Question

Electrospray mass spectrometry is a recently developed technique for looking at large molecules with a mass spectrometer. In this technique, molecular ions, each associated with one or more \(\mathrm{H}^{+}\)ions, are prepared under mild conditions in the mass spectrometer. As an example, a protein (P) with a molecular mass of 11,812 gives clusters of the type \((\mathrm{P}+8 \mathrm{H})^{8+},(\mathrm{P}+7 \mathrm{H})^{7+}\), and \((\mathrm{P}+6 \mathrm{H})^{6+}\). At what mass-to-charge values do these three clusters appear in the mass spectrum?

Short answer

Answer: The mass-to-charge ratios for the three clusters are as follows: Cluster 1 (\((P+8H)^{8+}\)): 1,477.5 Cluster 2 (\((P+7H)^{7+}\)): 1,688.43 Cluster 3 (\((P+6H)^{6+}\)): 1,969.667

Step-by-step solution

Determine the mass of a single \(\mathrm{H}^{+}\) ion

Firstly, we need to determine the mass of a single \(\mathrm{H}^{+}\) ion, which will be used in later calculations. Since the exercise is operating in units of Daltons, we find that the mass of the H+ ion is 1 Da.

Calculate the mass of each cluster

We will now calculate the total mass of each cluster by adding the mass of protein and the mass of hydrogen ions present in the cluster. For Cluster 1 (\((P+8H)^{8+}\)): Total mass = mass of protein + (8 * mass of H+ ion) = 11,812 + (8 * 1) = 11,820 Da For Cluster 2 (\((P+7H)^{7+}\)): Total mass = mass of protein + (7 * mass of H+ ion) = 11,812 + (7 * 1) = 11,819 Da For Cluster 3 (\((P+6H)^{6+}\)): Total mass = mass of protein + (6 * mass of H+ ion) = 11,812 + (6 * 1) = 11,818 Da

Calculate the mass-to-charge ratio for each cluster

Now we will calculate the mass-to-charge ratio (m/z) for each cluster by dividing the mass of each cluster by its respective charge. For Cluster 1 (\((P+8H)^{8+}\)): Mass-to-charge ratio = mass of cluster / charge of cluster = 11,820 / 8 = 1,477.5 For Cluster 2 (\((P+7H)^{7+}\)): Mass-to-charge ratio = mass of cluster / charge of cluster = 11,819 / 7 = 1,688.43 (rounded to 2 decimal places) For Cluster 3 (\((P+6H)^{6+}\)): Mass-to-charge ratio = mass of cluster / charge of cluster = 11,818 / 6 = 1,969.667 (rounded to 3 decimal places) As a result, the mass-to-charge ratios for the three clusters are as follows: Cluster 1 (\((P+8H)^{8+}\)): 1,477.5 Cluster 2 (\((P+7H)^{7+}\)): 1,688.43 Cluster 3 (\((P+6H)^{6+}\)): 1,969.667

Key concepts

Mass-to-Charge Ratio

Understanding the mass-to-charge ratio is fundamental when working with electrospray mass spectrometry. It's denoted as m/z and is crucial for determining the molecular weight of ions within a sample. This ratio is calculated by dividing the mass of an ion by its charge number. In the context of proteins, where large molecules are often involved, the mass-to-charge ratio offers a detailed view of the protein's structure and composition.

For instance, let's consider molecular ions of a protein with variations in their associated hydrogen ions. Using simple addition and division, we determine the mass of each protein-hydrogen cluster and then divide this by the charge, which is the number of hydrogen ions plus one for the protein itself. A protein ion P, clustered with 8 hydrogen ions and a charge of 8+, would have its mass-to-charge ratio calculated by summing the mass of the protein and 8 times the mass of a hydrogen ion, then dividing by 8. These calculations allow scientists to effectively characterize molecular ions in complex biological samples.

Protein Mass Spectrometry

Protein mass spectrometry is a powerful analytical technique used to determine the mass of proteins. This method allows for the analysis of complex mixtures, identification of proteins, and the study of post-translational modifications. Proteins, due to their size and complexity, require a gentle ionization technique, such as electrospray, to prevent fragmentation. This results in intact molecular ions that can be analyzed.

These molecular ions are then introduced into a mass spectrometer, where their mass-to-charge ratios are measured. The data are interpreted to provide insights into the protein's molecular weight and structure. In exercises involving electrospray mass spectrometry, understanding how to calculate the m/z values of various charged states of a protein ion can reveal detailed information about the protein's composition and structure in the biological system being studied. This is particularly useful in biotechnological and pharmaceutical research, where precise molecular understanding is critical.

Molecular Ions

In mass spectrometry, molecular ions are formed when a molecule loses or gains an electron through ionization, which can occur under different conditions based on the method used. In electrospray ionization, the process is soft enough to produce intact molecular ions that have not been fragmented, which is essential when studying large molecules such as proteins.

These generated ions, often carrying multiple charges, are the primary focus during mass spectrometric analysis since they reflect the original molecular mass of the sample. By understanding how these ions are formed and what their mass-to-charge ratios represent, students can better interpret their significance in a spectrum. The presence and mass of these molecular ions, as shown in the given exercise, are indicative of the molecular structure and the charge states of the analyzed protein, offering profound insights into the respective molecular complexities.