Problem 88 - Chemistry An Atoms First Approach

Question

Draw the structures of the tripeptides gly-ala-ser and ser-alagly. How many other tripeptides are possible using these three amino acids?

Step-by-Step Solution

Answer

The tripeptide Gly-Ala-Ser has the structure: H2N-CH2-CO-NH-CH(CH3)-CO-NH-CH(OH)-CH2-COOH, and the tripeptide Ser-Ala-Gly has the structure: H2N-CH(OH)-CH2-CO-NH-CH(CH3)-CO-NH-CH2-COOH. There are 4 other possible tripeptides using the three amino acids Gly (glycine), Ala (alanine), and Ser (serine).

Step 1: Drawing the structure of Gly-Ala-Ser

First, we need to find the structure of each amino acid involved. Glycine (Gly) has the following structure: H2N-CH2-COOH, alanine (Ala) has the structure: H2N-CH(CH3)-COOH, and serine (Ser) has the structure: H2N-CH(OH)-CH2-COOH. To construct the tripeptide Gly-Ala-Ser, we need to connect amino acids through peptide bonds. A peptide bond forms by the reaction of the carboxyl group (-COOH) of one amino acid with the amino group (-NH2) of another amino acid, releasing a water molecule. The structure of Gly-Ala-Ser would be: H2N-CH2-CO-NH-CH(CH3)-CO-NH-CH(OH)-CH2-COOH

Step 2: Drawing the structure of Ser-Ala-Gly

Similarly, we can construct the tripeptide Ser-Ala-Gly by connecting serine, alanine, and glycine through peptide bonds. The structure of Ser-Ala-Gly would be: H2N-CH(OH)-CH2-CO-NH-CH(CH3)-CO-NH-CH2-COOH

Step 3: Calculate the number of other possible tripeptides

Now, we need to determine the number of other possible tripeptides using these three amino acids. To do this, we can use the permutation formula. We have three unique amino acids, and we want to arrange them in three positions, which will be calculated as 3 × 2 × 1. Number of other possible tripeptides = 3! = 3 × 2 × 1 = 6 However, we have already drawn two of these tripeptides (Gly-Ala-Ser and Ser-Ala-Gly), so the number of other possible tripeptides is: 6 - 2 = 4 So, there are 4 other possible tripeptides using the three amino acids Gly (glycine), Ala (alanine), and Ser (serine).