How do you draw a tetrapeptide from the one letter codes(Amino acids) and also give the pI, if given the pKa values?

For example the Amino acids: S,T,G,L

To draw a tetrapeptide from the one-letter codes of amino acids (S, T, G, L) and determine the isoelectric point (pI), follow these steps:

Step 1: Identify the amino acids:
- S represents Serine
- T represents Threonine
- G represents Glycine
- L represents Leucine

Step 2: Determine the pKa values for each amino acid:
- Serine (S): pKa1 = 2.21, pKa2 = 9.15, and pKa3 = 10.47
- Threonine (T): pKa1 = 2.09, pKa2 = 9.10, and pKa3 = 10.43
- Glycine (G): pKa1 = 2.35, pKa2 = 9.78
- Leucine (L): pKa1 = 2.36, pKa2 = 9.60, and pKa3 = 10.98

Step 3: Draw the tetrapeptide:
Based on the given amino acids, the tetrapeptide can be written as follows:
- Serine-Threonine-Glycine-Leucine (STGL)

Step 4: Calculate the pI:
pI can be calculated using the pKa values of the amino acids involved. The pI is the average of the pKa values of the ionizable groups that are still charged (deprotonated) at a given pH.

For the tetrapeptide STGL, we need to consider the ionizable groups of each amino acid:
- Serine: Side chain (-OH)
- Threonine: Side chain (-OH)
- Glycine: No ionizable side chain
- Leucine: No ionizable side chain

The two ionizable groups present are the side chains of Serine and Threonine. These side chains can be deprotonated at higher pH values.

The pI calculation involves averaging the pKa values that correspond to the acidic and basic forms of the amino acid side chains involved.

Using the pKa values:
- For Serine: Consider pKa2 (9.15)
- For Threonine: Consider pKa2 (9.10)

To calculate the pI, take the average of these pKa values:
pI = (pKa2 + pKa2) / 2
pI = (9.15 + 9.10) / 2
pI = 9.125

Therefore, the pI of the tetrapeptide STGL is approximately 9.125.

To draw a tetrapeptide from the one-letter codes for amino acids (S, T, G, L), you will need to know the sequence of the amino acids in the peptide. In this case, let's assume the sequence is "STGL".

To draw the tetrapeptide structure, you would start by identifying the chemical structure of each amino acid based on their one-letter codes:

- S represents Serine: Serine has a side chain consisting of a hydroxyl group (-OH) attached to the carbon atom.
- T represents Threonine: Threonine has a side chain consisting of a hydroxyl group (-OH) attached to the carbon atom and a methyl group (-CH3) attached to the same carbon atom.
- G represents Glycine: Glycine is the simplest amino acid and has a hydrogen atom (-H) as its side chain.
- L represents Leucine: Leucine has a side chain consisting of a branched alkyl group (-CH2-CH(CH3)2) attached to the carbon atom.

To draw the tetrapeptide structure, you can place the amino acids in a linear fashion, connecting them with peptide bonds. Since there are four amino acids, the structure will have three peptide bonds connecting the amino acids:

S T G L
| | | |
N — C — C — C — C — C — C — C — C — C — C
| | | |
H OH H CH2-CH(CH3)2

(Note: The "N" on the left and "H" on the right represent the N-terminus and C-terminus of the peptide, respectively, as these are the ends of the amino acid chain.)

To calculate the isoelectric point (pI) of the tetrapeptide, you need to know the pKa values of the ionizable groups (amino and carboxyl groups) in the side chains of the amino acids. The pKa values for the amino and carboxyl groups of the common amino acids are typically around 2 and 9, respectively. However, for a more accurate calculation, you could consult a pKa table or database specific to the amino acids you are working with.

Once you have the pKa values, you can determine the charge of the peptide at different pH levels. The pI is the pH at which the overall charge of the peptide is neutral (zero charge). To calculate the pI, you can follow these steps:

1. Identify the ionizable groups in the peptide sequence. In this case, you have the N-terminal amino group and the C-terminal carboxyl group, as well as the side chains of Serine (hydroxyl group) and Threonine (hydroxyl and methyl groups).
2. Determine the charge of each ionizable group at different pH levels using the Henderson-Hasselbalch equation:

Charge = (10^(pH - pKa)) / (1 + 10^(pH - pKa))

3. Calculate the net charge of the peptide at different pH levels by summing up the charges of all ionizable groups.
4. Identify the pH at which the net charge of the peptide is closest to zero. This pH value corresponds to the pI of the tetrapeptide.

By applying these steps, you can determine the pI of the tetrapeptide based on the given pKa values for the ionizable groups.