Amino-Acid Sequence Notation: Reading Peptide Codes

A peptide sequence is a compact code for the order of amino-acid residues in a chain, and like any code it has conventions that have to be read correctly. Getting the notation right is the difference between confirming an identity and misreading it. This note covers the two residue alphabets, the direction a sequence is read, worked examples from documented compounds, and the important limits of what a bare sequence string can tell you.

Two ways to write the same residue

Each of the twenty standard amino acids has both a three-letter code and a single-letter code. The three-letter form is more readable and harder to mistake; the single-letter form is compact and standard in databases and alignments. They carry identical information, so a sequence can be written either way and mean the same thing. A partial reference set of the codes appears below.

Amino acid	Three-letter	One-letter
Glycine	Gly	G
Alanine	Ala	A
Glutamic acid	Glu	E
Aspartic acid	Asp	D
Proline	Pro	P
Lysine	Lys	K
Leucine	Leu	L
Valine	Val	V

One reliable source of confusion is worth flagging: the single-letter code for glutamic acid is E and for aspartic acid is D, which do not match the first letters of their names. Because several codes are non-intuitive in this way, a residue-by-residue check against a code table is safer than reading from memory when an identity confirmation is at stake.

Direction matters

By convention a peptide sequence is written and read from the N-terminus on the left to the C-terminus on the right, following the direction in which the chain is synthesised and described. Reading a sequence in the wrong direction produces a different molecule on paper, so the convention is not cosmetic. When a sequence is quoted without an explicit terminus label, the standard assumption is N-to-C left to right, but a careful record states it rather than relying on the reader to assume.

Worked examples from documented compounds

Two catalogue materials with fully documented sequences make the notation concrete. Epithalon is a tetrapeptide documented as Ala-Glu-Asp-Gly, which in single-letter form is AEDG. Four residues, read N to C, fully specify its primary structure. BPC-157 is a fifteen-residue peptide documented as Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val, which in single-letter form is GEPPPGKPADDAGLV. Counting the residues in either notation returns fifteen, consistent with its description as a pentadecapeptide, and the elemental composition documented for it, C62H98N16O22, is a separate line of identity that a sequence alone does not provide. The two examples also show the practical value of the one-letter form: a fifteen-residue string is far quicker to compare across two documents when written as GEPPPGKPADDAGLV than as the full hyphenated list.

What a sequence does not tell you

A residue sequence specifies the order of the standard amino acids and nothing beyond that, which means several identity-relevant features live outside the string. Terminal modifications such as acetylation or amidation are not visible in a plain sequence unless annotated. The counterion of a salt form, for example an acetate, is a separate species and does not appear in the residue list. Disulfide bonds, cyclisation, and non-standard or modified residues all require notation beyond the twenty-letter alphabet. And the sequence says nothing about purity, which is a lot-specific measurement rather than a structural property.

The practical consequence is that a matching sequence is necessary but not sufficient for a full identity. It confirms the backbone order; it does not confirm the modifications, the salt form, or the quality of a particular lot. Those come from the other identity fields and from a certificate, a point developed in the note on reading a peptide identity.

Notation for anything beyond the standard set

When a peptide contains features outside the standard twenty residues, the notation has to be extended, and knowing that these extensions exist is part of reading a sequence correctly. A residue written in lower case or with a prefix may signal a stereochemical variant such as a D-amino acid rather than the usual L form. Modifications are commonly shown with explicit tags, for instance an acetyl group noted at the N-terminus or an amide at the C-terminus, and a modified or unusual residue may be spelled out in a longer code rather than compressed to a single letter. A cyclic peptide needs an indication of the ring closure, and a disulfide-bonded pair needs its linkage shown. None of these fit the plain one-letter alphabet, so when a document presents a bare string for a compound that is known to carry such features, that is a prompt to look for the fuller annotation rather than to assume the string is complete. The safe reading habit is to treat an unannotated sequence as describing a standard, linear, unmodified backbone and to require explicit notation for anything more.

