Amino Acid Side Chain Chart: Properties and a Practice Quiz

Phenylalanine and tyrosine both have an aromatic ring. Tyrosine also has an OH group, which gives it a polar part that phenylalanine lacks. That small difference explains why memorizing one label per amino acid can get confusing: a side chain can be aromatic, polar, and uncharged at the same time.

Use this chart to compare the 20 standard amino acids by structure, polarity, and charge. Then try the eight questions without looking back: each asks you to explain the chemical feature behind an answer.

Amino acid side chain chart

The side chain, or R group, is the part attached to the alpha carbon that distinguishes one amino acid from another. The chart uses common introductory biochemistry categories for side chains in water around pH 7. Its charge column gives the predominant side-chain charge; the whole amino acid or protein can have a different net charge. Protein surroundings can also change how readily a side chain gains or loses a proton.

The first dash in each condensed formula marks attachment to the alpha carbon. Ring names are recognition cues: use the linked IMGT structural reference for complete drawings. The chart omits stereochemistry.

Amino acid	Codes	R group or structural cue	Category	Charge at pH 7
Glycine	Gly, G	–H	Nonpolar; special case	0
Alanine	Ala, A	–CH₃	Nonpolar	0
Valine	Val, V	–CH(CH₃)₂	Nonpolar	0
Leucine	Leu, L	–CH₂–CH(CH₃)₂	Nonpolar	0
Isoleucine	Ile, I	–CH(CH₃)–CH₂–CH₃	Nonpolar	0
Proline	Pro, P	–(CH₂)₃– returns to backbone N	Nonpolar; special case	0
Methionine	Met, M	–CH₂–CH₂–S–CH₃; thioether	Nonpolar	0
Phenylalanine	Phe, F	–CH₂–phenyl (benzene ring)	Nonpolar; aromatic	0
Tryptophan	Trp, W	–CH₂–indole (fused rings, one NH)	Mostly nonpolar; aromatic	0
Serine	Ser, S	–CH₂–OH	Polar, uncharged	0
Threonine	Thr, T	–CH(OH)–CH₃	Polar, uncharged	0
Cysteine	Cys, C	–CH₂–SH; thiol	Polar, uncharged	Mostly 0
Tyrosine	Tyr, Y	–CH₂–phenyl–OH (opposite positions)	Polar, uncharged; aromatic	Mostly 0
Asparagine	Asn, N	–CH₂–C(=O)–NH₂; amide	Polar, uncharged	0
Glutamine	Gln, Q	–CH₂–CH₂–C(=O)–NH₂; amide	Polar, uncharged	0
Aspartate (aspartic acid)	Asp, D	–CH₂–COO⁻; carboxylate	Acidic; polar	−1
Glutamate (glutamic acid)	Glu, E	–CH₂–CH₂–COO⁻; carboxylate	Acidic; polar	−1
Lysine	Lys, K	–(CH₂)₄–NH₃⁺	Basic; polar	+1
Arginine	Arg, R	–(CH₂)₃–NH–C(=NH₂⁺)–NH₂; guanidinium	Basic; polar	+1
Histidine	His, H	–CH₂–imidazole (five-membered, two N)	Basic; polar; aromatic	Mostly 0; some +1

The broad categories follow Purdue's amino acid reference and the University of Wisconsin's polarity groups. The IMGT structural reference above supplies the structures and codes. Histidine's charge needs the extra qualification explained below.

For the ring cues, tyrosine's OH sits opposite the CH₂ attachment on its benzene ring. In histidine and tryptophan, the CH₂ attaches to a ring carbon, not a nitrogen. Arginine's formula shows one resonance form; its positive charge is spread across the guanidinium group.

Keep polarity, charge, and ring structure separate

Polar groups have an uneven charge distribution that helps them interact with water. Charged groups carry a net electrical charge. Serine's OH group is polar even though its side chain has no net charge around pH 7. Aspartate's carboxylate is both polar and negatively charged.

Aromatic describes a ring's electronic structure. The familiar aromatic trio is phenylalanine, tyrosine, and tryptophan. It doesn't follow that all three have identical polarity. Histidine's imidazole ring is aromatic too, although teaching charts usually place histidine with the basic amino acids.

A nonpolar classification doesn't mean that every bond in the side chain is nonpolar. Tryptophan's indole NH can donate a hydrogen bond, but its large ring system supports its usual placement in the nonpolar group. IMGT's classification reference explicitly discusses this exception.

The backbone is another separate issue. A free amino acid can carry a positive alpha-amino group and a negative alpha-carboxyl group while having an uncharged R group. When those opposite charges give the molecule no net charge, it is a zwitterion. For a peptide's total charge, consider its free N- and C-termini as well as its side chains, at the stated pH.

Why some charts disagree

Cysteine is polar and uncharged in many introductory charts, including this one. IMGT places it in its nonpolar class. Tyrosine may sit under “aromatic,” “polar,” or a broader hydrophobic grouping, depending on what a chart is trying to describe. For example, the University of Arizona's tyrosine page emphasizes its hydrophobic character while describing its phenolic OH group.

These labels compress several properties into a few boxes. On an exam, follow the classification scheme specified by your course. When explaining a structure, name the functional group: cysteine has a thiol; tyrosine has a phenol. Those features stay the same across classification schemes.

Glycine also needs care. Its R group is a hydrogen, so this chart places it with nonpolar side chains. Its tiny size makes it different from a bulky hydrophobic residue such as leucine. IMGT classifies glycine's hydropathy as neutral, and the University of Arizona describes it as able to occur inside or outside a protein. “Nonpolar” isn't a promise about where every glycine will sit.

