A peptide has five residues. Count the possible sequences in three cases…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A peptide has five residues. Count the possible sequences in three cases. In the first, each place can hold any of the 20 standard amino acids. In the second, the peptide contains five different amino acids, each used exactly once, and only the order changes. In the third, it contains two glycine, two alanine and one serine residues. Remember that Gly-Ala and Ala-Gly are different.
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. Use the counting rules.
Step 1. Free choice. .
Step 2. Five different, each once. .
Step 3. With repeats. .
Answer: , and sequences.
Check. The count with repeats is smaller than , because swapping the two glycine residues gives the same peptide.
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