why does lugol's iodine with glycogen is a brown color while lugol's iodine with starch is a color blue?

Question

it is used in structural staining.

Answer 1

First of all lugol's iodine is the complex ion I3(-)

It's colour in water is brown.

Starch is composed of amylose and amyolpectin.

Amylose is a non-branched polysaccharide and the linear polymer forms coils. I3(-) enters these coils, forms a complex with amylose, these changes the energy differency of the orbitals of I3(-) and thus change the colour to blue.

Glycogen and amylopectin are branched and actually glycogen is even more branched than amylopectin. The branches do not allow the formation of a coiled structure and thus the blue I3(-)-polysaccharide complex cannot form.