Aspartame is the methyl ester of the dipeptide Asp-Phe…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Aspartame is the methyl ester of the dipeptide Asp-Phe. It forms from aspartic acid (), phenylalanine () and methanol () with the loss of two molecules of water ( each). Find its molar mass and the percentage of nitrogen by mass. Each of its two amino acid residues has a stereocentre. How many stereoisomers does aspartame have?
Need a reminder? The key ideaShow
A peptide bond forms with the loss of one water. A chain of residues has bonds, and its molar mass is the sum of the free amino acids minus for each bond. In an α-helix there are 3.6 residues per turn and a rise of 0.15 nm per residue. The C=O of residue bonds to the N-H of residue . The number of sequences with repeats is divided by the factorial of each repeat.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Add the three parts and take away two waters.
Step 1. Molar mass. . The formula gives , which agrees to rounding.
Step 2. Nitrogen. , which is .
Step 3. Stereoisomers. Two stereocentres give .
Answer: , nitrogen and 4 stereoisomers. Only the L,L form tastes sweet.
Check. The two losses of water explain why the molar mass is below the sum of the three parts, .
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