Precipitant fixatives

The concepts of coagulation and precipitation, common in the classical literature of fixation, are outmoded and confusing a coagulant fixative gels some, but not all proteins, while a precipitant fixative causes only certain proteins to fall out of solution. Instead, we will use terms that actually describe the chemical and physical reality of fixation at the molecular level. 

The slide preparation is fixed by using either precipitating fixatives such as ethanokacetic acid or cross-linking fixatives such as paraformaldehyde. They are further treated to remove protein with a mixture that includes proteinase K. Removal of proteins facilitates the access of probe to the sample DNA target. 

The non-ionic detergents Triton X-100 and NP40 are frequently used in studies of the cytoskeleton. They are applied either prior to fixation with a cross-linker in the presence or absence of precipitant fixatives or after fixation to remove remaining lipids. Generally, 0.3% of detergent in PIPES buffer containing 0.1 mM EGTA is used for 1 to 3 min. 

For DNA, fixation with a precipitant fixative is far superior to formaldehyde. RNA may yield false signals if DNA has to be detected since (i) the number of RNA transcripts exceeds the number of corresponding DNA sequences, (ii) the target RNA is sj and... 

Formalin fixation sterilizes tissue specimens in a more reliable way than precipitating fixatives, particularly for viruses. 

Melanoma Isopropanol precipitation Ammonium acetate precipitation PEG precipitation Fixed bed lEC SEC 48... 

Before or during the staining, the proteins are fixed in the gel (i.e., denatured and precipitated). Fixing is typically done with mixtures of ethanol, acetic acid, water, and a dye such as Coo-massie Blue. Trichloroacetic acid is sometimes used instead of acetic acid, because some people believe that trichloroacetic acid fixes the proteins better. 

The first step for a successful in situ hybridization is the fixation of the tissue. This will ensure target nucleic acid retention and preservation of the tissue morphology. Either crosslinking or precipitative fixatives can be used, and a preference for either of the two types of fixative has often been based on the different types of system under investigation (1-7). For hybridization of regulatory peptide mRNA, 4% paraformaldehyde appears to be the most effective, both on tissue blocks and on tissue culture preparations. 

A solution of Mo[HB C3H(CH3)2N2 3](NO)l2-C6H5CH3 (0.5 g) and an excess of ethylamine (0.1 mL) in dichloromethane (40 mL) is stirred at room temperature for 0.5 hour during which time the solution becomes deep red. The volume of the solution is then reduced by evaporation (to —20 mL), and diisopropyl ether is added dropwise to produce a white precipitate of ethylanunonium iodide. The mixture is then filtered and the dichloromethane is removed from the filtrate by evaporation, causing the crude, red product to precipitate fix>m the diisopropyl ether. This is collected by filtration and recrystallized from dichloromethatte/ diisopropyl ether. Yield 0.3 g (78%). Anal. Calcd. for MoC,7H2gNgBOI C, 34.36 H, 4.72 N, 18.86. Found C, 34.61 H, 4.72 N, 18.61. 

Cellulose is a partially crystalline polysaccharide and is the chief constituent of plant fiber. Cotton is the purest natural form of cellulose, containing 90 percent cellulose. Cellulose decomposes before melting and therefore cannot be melt processed. It is insoluble in organics and water and can only be dissolved in strong basic solutions. Regenerated cellulose, also known as rayon, is cellulose that has been precipitated form a basic solution. Cellulose is used in bandages and sutures. Cuprophan is cellulose precipitated fix>m copper hydroxide solutions to form hemo alysis membranes. 

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