Why are fibrous proteins insoluble in water?

Because of their different polarities, fibrous proteins and water do not dissolve in an aqueous solution. This is because, in accordance with chemical laws, "like dissolves like." Fibrous proteins are polar, and their surface is coated in non-polar amino acids.

Polarity is defined as the distribution of charges within a molecule, where a partial positive charge is held in areas of high electron density and a partial negative charge is held in areas of low electron density. The attraction between a partial positive and partial negative charge is similar to that of magnets with north and south poles, but instead of magnetic fields, the attraction is caused by charges, and it is typically strong enough to break the solute (the object added to the water/solvent) up into individual molecules.

Fibrous proteins lack partially charged zones on their surface, which prevents them from being drawn to the partial charges of the water molecules and preventing them from dissociating, maintaining the heterogeneity of the protein and water mixture.

Fibrous proteins are characterized by an elongated, highly ordered structure with strong intramolecular bonds (disulfide and hydrogen bonds) that hinder water molecules from interacting with the protein surface; in addition, hydrophobic regions of the protein frequently repel water molecules, making fibrous proteins insoluble in water.