Melzer's Reagent
Encyclopedia
Melzer's reagent is a chemical reagent used by mycologists to assist with the identification of fungi.
of chloral hydrate
, potassium iodide
, and iodine
. Depending on the formulation, it consists of approximately 2.50-3.75% potassium iodide and 0.75–1.25% iodine, with the remainer of the solution being 50% water and 50% chloral hydrate. Melzer's is toxic to humans if ingested due to the presence of iodine and chloral hydrate.
Melzer's reagent is part of a class of iodine/potassium iodide (IKI)-containing reagents used in biology; Lugol's iodine
is another such formula.
preparation, and looking for any of three color reactions:
Among the amyloid reaction, two types can be distinguished:
Melzer's reactions are typically almost immediate, though in some cases the reaction may take up to 20 minutes to develop.
The function of the chemicals that make up Melzer's reagent are several. The chloral hydrate is a clearing agent, bleaching and improving the transparency of various dark-colored microscopic materials. The potassium iodide is used to improve the solubility
of the iodine, which is otherwise only semi-soluble in water. Iodine is thought to be the main active staining agent in Melzer's; it is thought to react with starch-like polysaccharides
in the cell walls of amyloid material, however, its mechanism of action is not entirely understood. It has been observed that hemiamyloid material reacts differently when exposed to Melzer's then it does when exposed to other IKI solutions such as Lugol's, and that in some cases an amyloid reaction is shown in material that had prior exposure to KOH, but an inamyloid reaction without such pretreatment.
An experiment in which spores from 35 species of basidiomycetes were tested for reactions to both Melzer's and Lugol's showed that spores in a large percentage of the species tested display very different reactions between the two reagents. These varied from being weakly or non-reactive in Lugols, to giving iodine-positive reactions in Lugol's but not in Melzer's, to even giving dextrinoid reactions in Lugol's while giving amyloid reactions in Melzer's.
Melzer's degrades into a cloudy precipitate when combined with alkaline solutions, hence it cannot be used in combination or in direct series with such common mycological reagents such as potassium hydroxide or ammonium hydroxide
solutions. When potassium hydroxide is used as a pretreatment, the alkalinity must be first neutralized before adding Melzer's.
Melzer's reagent was first described in 1924 and takes its name from its inventor, the mycologist Václav Melzer, who modified an older chloral hydrate-containing IKI solution developed by botanist Arthur Meyer
. Melzer was a specialist in Russula
, a genus in which the amyloidy on the spore
ornamentation or entire spore is of great taxonomic significance.
Composition
Melzer's reagent is an aqueous solutionAqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
of chloral hydrate
Chloral hydrate
Chloral hydrate is a sedative and hypnotic drug as well as a chemical reagent and precursor. The name chloral hydrate indicates that it is formed from chloral by the addition of one molecule of water. Its chemical formula is C2H3Cl3O2....
, potassium iodide
Potassium iodide
Potassium iodide is an inorganic compound with the chemical formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with...
, and iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
. Depending on the formulation, it consists of approximately 2.50-3.75% potassium iodide and 0.75–1.25% iodine, with the remainer of the solution being 50% water and 50% chloral hydrate. Melzer's is toxic to humans if ingested due to the presence of iodine and chloral hydrate.
Melzer's reagent is part of a class of iodine/potassium iodide (IKI)-containing reagents used in biology; Lugol's iodine
Lugol's iodine
Lugol's iodine, also known as Lugol's solution, first made in 1829, is a solution of elemental iodine and potassium iodide in water, named after the French physician J.G.A. Lugol. Lugol's iodine solution is often used as an antiseptic and disinfectant, for emergency disinfection of drinking water,...
is another such formula.
