Using nonlocal density functional theory and extended basis sets, full geometry optimizations have been performed on two stationary points on the potential energy surface of monodeprotonated porphine, viz. the ground state and the transition state for NH tautomerism. The transition state is calculated to be approximately 11.84 kcal/mol above the ground state. The porphyrin framework deforms considerably along the reaction pathway of the proton transfer, with the internal angles of the central C12N4 macrocycle undergoing the maximum changes.