Quiz · peptide basics
Research quiz
Twelve questions on what peptides are, how they are written and how they are made. Each answer comes with an explanation and the guide that covers it.
Question 1 of 12
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1. What joins one amino acid to the next in a peptide?
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Answer A peptide bond: an amide link formed with the loss of one water molecule
A peptide bond forms when the carboxyl group of one amino acid reacts with the amino group of the next, releasing a molecule of water. The result is an amide link, and a chain of them is the backbone of every peptide and protein. Disulfide bridges and hydrogen bonds shape a chain once it exists; they are not what holds its residues in order.
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2. What mainly separates the words peptide and protein?
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Answer The length of the chain
Peptides and proteins are built from the same amino acids, joined by the same peptide bonds. The words differ by size: a short chain is called a peptide, and a long, folded one a protein. The line between them is a convention rather than a law of chemistry, and it is usually drawn at around fifty residues.
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3. A sequence is written from left to right. Which end of the chain comes first?
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Answer The N-terminus, with its free amino group
By convention a peptide sequence is written from the N-terminus, the end with a free amino group, to the C-terminus, the end with a free carboxyl group. The convention matters because a chain read backwards is a different molecule: the same residues in the opposite order do not have the same chemistry.
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4. How many amino acids does the standard genetic code specify?
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Answer 20
The standard genetic code specifies twenty amino acids, each with a three-letter and a one-letter abbreviation. Sixty-four is the number of three-base codons, several of which stand for the same amino acid. A synthetic peptide can also carry building blocks from outside the twenty, such as D-amino acids or Aib, because a chemist is not limited to what a ribosome can use.
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5. What does lyophilization remove from a peptide?
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Answer Water
Lyophilization, or freeze-drying, freezes a solution and then lowers the pressure so that the ice turns directly into vapour. What is left is a dry solid. Water is removed because most of the reactions that degrade a peptide need it. Counter-ions, and any impurities present before drying, stay in the dried material.
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6. What does Ac- at the start of a written sequence mean?
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Answer The N-terminus carries an acetyl group
Ac- is shorthand for an acetyl group attached to the amino group at the N-terminus, a modification called N-terminal acetylation. It removes the positive charge that end of the chain would otherwise carry and adds about 42 daltons to the mass. It has nothing to do with an acetate salt, in which acetate is a separate counter-ion and not part of the peptide.
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7. What does -NH2 at the end of a written sequence mean?
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Answer The C-terminus is an amide instead of a free acid
A sequence that ends in -NH2 has a C-terminal amide: the carboxyl group at the end of the chain has been converted to a carboxamide. Amidation removes the negative charge a free acid would carry at neutral pH, and it lowers the mass by about one dalton compared with the free acid.
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8. How does a D-amino acid differ from its L-form?
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Answer It is the mirror image: the same atoms with the opposite handedness
D- and L-amino acids are mirror images of each other, like a left and a right hand. They share a formula, a mass and a side chain, and differ only in the arrangement of groups around the alpha carbon. Ribosomes build proteins from L-amino acids. Chemists place D-residues in synthetic peptides because the enzymes that cut peptide chains are, for the most part, specific to the L-form.
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9. How are most research peptides manufactured?
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Answer By solid-phase synthesis: one residue at a time, on a chain anchored to resin beads
Most research peptides are made by solid-phase peptide synthesis. The first amino acid is anchored to an insoluble resin and the chain grows one protected residue at a time, with washing between the steps. At the end the chain is cleaved from the resin and purified. No coupling step is perfectly complete, and that is where most synthesis-related impurities come from.
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10. A finding is reported from an experiment in cultured cells. Which statement is correct?
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Answer It shows what happened in that cell system, under those conditions
Cell-culture work is the first rung of the evidence ladder. It can show that a compound binds a receptor, or changes what a cell does, under controlled conditions. It cannot show what happens in a whole organism, where absorption, distribution, metabolism and many interacting tissues come into play. Animal studies and controlled clinical trials are separate, higher levels of evidence, and a finding at one level does not carry over to the next by itself.
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11. What is a disulfide bridge?
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Answer A covalent bond between the sulfur atoms of two cysteine residues
A disulfide bridge is a covalent bond between the sulfur atoms of two cysteine side chains. It can link two parts of one chain, closing a loop, or join two separate chains. Oxytocin is an example: a nine-residue peptide whose two cysteines are joined in this way. Methionine also contains sulfur, but it cannot form the bridge.
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12. Which of these is a chemical route by which a peptide degrades?
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Answer Oxidation of a methionine side chain
Oxidation is one of the common chemical routes of peptide degradation: methionine, and to a lesser extent cysteine and tryptophan, react with oxygen. The other main routes are deamidation of asparagine and glutamine, hydrolysis of the backbone, and aggregation. Water, heat, light and oxygen speed them up, which is why peptides are kept dry, cold and in the dark.
How this works
A short check of what you know, built to teach as it goes.
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One question at a time
Choose an answer and you see at once whether it was right, with a paragraph on why. There is no timer and no trick wording.
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Settled science only
Every question has one textbook answer: bonds, units, methods. Nothing here is about what a compound does, and nothing is medical.
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A reading list at the end
The result lists the guides behind the questions you missed first. No sign-up is asked for and your answers are not stored.
Related guides
The reading behind this tool, written as plain laboratory science.
- What Are Peptides? Chains of amino acids, and how research peptides are made.
- Peptide vs Protein vs Amino Acid Three words for one family of molecules, by length.
- Peptide Nomenclature and Modifications One-letter codes, acetyl and amide caps, D-residues, Aib, rings and lipid chains: what a peptide's written name says about its structure.
- Solid-Phase Peptide Synthesis How a peptide is built one residue at a time on a resin, and where deletion sequences, epimers and other impurities come from.
- Evidence Levels in Peptide Research Cell work, animal work and clinical trials: what each can and cannot show, and how to read an abstract critically.
More research tools
This quiz covers research peptides as laboratory chemistry. It contains no dosing information and no guidance on use in people or animals.