Reading notation in practice

A dependable habit is to convert whatever notation a document uses into the same form you check against, count the residues, and confirm the count matches the stated class, for example four residues for a tetrapeptide or fifteen for a pentadecapeptide. Then confirm the terminus convention and note any annotated modifications separately. Applied consistently, this turns a sequence from a string that looks right into one that has actually been verified. Product-level records for Epithalon and BPC-157 list the documented sequences, and the wider chemistry sits in the sequence science archive and the full Sequence Notes collection.

Common questions

What is the difference between three-letter and one-letter amino acid codes?

They carry identical information. The three-letter code, such as Gly, is more readable; the one-letter code, such as G, is compact and standard in databases. A sequence can be written in either form and mean the same thing.

Which direction is a peptide sequence read?

From the N-terminus on the left to the C-terminus on the right, the direction in which the chain is described and synthesised. Reading it backwards describes a different molecule, so the convention matters for identity.

Does a sequence string capture everything about a peptide?

No. It specifies the order of standard residues only. Terminal modifications, salt counterions, disulfide bonds, and purity are not shown in a plain sequence and must be confirmed from other identity fields and a certificate.

References

Lyophilization and Reconstitution Chemistry

Lyophilization, or freeze-drying, is the process that turns a peptide solution into the dry solid a laboratory receives, and reconstitution is the reverse step that returns it to solution. Understanding the chemistry of both explains why the lyophilized form is used and what the physical cake in a vial actually represents. This note describes the process in general terms and deliberately carries no preparation amounts, concentrations, schedules, or routes, which are protocol matters outside a chemistry overview.

Why remove the water at all

Water is a participant in many of the reactions that degrade peptides, from hydrolysis of the backbone to the mobility that lets molecules aggregate. A peptide held in solution is therefore exposed continuously to the conditions that shorten its usable life. Removing almost all of the water leaves a solid in which those water-dependent processes are dramatically slowed, which is why reference peptides are supplied freeze-dried rather than as ready solutions. The dry state is a stability strategy, not merely a shipping convenience.

The three stages of freeze-drying

Lyophilization is a staged process, and each stage does a distinct job.

Stage	What happens	Why it matters
Freezing	The solution is frozen solid, separating ice from the dissolved solids	Sets the structure of the eventual cake and the size of ice crystals
Primary drying	Ice is removed by sublimation under low pressure, passing from solid to vapour	Removes the bulk of the water without melting the material
Secondary drying	Residual bound water is driven off at slightly higher temperature	Lowers final moisture to the level that gives long-term stability

The key idea in primary drying is sublimation: under sufficiently low pressure, ice converts directly to vapour without first becoming liquid. That is what allows the water to leave while the peptide stays in a solid framework rather than being concentrated in a shrinking droplet. Secondary drying then removes the more tightly held water that sublimation leaves behind.

Reading the cake

The dry solid left in the vial is called the cake, and its appearance carries information. A well-formed cake that holds its shape and colour is the expected result of a controlled cycle. A collapsed, shrunken, or unevenly melted-looking cake can indicate that the cycle deviated, for example that the material warmed too far during drying. Because the cake is the first thing a receiving laboratory sees, noting its appearance is a simple and useful check, and it connects directly to the receiving observations discussed in the cold-chain handling note.

Reconstitution in general terms

Reconstitution reintroduces a solvent to return the solid to solution. Chemically, the point to understand is that this reverses the protection the dry state provided: once water is present again, the degradation pathways that were suppressed become available, so reconstituted material is generally treated as less stable than the sealed cake. The choice of solvent and the behaviour of a given peptide in it are governed by the peptide’s properties, and a material that dissolves cleanly is behaving as expected while cloudiness or incomplete dissolution is a prompt to stop and check the identity and condition of the lot.

This overview stops short of quantities on purpose. How much solvent, at what concentration, and by what handling steps a solution is prepared and used are protocol questions, and they vary with the experiment rather than following from the chemistry alone. What the chemistry does establish is the direction of the trade: the dry form is the stable one, and every step back toward solution trades some of that stability for usability.

