# DBT JRF 2020 — Question 26

Exam: DBT JRF 2020 · Section: A · Topic: Biochemistry · Difficulty: Easy

## Question

The molecular weight of Val and Ser are 117 Dalton and 105 Dalton, respectively. Val and Ser form a dipeptide Val-Ser. The molecular weight (in Daltons) of the dipeptide is

## Options

- **A.** 204
- **B.** 186
- **C.** 240
- **D.** 222

## Answer

Option A

## Explanation

Dipeptide MW calculation: When two amino acids form a peptide bond, one water molecule ( $\text{H}_2\text{O}$ , MW = 18 Da) is lost. $\text{MW}_{\text{dipeptide}} = \text{MW}(\text{Val}) + \text{MW}(\text{Ser}) - \text{MW}(\text{H}_2\text{O}) = 117 + 105 - 18 = 204 \text{ Da}$ . This applies to every peptide bond formed: for a polypeptide of $n$ amino acids, the mass loss is $(n-1) \times 18$ Da due to the $(n-1)$ water molecules eliminated during condensation reactions.

Source: https://mypyqbuddy.com/questions/the-molecular-weight-of-val-and-ser-are-6a05a8a02adafec470293200