not strongly interact with water through hydrogen bonding. The side chain of lysine (K) contains an amino group (NH2) which can also form hydrogen bonds with water. Therefore, both threonine and lysine can form hydrogen bonds with water at pH 7.4.
4. To determine the net charge on the most abundant chemical species of the tripeptide dissolved in a pH 8.5 buffer, we need to consider the pKa values of the functional groups of the amino acids. At pH 8.5, the carboxyl group (COOH) will be partially deprotonated, while the amino group (NH2) will be protonated.
In T-A-K, the terminal carboxyl group (T-COOH) will lose some of its protons and become partially negatively charged. The amino group (A-NH2) will be protonated and positively charged. The side chain amino group of lysine (K) will also be protonated and positively charged at pH 8.5.
Therefore, in the most abundant chemical species of the tripeptide at pH 8.5, the overall charge will be +3.
To answer these questions, let's break down the information step by step:
1. The overall charge of a tripeptide at a specific pH is determined by the ionization state of its constituent amino acids. Below pH 1.0, the amino acid residues can be assumed to be fully protonated, which means they have a positive charge.
- The first amino acid in the tripeptide is T (Threonine), with a pKa value of around 2.1 for its side chain. At pH 1.0, the Threonine side chain will likely be fully protonated with a positive charge.
- The second amino acid in the tripeptide is A (Alanine), with a pKa value of around 2.3 for its side chain. Similarly, at pH 1.0, the Alanine side chain will also likely be fully protonated with a positive charge.
- The third and last amino acid in the tripeptide is K (Lysine), with a pKa value of around 10.5 for its side chain. At pH 1.0, the Lysine side chain will likely be fully protonated with a positive charge.
Therefore, at pH 1.0, the overall charge of the tripeptide is likely +3 (three positive charges).
2. At pH 12, the situation is slightly different. The amino acid residues can be assumed to be fully deprotonated, which means they have a negative charge.
- At pH 12, Threonine's side chain would likely be fully deprotonated with a negative charge.
- Alanine's side chain does not have any ionizable groups, so it will not contribute to the overall charge.
- Lysine's side chain, with a pKa of around 10.5, will likely still have most of its side chains protonated with a positive charge.
Therefore, at pH 12, the overall charge of the tripeptide will likely be -1 (one negative charge).
3. At pH 7.4, intermediate to acidic and basic conditions, the ionization states of the amino acids' side chains will be different.
- Threonine's side chain will likely still be mostly protonated at pH 7.4, and thus can form hydrogen bonds with water.
- Alanine's side chain does not have any ionizable groups and, therefore, will not form any specific attractions with water.
- Lysine's side chain will likely be partially protonated at pH 7.4. The positively charged part of the side chain can form hydrogen bonds with water, while the amino group can donate a hydrogen bond to water.
Thus, Threonine and Lysine can form intermolecular attractions with water in the form of hydrogen bonds.
4. If the tripeptide is dissolved in a buffer at pH 8.5, the ionization states of the amino acids' side chains will be different again.
- Threonine's side chain will likely be mostly protonated at pH 8.5, contributing a positive charge.
- Alanine's side chain does not have any ionizable groups and, therefore, will not contribute to the overall charge.
- Lysine's side chain with a pKa of around 10.5 will likely be partially deprotonated at pH 8.5, contributing a negative charge to the tripeptide.
Therefore, at pH 8.5, the net charge on the tripeptide would depend on the proportion of the partially protonated Lysine side chains. It could be neutral if the positive and negative charges balance each other out or slightly negative if there are more deprotonated Lysine side chains.
Please note that these predictions are based on general knowledge of amino acid properties and pH-dependent ionization tendencies. Experimental measurement and parameterization for specific residues may yield slightly different results.