Q.42 Shown below is an electrospray ionization mass spectrum of a protein:
The numbers written on top of the peaks are the m/z values. The mass of the protein deduced from
the given data is ___________ kDa.
Observed m/z Values
| Peak | m/z Value |
|---|---|
| 1 | 1813.50 |
| 2 | 2072.42 |
| 3 | 2417.67 |
| 4 | 2901 |
| 5 | 3626 |
Concept: Electrospray Ionization (ESI)
In ESI mass spectrometry, proteins acquire multiple charges due to protonation.
Each peak corresponds to a different charge state of the same protein.
The relationship between molecular mass and m/z is:
m/z = (M + zH) / z
where M is the molecular mass of the protein and z
is the charge state.
Step 1: Choose Two Adjacent Peaks
Two neighboring peaks correspond to consecutive charge states.
Consider the peaks at:
- m/z = 2072.42
- m/z = 2417.67
Step 2: Calculate the Charge State
The charge (z) is calculated using:
z = (m/z)2 / [(m/z)2 − (m/z)1]
z = 2417.67 / (2417.67 − 2072.42)
z = 2417.67 / 345.25 ≈ 7
Step 3: Calculate Molecular Mass
Molecular mass is given by:
M = z(m/z) − z
M = 7 × 2417.67 − 7
M = 16923.7 − 7 = 16916.7 Da
Final Answer
The molecular mass of the protein is approximately
16.9 kDa.
Why Other Values Would Be Incorrect
- Confusing m/z value directly with molecular mass
- Ignoring multiple charge states in ESI spectra
- Using non-adjacent peaks without accounting for charge difference
Conclusion
Electrospray ionization produces multiple peaks corresponding to different
charge states of the same protein. By identifying two adjacent peaks and
calculating the charge state, the molecular mass of the protein can be
accurately determined. In this case, the protein has a mass of
approximately 16.9 kDa.
