Final answer:
The molecular weight of the tripeptide ala-ala-ala is calculated by adding the weights of three alanine molecules and subtracting the weight of the two water molecules lost during peptide bond formation. The correct molecular weight of ala-ala-ala is 267 daltons. Option a.
Step-by-step explanation:
The question is about calculating the molecular weight of a tripeptide composed of three alanine amino acids (ala-ala-ala). The molecular weight of a single alanine molecule is given as 89 daltons. For a peptide, we need to account for the loss of water molecules during peptide bond formation.
Each peptide bond formed releases one water molecule (H2O), which has a molecular weight of 18 daltons.
To calculate the molecular weight of ala-ala-ala, we add the molecular weights of three alanine molecules and then subtract the weight of the two water molecules that would be released during the formation of the tripeptide:
Molecular weight of three alanines: 3 × 89 daltons = 267 daltons
Molecular weight of two water molecules: 2 × 18 daltons = 36 daltons
We subtract the weight of the water from the total weight of alanines:
267 daltons - 36 daltons = 267 daltons
The molecular weight of ala-ala-ala is 267 daltons. Option a.