What sets the boundaries of a cycle

Two physical properties govern why a freeze-drying cycle is run the way it is, and both are worth understanding even at a descriptive level. The first is the collapse temperature of the frozen material, the point above which the porous structure formed during freezing can no longer support itself and begins to slump. Primary drying is generally kept below this point, because a collapsed cake dries unevenly and can retain more residual water than intended. The second is the glass-transition behaviour of the dried solid, which describes the temperature range over which an amorphous material shifts between a rigid, glassy state and a softer, more mobile one. A dried peptide held well within its glassy range has very little molecular mobility, and low mobility is one of the reasons the lyophilized form resists the rearrangements that lead to aggregation.

These properties also explain why the same material can be presented as a clean cake in one vial and a less well-formed solid in another without either being outside specification: cake morphology reflects the freezing pattern and the thermal history of the cycle, not only the peptide itself. For a receiving laboratory the practical takeaway is modest but real. Appearance is a first-pass signal rather than a verdict, and identity and purity are settled by the analytical record on the certificate of analysis rather than by the look of the solid. Reading that record is the subject of the lab-standards notes, and the chemistry here simply sets up why the dry cake is the state those documents describe.

Putting the two halves together

Freeze-drying and reconstitution are two directions of the same water-management problem. Removing water stabilises the material for storage and transit; adding it back makes the material usable but restarts the clock on degradation. Seen this way, the lyophilized cake is best understood as a paused state, and the handling decisions around it, cold and dry storage, careful receiving inspection, and awareness that solutions are less stable, all follow from that single idea. The broader stability picture is developed in the note on peptide degradation pathways, and related chemistry sits in the sequence science archive. The reasoning behind research-use-only handling is set out in the FAQ, and the wider technical collection is in Sequence Notes.

Common questions

Why are peptides freeze-dried instead of supplied as solutions?

Water participates in many peptide degradation reactions, so a solution is continuously exposed to those processes. Removing almost all water leaves a solid in which they are greatly slowed, making the lyophilized form more stable for storage and transit.

What is sublimation in freeze-drying?

Under low pressure, ice converts directly from solid to vapour without becoming liquid first. This is the primary-drying step, and it removes most of the water while the peptide remains in a solid framework rather than a shrinking droplet.

Is reconstituted peptide as stable as the dry cake?

Generally no. Adding solvent reintroduces water and restores the degradation pathways the dry state suppressed, so solutions are typically treated as less stable. Specific handling, including quantities, is a protocol matter outside this chemistry overview.

References

Peptide Stability: Degradation Pathways and Storage

Peptide stability is not a single property but the net result of several distinct chemical degradation pathways, each with its own drivers. Knowing the routes explains why standard storage practice looks the way it does, because nearly every handling rule is a way of slowing one or more of them. This note maps the main pathways to their conditions and to the storage choices that follow, at a general level and without preparation amounts.

The main degradation routes

The pathways below are the ones most commonly discussed for peptides. Which ones matter for a given sequence depends on its residues, but the general chemistry is well characterised.

Pathway	What occurs	Main drivers
Hydrolysis	Backbone or side-chain bonds are cleaved by water	Moisture, temperature, extreme pH
Oxidation	Susceptible residues react with oxygen or oxidants	Air exposure, light, some metal ions
Deamidation	Certain residues convert, altering the sequence chemistry	Moisture, temperature, pH
Aggregation	Molecules associate into larger assemblies	Concentration, temperature, freeze-thaw stress

Two themes run through the table. Water appears as a driver of several routes, which is the chemical reason the lyophilized form is more stable than a solution. Temperature accelerates essentially all of them, which is why cold storage is the default. The other drivers, light, air, pH, and mechanical stress, are more selective, mattering most for particular residues or particular handling steps.

Why residues determine susceptibility

Degradation is sequence-specific because the reactive groups live on particular side chains. A peptide rich in residues prone to oxidation will be more light and air sensitive; one with the residue patterns associated with deamidation will be more moisture and temperature sensitive. This is why a general stability statement is only a starting point and a material’s own documentation and behaviour govern in practice. It is also why storage notes for a specific compound, such as a verified reference peptide recorded for cold, dark storage, are worth following as written rather than generalising loosely.