Histidine is basic without always being positive

Histidine's side-chain pKa is commonly approximated as 6.0, compared with about 10.5 for lysine and 12.5 for arginine in the National Cancer Institute's amino acid reference. The pKa marks the pH at which the protonated and unprotonated forms are equally abundant. Above that pH, the unprotonated form predominates.

Using pKa 6.0 at pH 7, the Henderson–Hasselbalch relation gives an unprotonated-to-protonated ratio of 10^(7 − 6) = 10:1. In that simplified calculation, about 9% of histidine side chains are positively charged and about 91% are neutral. That percentage follows from the assumed pKa; it isn't fixed for every histidine in a protein.

Answer “basic” when asked for histidine's acid–base category. For its predominant side-chain charge at a specified pH, use the supplied pKa. Cysteine and tyrosine can also lose a side-chain proton as pH rises, so the chart's “uncharged” labels apply around pH 7.

Learn the differences that change the answer

Several pairs are easier to remember together than as isolated names:

• Aspartate and asparagine: –COO⁻ versus –C(=O)–NH₂. The first is a negatively charged carboxylate; the second is an uncharged amide. Glutamate and glutamine repeat the same distinction with one extra CH₂.
• Leucine and isoleucine: both have four-carbon side chains. In isoleucine, branching starts at the side-chain carbon nearest the backbone. Leucine has a CH₂ before the branch.
• Cysteine and methionine: both contain sulfur. Cysteine has an SH group; methionine's sulfur lies between carbons. Two cysteine thiols can form a disulfide bond on oxidation, as described in OpenStax's protein chapter.
• Phenylalanine and tyrosine: adding the ring OH gives the tyrosine side chain a hydrogen-bonding group. The ring remains aromatic.

Amino acid classification quiz

Cover the chart and write an answer plus one chemical reason for each question. Use this article's classification scheme and pH 7 unless the question says otherwise.

1. A side chain is –CH₂–CH₂–C(=O)–NH₂. Name the amino acid, give its one-letter code, and state its side-chain charge.
2. Change the terminal amide in question 1 into a carboxylate. Which amino acid do you now have, and what changes about its charge?
3. A student groups phenylalanine and tyrosine as aromatic, then concludes that neither has a polar functional group. Which part of the answer needs correcting?
4. Identify –CH(CH₃)–CH₂–CH₃. What structural change would give you leucine's side chain instead?
5. Two residues form an S–S bridge through their side chains. Which amino acid supplies them, and why doesn't “contains sulfur” identify methionine as an equivalent answer?
6. Assume histidine has a side-chain pKa of 6.0. At pH 7, is its side chain predominantly neutral or positive? What happens to the positive fraction if the pH falls to 6?
7. A free alanine molecule has an NH₃⁺ group and a COO⁻ group. Does that make alanine's R group charged?
8. A five-membered ring connects a side chain back to the backbone nitrogen. Identify the amino acid. Would a ring alone be enough to call a side chain aromatic?

Answers and the mistake each one catches

1. Glutamine, Q, charge 0. Two CH₂ groups lead to an amide. Asparagine has only one CH₂. The NH₂ in this amide doesn't behave like lysine's terminal amino group.
2. Glutamate, E, charge −1. The terminal group is now –COO⁻. If you answered aspartate, check the number of CH₂ groups before the terminal group.
3. Both are aromatic, but tyrosine has a polar OH group. Aromaticity and polarity answer different questions. This chart therefore places tyrosine among polar, uncharged amino acids.
4. Isoleucine, I. Leucine is –CH₂–CH(CH₃)₂: move the branch one carbon farther from the backbone. Both remain nonpolar.
5. Cysteine, C. Oxidation joins two cysteine thiols into a disulfide, –CH₂–S–S–CH₂–. Methionine has a thioether, with sulfur bonded between carbons, rather than an SH group.
6. Predominantly neutral at pH 7. With the assumed pKa, the positive fraction is about 9%. At pH 6, pH equals pKa, giving equal protonated and unprotonated populations: about 50% positive. “Basic” doesn't mean “always +1.”
7. No. Alanine's R group is –CH₃, with charge 0. The charges in the question belong to the shared amino acid backbone groups.
8. Proline, P. Its saturated ring closes onto the backbone nitrogen. A ring can be nonaromatic; the presence of a ring alone doesn't establish aromaticity.

Turn the missed distinction into a flashcard

Match the card to the mistake. If you confused glutamine with glutamate, ask yourself to distinguish an amide from a carboxylate and explain their charges. Recalling Q's full name tests a different skill.

If you missed…	A useful card front	What the answer should include
Amide versus acid	At pH 7, compare the side-chain charges of Q and E.	Q: 0, amide. E: −1, carboxylate.
Branch position	Draw only the R groups of leucine and isoleucine.	Leu: CH₂ before the branch. Ile: branch at the first side-chain carbon.
Aromatic versus polar	Which group gives tyrosine a polar part that phenylalanine lacks?	The ring OH; both remain aromatic.
Basic versus positive	Histidine side-chain pKa = 6.0, pH = 7: which form predominates?	Neutral; unprotonated-to-protonated ratio 10:1.
Sulfur chemistry	Which side-chain feature allows cysteine to form a disulfide?	Its thiol, –SH.

Nibomo's 120-card amino acid set covers names, codes, structure recognition, and drawing for the 20 standard amino acids. Its diagrams show neutral connectivity, and the set excludes classification labels, pKa, and charge states. Use it for structural recall, then add your own cards for the distinctions above. The MCAT flashcard guide explains how to fit those cards around practice questions.

On your next review, redraw or explain the pair you confused before reopening the chart. Check both the name and the chemical difference.