How does Congo red work?
Congo red demonstrates the accumulation of amyloid. With light microscopy it stains a pink or light red colour and shows green birefringence with polarized light. With rhodamine or Texas fluorescence the deposits show enhanced red fluorescence.
How does Congo red bind to amyloid?
CR binds to amyloid fibrils formed by both hydrophobic peptides (such as Aβ peptide) and hydrophilic peptides (such as GNNQQNY) and induces green-yellow birefringence under polarized light. Two binding modes have been proposed. One mode is β-sheet specific in which the CR molecules are aligned along the fibril axes.
Why does Congo have a red stain?
Staining with Congo Red (CR) is a qualitative method used for the identification of amyloids in vitro and in tissue sections. However, the drawbacks and artefacts obtained when using this dye can be found both in vitro and in vivo.
How does Congo red stain amyloid?
Congo red stain applied to the tissue gives the amyloid protein a salmon-pink color and placed under polarized light the amyloid proteins have an apple-green birefringence. This apple-green birefringence is considered pathognomonic for amyloid fibril deposits.
What is the structure of Congo red?
C32H22N6Na2O6S2Congo red / Formula
Why does Congo red turn black?
This is because these bacteria do not ferment any other sugars that may be necessary to release certain metabolites, which combine with Congo red to impart a black colour to the colonies indicating slime production.
What type of stain is Congo red?
It is an azo dye. Congo red is water-soluble, yielding a red colloidal solution; its solubility is greater in organic solvents.
How is Congo red synthesized?
Congo Red is synthesised by coupling bis-diazotised benzidine with two molecules of napthionic acid. The blue dye so obtained is converted into its red disodium salt during its salting out with sodium chloride (Fig.
Is Congo red cationic or anionic?
Congo red (CR) is a benzidine-based anionic diazo dye that can cause allergic reactions and can be metabolized to benzidine, a carcinogenic product19.
Which chromophore is present in Congo red?
Congo red contains an azo (-N=N-) chromophore and an acidic auxochrome (-SO3H) associated with the benzene structure. CR is also called acidic diazo dye. The azo structure is more important because of its high ability to impart colour to the compound .
Is Congo Red acidic or basic?
Congo red is an acid-base indicator dye.
Why Congo red is used as acid-base indicator?
It was used as a direct textile dye for Cotton. Congo red is now used as an acid-base indicator. It is prepared as a 0.1% aqueous solution that is red above pH 5 and blue below pH 3. Congo red is only used for the detection of inorganic acids since it is not sensitive to the presence of organic acids.
What is Congo red used for in histology?
Micrograph demonstrating amyloid deposition (red-orange) with Congo red staining in cardiac amyloidosis. In histology and microscopy, Congo red is used for staining in amyloidosis, and for the cell walls of plants and fungi, and for the outer membrane of Gram-negative bacteria.
Does Congo red stain amyloid?
In aqueous solution Congo red stains many tissue components, although there is always a greater affinity for amyloid.
What is the solubility of Congo red?
Congo red is water-soluble, yielding a red colloidal solution; its solubility is greater in organic solvents. However, the use of Congo red has long been abandoned, primarily because of its carcinogenic properties.
Is Congo red made from benzidine?
Congo red, an azo dye derived from benzidine, and 2-azoxyfluorene, a derivative of 2-aminofluorene, were reduced during overnight incubation with a suspension of rat intestinal bacteria. HPLC and uv spectral analysis verified the presence of benzidine in the extracts of the Congo red incubations.