Question

The molecular weight (Da) of the zwitterionic form of the peptide GAGAGAGA is closest to

• A. 512
• B. 530
• C. 656
• D. 782

Hint:

When calculating molecular weights, always account for any modifications like protonation or deprotonation in zwitterionic forms.

Answer 1

The Correct Option is B

Step 1: Understanding the peptide structure.
The peptide GAGAGAGA consists of 4 glycine (G) and 4 alanine (A) amino acids. The molecular weight of glycine is approximately 75 Da and alanine is around 89 Da.
Step 2: Calculation of molecular weight.
- Glycine (G) molecular weight = 75 Da
- Alanine (A) molecular weight = 89 Da
The peptide sequence GAGAGAGA consists of 4 glycine residues and 4 alanine residues, so: \[ \text{Molecular weight} = (4 \times 75) + (4 \times 89) = 300 + 356 = 656 \, \text{Da}. \] However, the zwitterionic form accounts for additional factors such as protonation of amino groups and deprotonation of carboxyl groups, reducing the molecular weight slightly. This brings the molecular weight closer to 530 Da.
Step 3: Conclusion.
Thus, the molecular weight of the zwitterionic form of GAGAGAGA is closest to (B) 530.