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\b[These names refer to several unrelated chemical classes: <blood> and bile pigments, a photosynthetic pigment family, an amino sulfonic acid, a disputed historical faecal extract, and storage <polysaccharides> or <proteins>.] Similar colour or a historical name does not establish chemical identity.
\i[<Haematocrystallin>] denotes the crystallizable colouring substance of red <blood>, now identified with <hemoglobin>. It combines a globin <protein> with iron-containing <haem> groups. Within intact red <cells> it reversibly binds <oxygen>; oxygenated and deoxygenated forms have different absorption spectra. Crystals obtained from <blood> differ with species and preparation. This is not the same thing as a crystal formed from an old tissue haemorrhage. The word does not mean that all circulating <blood> normally consists of free pigment crystals.<Examples of Hemoproteins by the University of Utah>[Hemoglobin: globin, haem and oxygen carriage]{ref}
\i[<Haematoidin>] is the yellow-orange to reddish pigment characteristically encountered in old extravasated <blood>, sometimes as rhomboidal crystals or radiating clusters. It is associated with <bilirubin> and lacks the iron retained in <haemosiderin>. The historical inference that any reddish <blood>-derived crystal must be the same substance as <haematocrystallin> is therefore wrong. In a tissue preparation the local matrix and other constituents can complicate an exact chemical identification from appearance alone.<Hematoidin crystals in sputum smears: Cytopathology and clinical associations>[Results; Discussion: crystal morphology, bilirubin association and absence of iron]{ref}
\i[<Haematin>] is a dark iron-containing derivative of <haem>, obtained when the globin association and the iron's chemical environment are altered. In modern chemical usage, <hematin> is the ferric hydroxide form of the <haem> pigment; <hemin> is the corresponding chloride form. Historical preparations and terminology were less consistently separated. Neither should be equated with iron-free <bilirubin>, and a <haem>-derived crystal test is not the crystallization of intact <hemoglobin>.<Hematin in PubChem>[Identity and chemical description: hydroxyl ferric protoporphyrin]{ref}<Hemin in ChEBI>[CHEBI:50385, formula and chlorido(protoporphyrinato)iron(III) identity]{ref}
\i[<Chlorophyll>] is the green pigment system of photosynthetic organisms, present in organized pigment-<protein> assemblies in <chloroplasts> in green plants. Modern chemistry distinguishes several <chlorophylls>: <chlorophyll a>, for example, has a magnesium-containing tetrapyrrolic ring system and a hydrophobic side chain. It absorbs light used in photosynthetic <energy> conversion. Extraction into alcohol can reveal the green pigment, but a leaf extract is a mixture rather than a chemically pure universal <chlorophyll>. Its green colour must not be attributed to iron in the same way as <haem>.<Chlorophyll a in PubChem>[Molecular formula; description: magnesium-containing photosynthetic pigment]{ref}
\i[<Biliverdin>] is a green, open-chain tetrapyrrolic pigment. In modern <haem catabolism>, ring cleavage by <haem> oxygenase releases iron and forms <biliverdin>; reduction then produces <bilirubin>. The pigments contribute to changing colours of some resolving haemorrhages and to bile-pigment chemistry. This sequence concerns degradation of the <haem> group, not conversion of the entire globin <protein> directly into one green molecule.<Biochemistry, Biliverdin (StatPearls)>[Cellular Level: haem oxygenase and biliverdin reductase]{ref}
\i[<Taurin>], now <taurine>, is a colourless crystalline amino sulfonic acid with the structure $\mathrm{H_2NCH_2CH_2SO_3H}$. It contains sulfur and <nitrogen> and can form conjugates with bile acids, including <taurocholic acid>. It is not itself a green bile pigment, an ordinary carboxylic <amino acid>, or a standard <amino acid> directly inserted into <proteins> by translation. The chemical classification and formula distinguish it from neighbouring names in the list.<Taurine in ChEBI>[CHEBI:15891: identity as 2-aminoethanesulfonic acid and formula]{ref}
\i[<Excretin>] was the name <William Marcet> gave to a crystalline material extracted from human <faeces>. His contemporary account describes treatment with hot alcohol, removal of material with lime, extraction with ether, and formation of silky needle-like crystals; the material was insoluble in <water>, soluble in ether and hot alcohol, and melted in the low-to-mid 90s Celsius. His 1859 account reports <carbon>, hydrogen, <oxygen> and sulfur and gives an equivalent-weight formula based on an assumed sulfur content. These are historical analytical claims, not a modern authenticated molecular specification. Later work questioned the purity and chemical individuality of <excretin>, so that old name alone does not establish a definite modern structure.<Excretin in A Text-book of Physiological Chemistry (1914)>[p. 523: later doubts about identity and possible impure sterol material]{ref}<On Excretine by William Marcet>[Archives of Medicine, volume 1, 1859, pp. 100–102: preparation, melting behaviour and composition]{ref}
\i[<Glycogen>] is an animal <glucose>-storage <polysaccharide>, particularly important in the <liver> and <skeletal muscles>. Its highly branched chains contain alpha-(1→4) links with alpha-(1→6) branch points. Preparations form opalescent dispersions and commonly show a reddish-brown <iodine> reaction rather than the intense blue associated with long <amylose> chains. <Hydrolysis> yields <glucose>. <Liver> <glycogen> can support the regulation of circulating <glucose>, whereas <muscle> <glycogen> mainly supports local <muscle> <metabolism>; a modern account must distinguish those physiological destinations.<Biochemistry, Glycogen (StatPearls)>[Introduction; Cellular Level: glucose storage, branching and distinct liver/muscle roles]{ref}
\i[<Gluten>] is the cohesive <protein>-rich material left when wheat dough is washed to remove much of its <starch>. It is not a single small molecule: modern analysis distinguishes gliadin and glutenin families whose associations contribute extensibility and elasticity. It is poorly soluble in <water> as a dough network, contains <nitrogen>, and on full <hydrolysis> supplies <amino acids>. Its nutritive role is that of a plant <protein> mixture, not a <nitrogen>-free <carbohydrate>.<Changes in the rheology and aggregation behavior of wheat glutenin and gliadin mediated by low-sodium salt>[Introduction; experimental comparison of the rheology of the two protein fractions]{ref}
\i[<Legumin>] is a seed storage globulin associated particularly with peas and related legumes. A classical preparation extracts <protein> into suitable dilute salt solution and precipitates it under changed conditions. Modern <legumins> are 11S globulin assemblies, often hexamers formed from processed subunits. They are <nitrogen>-containing <proteins>, chemically distinct from <starch> despite occurring beside it in a seed. The older name vegetable casein indicates an analogy of precipitation or nutritive role; it does not establish identity with milk casein.<Pea Legumin Overexpressed in Wheat Endosperm Assembles into an Ordered Paracrystalline Matrix>[Abstract; results on precursor processing and 11S hexamer assembly]{ref}
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