What is the one-letter code for
Glycine?
The 20 standard amino acids: reference table
Use this chart alongside the quiz. The one-letter codes are what trip most students up, so the quiz drills those first.
| Name | 3-letter | 1-letter | Class | Charge |
|---|
*His is the one to watch: its imidazole side chain has a pKa of about 6.0, so at physiological pH 7.4 it's only around 10% protonated, reliably "basic" as a class, but not reliably charged like Lys and Arg. That gap is exactly why it's a favorite MCAT trick answer. See the amino acid titration curve tool for the full breakdown.
How to memorize the amino acids fast
Most students try to memorize all 20 at once and burn out. The trick is to group them by class first, nonpolar, polar, acidic, basic, then drill the one-letter codes separately, because those are nearly random and need pure repetition. This quiz is built around exactly that order. Run "Name → class" until it's automatic, then switch to "Name → 1-letter code."
More free tools: all biochem tools · peptide charge & pI calculator · enzyme kinetics simulator
How the 20 amino acids break down by class
Grouping first is faster than memorizing 20 flat facts. The 20 standard amino acids split into exactly 5 classes:
| Nonpolar | 7, Gly, Ala, Val, Leu, Ile, Pro, Met |
| Aromatic | 3, Phe, Trp, Tyr |
| Polar | 5, Ser, Thr, Cys, Asn, Gln |
| Basic | 3, Lys, Arg, His |
| Acidic | 2, Asp, Glu |
7 + 3 + 5 + 3 + 2 = 20. Nonpolar is the biggest group by far, which is why "is it nonpolar?" is the single most useful first question when you're unsure of an amino acid's class.
The 9 one-letter codes that do not match the first letter
Eleven codes are simply the first letter of the name. The other nine had to pick something else, because the obvious letter was already taken by another amino acid, and each one has a mnemonic that goes with it. They now have their own page with a drill for just the one-letter codes: amino acid one-letter code quiz.
FAQ
Why don't all codes match the first letter of the name?
Three names start with A, three start with G, only one of each group could keep the obvious letter. The rest got distinctive alternates, listed in the table above.
Is side chain charge always fixed, or does it depend on pH?
The charges here assume physiological pH (~7.4). Technically it depends on the side chain's pKa relative to the surrounding pH. See the peptide charge calculator to compute it at any pH.
Why memorize class before the one-letter codes?
Class sorts 20 amino acids into 5 groups with clear chemical logic, far easier to reason through than 20 arbitrary letters, which benefit more from pure repetition once classes are automatic.
Why "Aspartate/Glutamate" instead of "Aspartic acid/Glutamic acid"?
Same molecule, different naming convention. At physiological pH the acidic side chain is deprotonated (ionized), so biochemists typically use the "-ate" name for the ionized form, the same pattern as lactate vs. lactic acid.
Want the by-hand method for pI? See how to calculate pI without a calculator.
Related tools: Peptide charge & pI calculator · Amino acid titration curve · Amino acid catabolism explorer · Amino acid chart (all 20) · all biochem tools.