Instructor Notes — Chapter 32

How Peptides Are Made — Solid-Phase Synthesis, Recombinant Production, and Why Peptide Drugs Cost What They Cost

Instructor copy. Not published in the student edition.

Part VI opener. Advanced. Prerequisites: Chapters 1, 4, 19. Estimated student time 6–8 hours.

Teaching Notes

What this chapter is actually for

It looks like a chemistry chapter. It is really an epistemics chapter wearing a chemistry costume, and if you teach it as a survey of manufacturing methods you will get compliance and no transfer.

The through-line is this: a molecule's identity and a preparation's identity are different things. Students arrive believing that if two vials say "semaglutide," the question of what is in them has been answered. The chapter dismantles that belief with arithmetic rather than with suspicion, which is why it works on students who are resistant to being told that a market they use is unreliable.

Teach toward §32.9 from the first minute. Everything before it is evidence for one sentence.

The one thing to protect: §32.3

If your term is compressed and you must cut, cut §32.1 and §32.6. Do not cut §32.3, and do not summarize it — make the students compute.

The reason is pedagogical rather than mathematical. Every student can be told that yield falls with length. Almost none of them feel it until they have personally typed 0.99 into a calculator and raised it to the 50th power and seen 61%. The gap between "yields decline" and "61%" is the whole chapter. Budget class time for the calculation. Have them do it wrong first at 0.99 × 50.

A useful board sequence:

1. Ask: "If each step works 99% of the time, what fraction of a 50-residue peptide comes out right?" Collect guesses before anyone computes. You will get answers clustered around 95% and around 50%, almost nothing at 61%.
2. Compute it together.
3. Then ask the question that does the actual work: "Where did the other 39% go?" Most students say "it failed" or "it's waste." Push until someone says it is still in the vessel.
4. Draw the deletion sequence. Ask what property distinguishes it from the product. Let the class run out of answers.

That fourth step is the moment the chapter lands.

Common misconceptions, in the order they will surface

"Failed reactions produce nothing." The single most important correction. A failed coupling produces a chain that continues growing. Students consistently model failure as absence; the chapter models it as a different product. Do not move on until this is secure.

"Purity means how much of it is the thing." Conflating area percent with mass fraction. The counterion point in §32.4 usually fixes this, but only if you make it concrete — write "10 mg" on the board, then subtract water and TFA, then ask what number should have been on the label.

"Automated means easy." Students infer that because synthesis is automated, peptides are trivial to make. The difficult-sequence material in §32.2 and the exponential in §32.3 are the corrections. Exercise G.e is built for this misconception.

"The shortage was about making the drug." Nearly universal, and it is the belief that makes the gray-market inference feel reasonable. §32.7 is the correction, and it is worth spending real time on because it is the point at which a plausible-sounding argument (it's just a peptide, anyone can make it) is shown to be answering the wrong question.

"So the pharmacy price is a ripoff" / "So the gray market is fine." Both overshoots of §32.9. The section is written to block both, and students will still take one. Use the F.g exercise.

"Recombinant means natural, synthetic means artificial." Occasionally appears. Both produce the same molecule; the distinction is a process one. Chapter 1's "peptides are natural, so they're safe" Hype Check is the reference.

Sequencing across a term

• If teaching Part VI in order (32 → 33 → 34): this chapter sets up both successors explicitly. Preserve the handoffs; they are load-bearing.
• If your students are non-chemists: teach §32.3, §32.4, §32.7, §32.9 and assign the rest as reading. Those four are self-contained and carry the transfer.
• If you have chemists: §32.2 and §32.6 will be partly review, and the interesting work is in §32.4's purity/content distinction, which chemistry curricula frequently skip.
• Pairing with Chapter 19: this chapter is the mechanistic explanation of Ch 19's findings. Teaching them adjacently is strongly recommended; Exercise G.b assumes it.

On the no-protocol constraint

Students, especially chemistry students, will ask for more procedural detail. The chapter is written to explain the logic without becoming followable, and this is a deliberate design constraint of the whole book, not an oversight.

The line to hold, and it is defensible on pedagogical grounds rather than only on safety grounds: understanding why capping exists is the transferable skill; knowing which reagent to cap with is not, unless you are in a laboratory with supervision, in which case you have a protocol book. Tier 3 of further-reading.md names one and says exactly this.

On the case studies

Case Study 32.1 (insulin) is a history case and reads easily. Its real function is the expression/folding distinction, which transfers to any biologic. Question 6 is the one that connects it to the rest of the book — do not skip it.

Case Study 32.2 (Compound P) is a worked arithmetic case and is where the chapter's abstraction becomes concrete. It is labeled [constructed teaching example] throughout, and you should say aloud why: the arithmetic is universal, so a constructed case lets the class follow every chain without arguing about whose product it is. Some students read "constructed" as "made up therefore unimportant." Head that off.

Note the deliberate surprise in Exercise B.f: at 99.5% per step, the fragment-condensation route loses. Students expect the chapter's own technique to win. The lesson is that fragment condensation exists for a specific regime, and that a technique's existence is not an argument for using it.

Assessment guidance

• The quiz is recall-plus-arithmetic and should take 25–30 minutes. Questions 7–10 are the load bearing ones; a student who gets 7–10 right and everything else wrong has understood the chapter, and the reverse is not true.
• The exercises are not all assignable. A reasonable problem set is Set B entire, Set C a–d, Set D c and e, Set F a, e, f. The † items work well as discussion prompts or as a single extended written response.
• Best single essay prompt: Exercise F.g. It forces students to hold two claims simultaneously that most public discussion treats as opposites, and the good answers locate the genuine disagreement, which is narrow.

Timing

A 3-session plan for 75-minute sessions:

If you have only one session: §32.3 for 30 minutes, §32.7 for 15, §32.9 for 20, discussion for 10.