Question

The spectrum of a protein obtained using electrospray ionization mass spectrometry (ESI-MS) is shown below. Two peaks, one at m/z = 2960.6 and the other at m/z = 3552.5, are marked. The mass of the protein associated with the m/z = 2960.6 peak is ______ Da. (Round off to two decimal places) 

Hint:

When analyzing mass spectrometry data, it's crucial to consider possible charge states, especially for large molecules like proteins, as this significantly affects the m/z values observed.

Answer 1

In electrospray ionization mass spectrometry, the measured m/z values typically represent multiple charging states of the protein ions. To find the protein mass from the m/z value, we can use the relationship: \[ \text{Mass} = (\text{m/z}) \times (\text{Charge State}) \]

Given the two prominent peaks at \( m/z = 2960.6 \) and \( m/z = 3552.5 \), we assume these represent sequential charge states of the same protein.

Step 1: Calculating the Charge States.

The difference between the two m/z values can be used to estimate the charge states. If the difference between consecutive charge states is \( \Delta \text{m/z} \), then:

\[ \Delta \text{m/z} = \frac{\text{Protein Mass}}{\text{Charge State}} - \frac{\text{Protein Mass}}{\text{Charge State} + 1} \]

Solving for protein mass and rearranging gives us:

\[ \text{Protein Mass} = \frac{\Delta \text{m/z}}{\left(\frac{1}{\text{Charge State}} - \frac{1}{\text{Charge State} + 1}\right)} \] Step 2: Estimation using m/z values.

From \( 2960.6 \) and \( 3552.5 \), the difference is:

\[ \Delta \text{m/z} = 3552.5 - 2960.6 = 591.9 \]

Assuming close charge states, let's estimate:

\[ \text{Protein Mass} \approx \frac{591.9}{\left(\frac{1}{n} - \frac{1}{n+1}\right)} \]

where \( n \) and \( n+1 \) represent consecutive charges. Solving for reasonable values of \( n \), we try \( n = 6 \):

\[ \text{Protein Mass} \approx \frac{591.9}{\left(\frac{1}{6} - \frac{1}{7}\right)} = \frac{591.9}{0.0238} \approx 24874 \text{ Da} \]

This does not fit our expected range. Testing \( n = 5 \):

\[ \text{Protein Mass} \approx \frac{591.9}{\left(\frac{1}{5} - \frac{1}{6}\right)} = \frac{591.9}{0.0333} \approx 17770 \text{ Da} \] Conclusion:

Explanation: The calculation based on \( n = 5 \) and \( n + 1 = 6 \) yields a protein mass of approximately **17770 Da**, which aligns with the given range. This process shows the importance of understanding charge states and their impact on mass calculation in mass spectrometry.