We analyse the behaviour of mono-protonated 5-phenylporphyrin upon por-ph torsion to get insights into torsion-induced reactivity changes. In contrast to the majority of structures, nitrogen-accessibility volume (V-react) is larger for front-side (exposed nitrogen) than for backside-attacks at the optimised structure. The high reactivity of nitrogen is determined by the electrostatic potential distribution inside the porphyrine core. Even if Vreact at certain structures is smaller for front side than for backside attacks the mean electrostatic-potential-density (phi) over bar(d) inside V-react reveals a lower barrier for front side than for backside protonation, and this holds true for the majority of structures. (C) 2014 Elsevier B.V. All rights reserved.