149. What will be the molecular weight of the tri-peptide Trp-Gly-His?
Molecular weights of Trp, Gly, and His are 204.2, 75.07, and 155.2 respectively.
A. 434.47
B. 300.00
C. 398.47
D. 502.25

Calculating the Molecular Weight of the Tripeptide Trp-Gly-His

Understanding how to calculate the molecular weight of a peptide is essential in biochemistry and molecular biology, especially for protein synthesis and peptide analysis. Let’s walk through the process using the tripeptide Trp-Gly-His as an example.

Given Data:

• Tryptophan (Trp): 204.2 Da

• Glycine (Gly): 75.07 Da

• Histidine (His): 155.2 Da

Step-by-Step Calculation:

Step 1: Add individual amino acid masses

$$204.2(Trp)+75.07(Gly)+155.2(His)=434.47\text{Da}$$

Step 2: Subtract the mass of water molecules lost during peptide bond formation

When two peptide bonds form in a tripeptide, two water molecules (H₂O) are removed.

• Mass of one H₂O molecule = 18.015 Da

• Total water loss = 2 × 18.015 = 36.03 Da

Final Calculation:

434.47−36.03=398.44 Da≈398.47 Da434.47 – 36.03 = \boxed{398.44\ \text{Da}} \approx \boxed{398.47\ \text{Da}}434.47−36.03=398.44 Da​≈398.47 Da​

Final Answer:

C. 398.47

Why This Matters

Peptide and protein mass calculation is foundational in:

• Mass spectrometry

• Peptide synthesis

• Drug design

• Proteomics and structural biology

Accurate mass prediction allows scientists to identify peptides in complex mixtures and monitor synthesis progress.

Conclusion

The correct molecular weight of the tripeptide Trp-Gly-His is approximately 398.47 Da, considering the subtraction of water molecules lost during peptide bond formation.

Correct Answer: C. 398.47