Dipole theory:
Electron donating groups ★★★★★★★★ attach to the aromatic tannin molecule & increase electron density in the ortho & pera position of benzene ring due to accumulation of π-electron.
When a group is attached to benzene ring, the molecules becomes slightly negatively charged at the point of higher electron densities, at the same time the point from where the electron is removed, it becomes slightly positively charged & the molecule behave like a dipole.
Naturally the CO-NH linkage behave like dipole due to uneven distribution of electron & shows good resonance between C & O and instantly the positive negative charges.
***** figure *****