Reactions
Melzer's is used by exposing fungal tissue or cells to the reagent, typically in a microscope slideMicroscope slide
A microscope slide is a thin flat piece of glass, typically 75 by 25 mm and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is placed or secured on the slide, and then both are inserted together in the microscope for viewing...
preparation, and looking for any of three color reactions:
- AmyloidAmyloid (mycology)In mycology the term amyloid refers to a crude chemical test using iodine in either Melzer's reagent or Lugol's solution, to produce a black to blue-black positive reaction. It is called amyloid because starch gives a similar reaction, and that reaction for starch is also called an amyloid reaction...
or Melzer's-positive reaction, in which the material reacts blue to black. - Pseudoamyloid or dextrinoid reaction, in which the material reacts brown to reddish-brown.
- Inamyloid or Melzer's-negative, in which the tissues do not change color, or react faintly yellow-brown.
Among the amyloid reaction, two types can be distinguished:
- Euamyloid reaction, in which the material turns blue without potassium hydroxidePotassium hydroxidePotassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
(KOH)-pretreatment. - Hemiamyloid reaction, in which the material turns red in Lugol's solution, but shows no reaction in Melzer's reagent; when KOH-pretreated it turns blue in both reagents.
Melzer's reactions are typically almost immediate, though in some cases the reaction may take up to 20 minutes to develop.
The function of the chemicals that make up Melzer's reagent are several. The chloral hydrate is a clearing agent, bleaching and improving the transparency of various dark-colored microscopic materials. The potassium iodide is used to improve the solubility
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
of the iodine, which is otherwise only semi-soluble in water. Iodine is thought to be the main active staining agent in Melzer's; it is thought to react with starch-like polysaccharides
Iodine test
The Iodine test is used to test for the presence of starch. Iodine solution — iodine dissolved in an aqueous solution of potassium iodide — reacts with the starch producing a purple black color. The colour can be detected visually with concentrations of iodine as low as 0.00002M at 20°C...
in the cell walls of amyloid material, however, its mechanism of action is not entirely understood. It has been observed that hemiamyloid material reacts differently when exposed to Melzer's then it does when exposed to other IKI solutions such as Lugol's, and that in some cases an amyloid reaction is shown in material that had prior exposure to KOH, but an inamyloid reaction without such pretreatment.
An experiment in which spores from 35 species of basidiomycetes were tested for reactions to both Melzer's and Lugol's showed that spores in a large percentage of the species tested display very different reactions between the two reagents. These varied from being weakly or non-reactive in Lugols, to giving iodine-positive reactions in Lugol's but not in Melzer's, to even giving dextrinoid reactions in Lugol's while giving amyloid reactions in Melzer's.
Melzer's degrades into a cloudy precipitate when combined with alkaline solutions, hence it cannot be used in combination or in direct series with such common mycological reagents such as potassium hydroxide or ammonium hydroxide
Ammonium hydroxide
Ammonia solution, also known as ammonium hydroxide, ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or simply ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3...
solutions. When potassium hydroxide is used as a pretreatment, the alkalinity must be first neutralized before adding Melzer's.
History
The use of iodine-containing solutions as an aid to describing and identifying fungi dates back to the mid-19th century.Melzer's reagent was first described in 1924 and takes its name from its inventor, the mycologist Václav Melzer, who modified an older chloral hydrate-containing IKI solution developed by botanist Arthur Meyer
Arthur Meyer (botanist)
Arthur Meyer was a German botanist, cell biologist, and pharmacognosist. Meyer is known for his pioneering work describing the structure of chloroplasts and other plastids. He was the first to name and describe the chlorophyll-containing structures in chloroplasts known as grana.Meyer spent his...
. Melzer was a specialist in Russula
Russula
Around 750 worldwide species of mycorrhizal mushrooms compose the genus Russula. They are typically common, fairly large, and brightly colored - making them one of the most recognizable genera among mycologists and mushroom collectors...
, a genus in which the amyloidy on the spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
ornamentation or entire spore is of great taxonomic significance.
Further reading
- Blackwell M, et al. 2001. The presence of glycine betaine and the dextinoid reaction in basidiomata. Harvard Papers in Botany 6:35–41.
- Rossman AY. 1980. The iodine reaction: Melzer's vs. IKI. MSA newsletter 31:22.