To calculate the molecular weight of a protein from an Electrospray Ionization (ESI) spectrum, we can derive the protein's molecular weight by identifying the charge states of the detected ions.
The peaks you provided correspond to the detected m/z (mass-to-charge ratio) values in the spectrum:
- 893.17
- 942.71
- 848.54
- 1060.44
- 1131.05
- 998.13
- 1211.77
- 808.17
- 1304.89
- 771.50
- 1413.58
- 737.99
Step 1: Identify Charge States
In ESI, ions can be detected in multiple charge states. The general idea is that if we assume the protein has a specific molecular weight (MW), then the m/z values can be used to estimate the charge state:
m/z = (molecular weight + charge) / charge
Rearranging this, we find:
molecular weight = m/z * charge - charge
To identify possible charge states (usually denoted as z), we can assume z-values of 1, 2, 3, etc. and assess the resulting molecular weight.
Step 2: Calculate Molecular Weights
We will calculate possible molecular weights for each m/z peak based on different charge states.
Let's try charge states z = 1, 2, and 3:
For z = 1 (monovalent)
- MW = m/z (since charge = 1)
- 893.17 (MW = 893.17)
- 942.71 (MW = 942.71)
- 848.54 (MW = 848.54)
- 1060.44 (MW = 1060.44)
- 1131.05 (MW = 1131.05)
- 998.13 (MW = 998.13)
- 1211.77 (MW = 1211.77)
- 808.17 (MW = 808.17)
- 1304.89 (MW = 1304.89)
- 771.50 (MW = 771.50)
- 1413.58 (MW = 1413.58)
- 737.99 (MW = 737.99)
For z = 2 (divalent)
- MW = m/z * 2 - 2
- 893.17 (MW = 1788.34 - 2 = 1786.34)
- 942.71 (MW = 1885.42 - 2 = 1883.42)
- 848.54 (MW = 1697.08 - 2 = 1695.08)
- 1060.44 (MW = 2120.88 - 2 = 2118.88)
- 1131.05 (MW = 2262.10 - 2 = 2260.10)
- 998.13 (MW = 1996.26 - 2 = 1994.26)
- 1211.77 (MW = 2423.54 - 2 = 2421.54)
- 808.17 (MW = 1616.34 - 2 = 1614.34)
- 1304.89 (MW = 2609.78 - 2 = 2607.78)
- 771.50 (MW = 1543.00 - 2 = 1541.00)
- 1413.58 (MW = 2827.16 - 2 = 2825.16)
- 737.99 (MW = 1475.98 - 2 = 1473.98)
For z = 3 (trivalent)
- MW = m/z * 3 - 3
- 893.17 (MW = 2679.51 - 3 = 2676.51)
- 942.71 (MW = 2828.13 - 3 = 2825.13)
- 848.54 (MW = 2545.62 - 3 = 2542.62)
- 1060.44 (MW = 3181.32 - 3 = 3178.32)
- 1131.05 (MW = 3393.15 - 3 = 3390.15)
- 998.13 (MW = 2994.39 - 3 = 2991.39)
- 1211.77 (MW = 3635.31 - 3 = 3632.31)
- 808.17 (MW = 2424.51 - 3 = 2421.51)
- 1304.89 (MW = 3914.67 - 3 = 3911.67)
- 771.50 (MW = 2314.50 - 3 = 2311.50)
- 1413.58 (MW = 4240.74 - 3 = 4237.74)
- 737.99 (MW = 2213.97 - 3 = 2210.97)
Step 3: Compile Results
Looking at the MW values calculated for different charge states, you will observe that multiple m/z values correspond to a protein that can exist in different charge states. The highest molecular weight value (when considering different charge states) will likely represent the protein's molecular weight in its intact state.
Summary of Possible Molecular Weights:
- At z = 1: Range from 737.99 to 1413.58
- At z = 2: Range from 737.99 * 2 down to peak proteins
- At z = 3: Larger molecular weights might indicate fragmentation or higher order structures.
Conclusion:
The most probable molecular weight of the protein can be identified near consistent values across charge states and examining the subsequent peaks (expected isotopes in the ESI spectrum). The highest peak is commonly assigned as the most prominent charge state; hence you would conclude the MW of the protein by correlating the scale of charge and its repeated observation.
Using the data you've provided, the protein's estimated molecular weight is likely to be within the range of approximately 1412 to 1413 Daltons with possible charge states (1 to 3), largely depending on specific experimental conditions.
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