L-Cysteine
Enable an AI agent to recognise L-cysteine, assess its identity and thiol availability in a particular material, and determine whether that material is suitable for a specified operation.
Research draft, second pass
A second pass drafted this model: the structure a model of this thing needs, and what is known about it in the world. The line under this one says how the second half was obtained - researched against sources, or recalled without web access, in which case nothing here was read anywhere and every claim is a lead to verify. Unreviewed either way.
Researched by: Codex + Grok
Purpose and description
Enable an AI agent to recognise L-cysteine, assess its identity and thiol availability in a particular material, and determine whether that material is suitable for a specified operation.
L-Cysteine is the proteinogenic, (R)-configured α-amino acid whose side chain is a thiol (-CH₂SH), making it the principal sulfur-containing residue used for disulfide bonding, metal coordination, and redox chemistry in peptides and proteins.
It can be Match a sample to the L-cysteine registry identity using structural and stereochemical evidence.; Calculate the required material quantity from verified L-cysteine content and chemical form.; Prepare or dilute a solution under a specified procedure and record its resulting conditions.; Assess remaining reduced L-cysteine and available thiol before a thiol-dependent operation.; Select release, retesting or rejection against an explicit application specification.; Record conversion to cystine or another product and link the resulting material to its own identity..
Distinguishing features
Confirm the free amino-acid identity against C3H7NO2S, CAS 52-90-4 and an appropriate structural reference; formula alone does not establish identity. [PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/cysteine)
Use stereochemistry-sensitive evidence to distinguish L-cysteine, whose systematic configuration is (2R), from D-cysteine and racemic material; an achiral identity assay cannot settle this distinction. [PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/cysteine)
Distinguish reduced L-cysteine from cystine by resolving the chemical species alongside thiol measurement; L-cysteine can oxidise to the disulfide cystine. [Sigma-Aldrich product information](https://www.sigmaaldrich.com/deepweb/assets/sigmaaldrich/product/documents/190/041/c7352pis.pdf)
Inspect the full chemical designation and composition to distinguish free L-cysteine from hydrochloride preparations, hydrated forms and covalently modified derivatives.
Establish whether the reported cysteine is present as free material or was released from a protein or other bound pool during sample preparation.
Scope
+ Identification of free L-cysteine and verification of its stereochemistry.
+ Amount, concentration and analytical purity of L-cysteine in a material.
+ Reduced thiol availability and conversion to cystine or other products.
+ Solid or solution state, preparation conditions and exposure history.
+ Evidence connecting a specific lot or preparation to an intended material use.
- Independent identities and full properties of D-cysteine, DL-cysteine, cystine and cysteine derivatives.
- Complete models of L-cysteine salts, hydrates and formulated mixtures.
- Cysteine residues within proteins, protein folding and sequence interpretation.
- Organism-level cysteine metabolism, nutritional requirements and clinical treatment.
- Complete manufacturing processes, reaction systems and finished-product approval.
Characteristics
- Chemical identity evidence
- Reference identity linked to sample-specific analytical evidence A supplier name or registry identifier does not establish the contents of a particular sample.
- Stereochemical composition
- L and D mole fractions or enantiomeric excess, with method and reporting basis Separates L-cysteine from its enantiomer and mixtures.
- Supplied chemical form
- Free amino acid; salt-associated preparation; hydrate-associated preparation; mixture; unresolved Determines whether the material is directly represented here or requires a related material model.
- L-cysteine content
- Mass fraction, mol/kg or mol/L, with as-received or dry basis and chemical form stated Supports quantity calculations without confusing gross material mass with L-cysteine amount.
- Available free thiol
- mol SH/kg or mol SH/L, with assay selectivity and sampling time Measures a functional property that may differ from nominal L-cysteine content.
- Cystine and other transformation products
- Species-specific mass or mole fractions, with detection limits Reveals conversion that a bulk assay or label may conceal.
