D. CHROMATOGRAPHY Chromatography is a general technique that is used to separate

Question

D. CHROMATOGRAPHY Chromatography is a general technique that is used to separate various materials. In this case, students can use the unique chemistry of the amino acid to determine its migration in a specific solvent. The solutions of amino acids can be applied to the plate and allowed to dry. The plate is placed in the solvent, and capillary action will carry the amino acids upward along the plate. The side chains, or R groups, can be characterized by their chemical proper- ties as nonpolar, polar uncharged, or polar charged. Those amino acids that are the most nonpolar will travel farther on the plate, while those that are charged and polar will stay near the bottom of the plate. This is because the plate is polar and more strongly attracts polar molecules, slowing their progress up the plate. In addition, the same amino acid will travel the same distance no matter what lane it is in. In this way, a mixture of amino acids may be separated and their identity matched with a standard. 1. Suppose you had the following resule (below).Answer che following questions. a. Which amino acid tested is most polar? Look up the structure of the amino acids. Explain why it makes sense that the amino acid you listed is more polar than the others. b. c. What amino acids are in the fruit juice?

Explanation / Answer

Q1

TLC stands for Thin Layer Chromatography; it is an analytical technique in which we exploit each sample's affinity toward different polar/nonpolar solvents.

We use a stationary phase: a phase which does NOT moves, as the name implies. It is typically the "thin layer" or the "plate". It is commonly to use silica gel plates, which are polar, but any species could be used, such as paper.

We also use a mobile phase: as the name implies, this phase will "move" and will carry along the path all the components from the solvent. Many times, you will be able to split in several components the "mixture" of the sample. Therefore, they can be seen in the stationary phase (plate). Typically, we allow the mobile phase to go up to the "Rfinal" mark, or final reference. This is the distance travelled by the solvent (mobile phase). It starts at solvent level, it is R0, therefore.

Rf = distance traveled by sample / distance traveled by solvent

Then, "Rf" value allows us to understand the affinities of anaytes towards the solvent.

As Rf increases, this implies higher affinity toward the mobile phase ( since it is carried all along the mobile phase)

If Rf is low, this implies a low affinity toward the mobile phase; therefore higher affinity toward the stationary phase, since it sticks to the plate rather than travel upwards.

Therefore...

the lower Rf values --> the more polar amino acids

then,

choose glutamic acid.

b)

the glutamic acid has NH2, COOH, and COOH groups present, therefore, in slution,

NH3+ and COO- are formed, those are pretty polar, which will have affinity towards the stationary phase, therefore, lower Rf

c)

from the TLC plate

Fruit juice has 2 points --> top one, isoleucine

lower one --> glutamic acid

therefore, the only not present is serine

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