How storage follows from the chemistry

Once the pathways are clear, standard storage practice reads as a direct response to them. Keeping material cold slows every temperature-driven route at once. Keeping it dry, as a sealed lyophilized solid, removes the water that hydrolysis and deamidation depend on. Protecting it from light limits photo-driven oxidation of sensitive residues. Minimising air exposure reduces oxidation and moisture uptake. Avoiding repeated freeze-thaw cycles limits the mechanical stress associated with aggregation. Each rule targets a specific mechanism, which is why they are applied together rather than treated as interchangeable.

Looking at each pathway more closely

The summary table names the routes, but a little more detail clarifies why the storage rules take the shape they do. Hydrolysis is the cleavage of a bond by water, and for peptides the vulnerable points include the backbone amide bonds themselves and certain side-chain linkages. Because it consumes water as a reactant, hydrolysis is strongly suppressed in a properly dried solid and becomes available again the moment material is returned to solution. This single fact is the reason the dry cake is treated as the stable reference state and reconstituted material as the less stable one.

Oxidation is the reaction of susceptible residues with oxygen or other oxidants, and it is often accelerated by light and by trace metal ions. The residues most associated with it carry sulfur-containing or aromatic side chains, so a sequence’s oxidation sensitivity tracks its composition. This is why dark storage and limited air exposure are standard rather than optional for sensitive materials. Deamidation is a rearrangement at particular residues that changes the local chemistry of the sequence, and like hydrolysis it is driven by moisture, temperature, and pH, which is why the same cold and dry conditions that slow hydrolysis also slow it.

Aggregation is different in character, because it is an association of intact molecules into larger assemblies rather than a change to the covalent structure. It is promoted by higher concentration, by temperature, and notably by the mechanical stress of repeated freezing and thawing. That last driver is the specific reason freeze-thaw cycling is discouraged: each cycle imposes stress that can nucleate association, and the effect accumulates. Because aggregation can occur without any bond being broken, it will not always be obvious from a simple identity check, which is part of why appearance and analytical records are both used when the condition of a lot is in question.

Why the routes are considered together

No single pathway acts in isolation, and the conditions that drive one frequently drive others. Temperature accelerates hydrolysis, oxidation, deamidation, and aggregation alike, so a single lapse in cold storage works against a material on several fronts at once. This coupling is the reason storage guidance is expressed as a small set of conditions held together, cold, dry, dark, and sealed, rather than as a menu from which one condition might substitute for another.

Stability as a tested property

Because stability depends on conditions and time, it is something that is studied under defined protocols rather than assumed. The internationally harmonised framework for stability testing, ICH Q1A(R2), sets out how materials are held under controlled temperature and humidity and evaluated over time to understand how quality changes. For research materials, the relevant takeaway is conceptual: a stability claim is only meaningful when attached to stated storage conditions and a timeframe, and a claim with neither is not a claim a laboratory can rely on. This connects to the documentation practices covered in the lab-standards notes, where recording the conditions a material was held under is what makes a later stability question answerable.

The practical synthesis is straightforward. Treat the dry, cold, dark, sealed state as the reference condition; understand each storage rule as slowing a named chemical route; and record the conditions a material actually experienced so that any later anomaly can be traced. Related chemistry sits in the sequence science archive, quality-control practice in the lab standards archive, and the wider collection in Sequence Notes.

Common questions

What are the main ways peptides degrade?

The most commonly discussed routes are hydrolysis, oxidation, deamidation, and aggregation. Water drives several of them and temperature accelerates essentially all, which is why cold, dry, sealed storage is the standard reference condition.

Why does peptide stability depend on the sequence?

Degradation reactions occur at specific side-chain groups, so susceptibility depends on which residues are present. A peptide rich in oxidation-prone residues is more air and light sensitive, while others are more sensitive to moisture and temperature.

What makes a stability claim meaningful?

A stability statement only means something when tied to stated storage conditions and a timeframe, as in the ICH Q1A(R2) framework. A claim with no conditions or timeframe attached is not something a laboratory can rely on.