- Physical preparation state
- Solid; fully dissolved; partially dissolved; precipitated; unresolved Determines whether sampling and dosing represent the whole preparation.
- Solution conditions
- pH, temperature in °C, L-cysteine concentration in mol/L and solvent composition Makes observations of solution behaviour and thiol reactivity interpretable.
- Exposure and preparation history
- Timestamped links to dissolution, air exposure, storage and treatment events Allows an agent to judge whether an earlier assay still describes the material.
- Application eligibility
- Unassessed; supported for specified use; requires retest; rejected for specified use Prevents a purity result or grade label from becoming unrestricted permission to use the material.
Where this came from
wikidata · CC0 1.0
Drafted structure
Bundle to layer to finding to question, as the second pass will find it: 6 bundles · 11 layers · 18 findings · 29 questions.
L-cysteine identity Establishes that the material contains the intended free amino acid with L stereochemistry.
Similar names, shared formulas and nonselective assays can conceal different chemical identities.
Molecular identity
Connects the registry concept to structural evidence for the sampled material.
Free cysteine assignment
Record evidence identifying free cysteine and distinguish a label claim from a measured assignment.
- Which reference identity and analytical observations support assigning this sample to free cysteine? definition
- Was cysteine already free in the sample, or did extraction, hydrolysis or reduction release it? boundary
Stereochemical identity
Determines whether the cysteine population satisfies the intended L-form requirement.
L-form verification
Record stereochemical evidence separately from total cysteine identity and content.
- Which measurement distinguishes L-cysteine from D-cysteine in this material? measurement
- What D-cysteine fraction or stereochemical uncertainty is acceptable for the intended use? boundary
Chemical form and quantity Establishes what supplied mass or solution volume represents in L-cysteine terms.
Free amino acid, associated components, water and impurities must not be counted interchangeably.
Supplied form
Separates the L-cysteine constituent from the identity of its containing material.
Form and component boundary
Record whether the material is free L-cysteine or a preparation requiring a related salt, hydrate or mixture identity.
- What complete chemical designation, counterions, water content and additional components describe the supplied material? definition
- Which related material model owns the preparation when it is not free L-cysteine alone? boundary
Content basis
Makes L-cysteine amount and purity claims usable in calculations.
Quantified L-cysteine
Record selective content measurements and their basis rather than treating all assay percentages as equivalent.
- Does the reported assay quantify L-cysteine specifically, total cysteine, thiol equivalents or another analytical response? measurement
- What water, purity and chemical-form corrections are needed to calculate the requested L-cysteine amount? action
Thiol and conversion state Tracks reduced cysteine availability and chemically distinct conversion products.
Nominal L-cysteine loading does not establish how much reduced material remains available.
Available thiol
Assesses the thiol population relevant to a proposed operation.
Selective thiol assessment
Record free-thiol results with matrix interference and any sample treatment that changes the measured pool.
- How much free thiol was measured, and what other sample constituents could contribute to that result? measurement
- Was the sample chemically reduced before measurement, making the result different from its original free-thiol state? provenance
Conversion products
Separates remaining L-cysteine from cystine and other detected products.
Cysteine and cystine accounting
Record resolved species and maintain an explicit basis when reporting cysteine equivalents.
- What quantities of L-cysteine, cystine and other identified products are present at the sampling time? measurement
- Are results expressed as species amounts or cysteine equivalents, and is conversion between those bases explicit? definition
Preparation and state persistence Connects physical state and preparation history to the validity of current L-cysteine measurements.
A solid lot and its later solution cannot automatically share the same concentration, homogeneity or chemical-state assessment.
Solution and phase condition
Records the conditions under which L-cysteine is dissolved, sampled and used.
Representative dissolved content
Distinguish nominal loading from measured dissolved content and investigate visible solids without assigning their identity by appearance.
- At what solvent composition, pH and temperature was dissolved L-cysteine concentration established? measurement
- If solids are present, what evidence identifies them and establishes whether the sampled liquid represents the intended preparation? boundary
Condition history
Relates storage and processing events to possible changes in L-cysteine content and thiol state.
