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What Effects Do Dimers Have on Experimental Molar Mass?

Dimer comes from the Greek for "two parts." As the name implies, dimers are pairs of molecules that join together to form a larger entity. In some cases, the two subunits are actually joined by covalent bonds so they form a single molecule; more frequently, however, the bonds between the two subunits are weak noncovalent bonds. In cases like these, dimers can affect the way you calculate molar mass --- the mass of one mole of molecules of a compound.

Types
- If the two subunits in a dimer are covalently bonded, they have no effect on the way you experimentally determine molecular mass because they have become a single molecule. You want to find the mass of one mole of molecules of the compound, and since the dimer is a compound, there are no considerations here that would not normally apply. Biochemists, however, often deal with proteins that form dimers which are weakly joined to each other by noncovalent bonds, and these are a little more interesting.
Dissociation
- If you calculated the molar mass of a protein dimer, your result would depend on whether you were looking at the mass of the dimer or its subunits. A homodimer made of two identical subunits, for example, would have a molar mass twice that of either subunit. A heterodimer made from nonidentical subunits, by contrast, is a little bit different, because each of the two subunits generally has a different molar mass.
Heating
- Heating protein dimers usually causes them to dissociate. Typically, it will also denature the proteins and cause them to lose their shape. If the two subunits are linked together by a kind of covalent bond called a disulfide bridge, treating them with mercaptoethanol breaks the disulfide link to free up the subunits. Biochemists usually heat proteins and add mercaptoethanol before conducting a procedure called a Western blot, so these two steps help ensure the proteins are completely denatured.
Mass Spectrometry
- Mass spectrometry is a very accurate way to determine the molar mass of proteins and other molecules. Vaporizing proteins in the same way that other compounds are vaporized for mass spectrometry could possibly fragment them, however, so mass spectrometry for proteins typically relies on either the electrospray or the matrix-assisted laser desorption (MALDI) techniques instead. These techniques can cause dimers to dissociate, although sometimes dimers can also form during the process, and scientists have to take these possibilities into account when interpreting their data.

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