Testing biomolecules

[nid404]

#Starch(iodine test)

to approx 2cm³ of test solution add two drops of iodine/potassium iodide soln. A blue black color indicates presence of startch as a a starch-polyiodide complex is formed. Starch is only slightly soluble in water,but the test works well in a suspension or as a solid.

#Reducing sugars(benedict's test)
All monosachharides and most disaccharides(except sucrose) will reduce copper(ii) sulphate, producing a precipitate  of copper(i) oxide on heating, so they r called reducing sugars. Benedict's reagent is an aqueous soln of copper(ii) sulpahte, sodium carbonate and sodium citrate. To approx 2cm3 soln add an equal quantity of benedicts soln. shake and heat for a few minutes at 95 degrees centigrade in a water bath. appt indicates reducing sugar. The color density of ppt gives an indication of the amount present...so this is semi-quantitative.the original pale blue color means no reducing sugar, a green ppt means relatively less reducing sugar, a brown or red ppt means progressively more sugar


more tests in a few min

[nid404]

#non-reducing sugars(benedict's test). Sucrose is a non-reducing sugar because it doesnot reduce copper sulphate.So,there is no direct test for sucrose.However if it is first hydrolysed( broken down) to its constituent monosaccharides(glucose and fructose), it will give a positive test for benedict's test. So sucrose is the only sugar which will give a negative benedict's test before hydrolysis and a positive test afterwards. First,test a sample for reducing sugars,to see if there are any present before hydrolysis.Then,using a separate sample,boil the test soln with dilute HCl for a few minutes to hydrolyse the glycosidic bond. neutralise the soln by gently adding small amounts of sodium hydrogencarbonate
until it stops fizzing. Then test as before for reducing sugars.

# Lipids(emulsion test)

Lipids do not dissolve in water,but do dissolve in ethanol.This property is used in emulsion test.Do not start by dissolvin gsample in water, but instead shake the sample with 4 cm3 of ethanol. Decant the liquid into a test tube of water, leaving any undissolved substances behind. If there r lipids dissolved in the ethanol, they will precipitate in the water,forming a cloudy white emulsion

#protein (biuret test)

To about 2cm3 of test soln ad an equal vol of biuret soln, down the side of the test tube. A blue ring forms at the surface of the soln,which disappears on shaking,and the soln turns lilac purple,indicating protein. The color is due to a complex between nitrogen atoms in th epeptide chain and Cu 2+ ions, so this is really a test for peptide bonds

[mousa]

Thanks alot nid!1.. but what do you mean by the "blue ring" ?

[nid404]

here biuret result....the ring forms at the surface(1st test tube) and then after shaking...it spreads(2nd tube)