Assay validity after exposure
Record elapsed time and relevant exposures so continued suitability is supported by evidence.
- What air exposure, temperature history, pH changes and processing treatments occurred after the last assay? provenance
- What stability evidence or retesting rule establishes whether that assay still supports the planned operation? action
L-cysteine use eligibility Links measured material properties and documented origin to a specific intended use.
Suitability depends on the required function and material specification, not merely the presence of L-cysteine.
Origin and grade evidence
Captures lot-specific support for origin and application-grade claims.
Supported material claims
Record the evidence behind claims such as non-animal origin or suitability for a named application.
- Which lot-linked records support the declared production origin and grade? provenance
- Which origin, impurity or microbiological requirements does the intended application impose on this L-cysteine material? boundary
Operation decision
Turns identity, content and state evidence into a bounded decision about the next operation.
Release for defined function
Assess the material against requirements for its specified role, including whether reduced thiol is necessary.
- Does the operation require a defined L-cysteine amount, available reduced thiol, stereochemical purity or a combination of these? definition
- Do current results satisfy the applicable acceptance limits, or must the material be retested or rejected for this use? action
- Which material-specific handling information and process instructions govern the proposed preparation or transfer? action
Evidence and external alignment What the world already says about this thing, gathered so the model can be checked against it.
A model that cannot be lined up against existing standards, identifiers and practice cannot be adopted by anyone who already uses them.
Reported evidence
Findings from the breadth pass, kept separate from the structural claims.
Kinds and varieties
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Free-base L-cysteine (anhydrous crystalline amino acid)
- L-Cysteine hydrochloride (anhydrous)
- L-Cysteine hydrochloride monohydrate (the usual commercial food/pharma salt)
- Food-additive grade (INS/E 920 dough conditioner)
- Parenteral/pharmaceutical grade (injection and amino-acid infusion solutions)
- Animal-derived industrial L-cysteine (keratin hydrolysate from hair, feathers, or bristles)
- Fermentation-derived L-cysteine (microbial, often marketed as vegan/halal/kosher)
- N-Acetyl-L-cysteine and other N- or S-substituted cysteine derivatives (related commercial forms, not the free amino acid)
- Which of these kinds and varieties hold for the sense of L-Cysteine this model covers, and on what evidence? provenance
Identifiers and schemes
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- IUPAC name - (2R)-2-amino-3-sulfanylpropanoic acid - Preferred systematic name for the free L-amino acid.
- CAS Registry Number - 52-90-4 - Free-base L-cysteine; hydrochloride salts have distinct CAS numbers (e.g. 52-89-1 anhydrous HCl, 7048-04-6 HCl monohydrate).
- PubChem CID - 5862 - NCBI PubChem compound for L-cysteine.
- Wikidata - Q186474 - Wikidata item for L-cysteine (distinct from Q234283 racemic/unspecified cysteine).
- ChEBI - CHEBI:17561 - L-cysteine as the zwitterion/proteinogenic residue.
- InChIKey - XUJNEKJLAYXESH-REOHCLBHSA-N - Stereospecific key; the racemate key differs in the protonation/stereo block.
- EC / EINECS - 200-158-2 - EC inventory number for L-cysteine.
- UNII - K848JZ4886 - FDA Substance Registration System UNII for cysteine (check salt-specific UNIIs for hydrochlorides).
- Amino-acid residue codes - Cys / C - IUPAC/IUBMB three- and one-letter codes in protein sequences.
- Codex INS / EU E-number - INS 920 / E 920 - Food-additive identity for L-cysteine and its hydrochlorides as flour-treatment agents.
- HS / tariff (typical) - 2922.49 (amino-acids and their esters, other) - Common heading for bulk amino-acid trade; national 8-10 digit splits vary.
- Which of these identifiers and schemes hold for the sense of L-Cysteine this model covers, and on what evidence? provenance
Standards and regulation
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- JECFA specifications and Codex GSFA: L-cysteine and L-cysteine hydrochlorides as INS 920 flour-treatment agent (FAO/WHO).
- EU: E 920 authorised as a food additive (flour treatment) under Regulation (EC) No 1333/2008 and the Union list; specifications under Regulation (EU) No 231/2012.
- United States: cysteine and cysteine hydrochloride listed among food additives / dough conditioners (21 CFR, e.g. 184.1271 cysteine and 184.1272 L-cysteine hydrochloride as GRAS for intended uses).
- Pharmacopoeias: USP/NF, Ph. Eur., and JP monographs for Cysteine Hydrochloride (and related parenteral amino-acid solutions) set identity, assay, specific rotation, and impurity limits.
- Flavouring: JECFA/EFSA evaluations of L-cysteine as a flavouring substance and as a precursor in process flavours (Maillard/meat notes).
- Religious/dietary certification is not a chemical standard but commercially decisive: animal-keratin vs fermentation origin for kosher, halal, and vegan claims.
- Which of these standards and regulation hold for the sense of L-Cysteine this model covers, and on what evidence? provenance
Real-world use
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Dough conditioner in industrial baking: reduces disulfide bonds in gluten, relaxing dough and shortening mixing time (E/INS 920, typically as the hydrochloride).
- Precursor in process flavours and savoury Maillard chemistry (meat, roast, and onion notes) via Strecker and sulfur-volatile pathways.
- Component of crystalline amino-acid infusion and parenteral-nutrition solutions, and a starting material for N-acetyl-L-cysteine (mucolytic / antidote).
- Reducing agent and processing aid in cosmetics (cold-wave permanent waving and hair-straightening formulations) and in some biomedical cell-culture media.
- Industrial intermediate for cysteine derivatives, flavour chemicals, and some pharmaceutical APIs.
- Which of these real-world use hold for the sense of L-Cysteine this model covers, and on what evidence? provenance
Typical measurements
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Molecular mass (free acid) - 121.16 (exact mass ~121.0197) - g/mol
- Specific optical rotation [α]D (free acid, aqueous) - about +8 to +10 (pharmacopoeial limits are salt- and concentration-specific) - degrees
- Side-chain thiol pKa - 8.3-8.5 - pKa
- Isoelectric point (pI) - about 5.0-5.1 - pH
- Melting / decomposition point (free acid) - decomposes around 220-240 - °C
- Assay (food/pharma grade, as C3H7NO2S or as hydrochloride salt) - typically ≥98.0-101.0 on the anhydrous/dried basis - % w/w
- Use level as flour-treatment agent - tens of mg/kg flour in baking practice; legal maxima are jurisdiction- and food-category-specific - mg/kg
- Aqueous solubility (free acid, 25 °C) - highly soluble, on the order of 100+ - g/L
- Which of these typical measurements hold for the sense of L-Cysteine this model covers, and on what evidence? provenance
Failure modes and hazards
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Air oxidation of the thiol to L-cystine (insoluble disulfide), especially in solution and on storage of damp product - assay drop and haze/precipitation.
- Racemisation and thermal decomposition on heating; sulfur stench (H2S, thiols) from overheating or microbial spoilage.
- Hygroscopicity of the hydrochloride salts: caking, water-content out of specification, and accelerated oxidation.
- Occupational hazard: respiratory and eye irritation from fine amino-acid dust; the free thiol has a characteristic unpleasant odour.
- Process-flavour overuse can generate high levels of sulfur volatiles and, in some Maillard systems, undesirable process contaminants - dose and process control matter.
- Parenteral use requires control of related substances (cystine, cysteic acid) and bacterial endotoxin; wrong salt or hydration form can fail identity/assay.
- Allergen/origin issue rather than intrinsic toxicity: undeclared animal-keratin origin conflicts with vegan, kosher, or halal specifications even when chemically identical.
- Which of these failure modes and hazards hold for the sense of L-Cysteine this model covers, and on what evidence? provenance
Regional variation
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Food-additive naming: E 920 in the EU/UK vs INS 920 in Codex-aligned markets; the United States labels the ingredient as cysteine / L-cysteine hydrochloride rather than an E-number.
- Permitted food categories and maximum levels differ: some jurisdictions restrict E 920 mainly to flours and bread, others have broader or narrower GSFA provisions.
- Sourcing practice: East Asian industrial production historically used human hair and now commonly duck feathers or pig bristles; European and multinational food brands increasingly specify fermentation-derived material for labelling and religious-diet reasons.
- Pharmacopoeial salt form: many markets trade the monohydrate hydrochloride as the default 'L-cysteine' commodity, so a purchase specification must name the salt and hydration.
- Which of these regional variation hold for the sense of L-Cysteine this model covers, and on what evidence? provenance
Neighbouring kinds and how to tell them apart
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- D-Cysteine / DL-cysteine - Opposite or mixed α-configuration; separated by specific optical rotation and by chiral HPLC or enzymatic L-amino-acid oxidase assays. Only the L-enantiomer is proteinogenic and the usual food/pharma article.
- L-Cystine (Cys-Cys disulfide) - Oxidised dimer, poorly soluble, no free thiol; distinguished by solubility, Ellman/thiol titration, and a different CAS (56-89-3) and residue code (not free Cys).
- L-Cysteine hydrochloride (anhydrous vs monohydrate) - Same amino acid, different counter-ion and water of crystallisation; water content (KF), chloride assay, and CAS/UNII identify the article. The monohydrate is the common bulk food grade.
- N-Acetyl-L-cysteine (NAC) - N-acetylated prodrug/mucolytic, not E 920; IR/NMR show the amide, and it has a distinct CAS (616-91-1) and pharmacopoeial monograph.
- L-Methionine - The other proteinogenic sulfur amino acid, with a thioether not a thiol; negative Ellman test, different residue code Met/M, and no dough-reducing action of the same kind.
- L-Homocysteine - One-carbon-longer thiol amino acid (clinical metabolite, not a coded protein residue); chromatographic retention and mass (135 vs 121) separate it from cysteine.
- Cysteamine (2-aminoethanethiol) - Decarboxylated analogue used as a drug (e.g. cystinosis); lacks the carboxylate, different CAS (60-23-1) and much more volatile/odorous.
- Which of these neighbouring kinds and how to tell them apart hold for the sense of L-Cysteine this model covers, and on what evidence? provenance
Sources
- L-Cysteine (Compound summary, CID 5862) - Chemical identity, formula, mass, CAS, InChIKey, pKa/physical properties, and biomedical/use notes.
- Cysteine - Proteinogenic status, thiol/disulfide chemistry, industrial production routes, food and process uses, and neighbour compounds.
- INS 920 L-Cysteine and its hydrochlorides - Codex GSFA / JECFA food-additive identity - Food-additive numbering, permitted food uses as a flour-treatment agent, and JECFA identity of the hydrochloride salts.
- Scientific Opinion on the safety of L-cysteine as a food additive / flavouring (EFSA FAF/CEF opinions) - European food-additive and flavouring assessment, specification expectations, and use as a process flavour precursor.
What the second pass must settle
- Does the registry or an existing world model already own L-cysteine, and how does it assign related salts, hydrates and solution preparations?
- Which analytical methods and acceptance limits adequately resolve L-cysteine, D-cysteine, cystine and interfering thiols in each intended matrix?
- What measured stability data establish retest intervals under the actual concentration, pH, temperature, oxygen exposure and packaging conditions?
- Which origin, impurity, microbiological and grade requirements apply to the intended applications, and what lot-specific evidence supports them?
- Which thiol-availability or conversion thresholds predict acceptable performance in the intended operation?