Atropisomers are stereoisomers resulting from restricted rotation around single bonds such that the rotational barrier is high enough to permit observation of the isomeric species. Atropisomers are numerous in number and type: among atropisomers of the sp 2 -sp 2 single bond type, a classical example is constituted by the biphenyls (or biaryls in general) as shown in figure 1A. When X ≠ Y and U ≠ V and if the steric interaction of X-U, X-V, and/or Y-V, Y-U is large enough, two nonplanar, axially chiral enantiomers exist [1]. Another example of atropisomerism, related to the biphenyltype, was first reported by Gottwald and Ullman in the porphyrin series (Fig. 1B) [2]. This type of atropisomerism in porphyrins has been utilised in the design of superstructured models of the active sites of hemoproteins [3][4][5][6][7].Porphyrins (see Fig. 3) are macrocycles consisting of four pyrroles linked by four methine bridges. Carbons 5,10, 15 and 20 are designated meso and carbons 2,3,7,8,12,13, 17 and 18 are the β pyrrole positions (Fisher's convention [8]). Porphyrins with four identical aryl groups in the meso position are derivatives with restricted rotation about the aryl-porphyrin bond due to interactions of the β-CH groups with the ortho-substituents of the meso-aryl groups (R or R' ≠ H, R and R' magnetically nonequivalent, Fig. 1B). At room temperature, the conformational rigidity is frequently sufficient to observe and also to isolate the corresponding atropisomers. These isomers may be described in terms of whether the ortho substituents are above (β) or below (α) the plane of the porphyrin ring [9]. When the porphyrin ring is formed by random linkages of pyrroles and mono-o-substituted benzaldehydes, the statistical ratio for the four possible isomers β 4 , αβ 3 , α 2 β 2 and αβαβ is 1:4:2:1, respectively (Fig. 2). Gottwald and Ullman [2] separated these isomers by chromatography in their study on meso-tetrakis (o-hydroxyphenyl)porphyrin. Recently, the structure of two atropisomers was assigned based on X-ray analysis [10]. The distribution of the four atropisomers was estimated in the case of the nickel complex of meso-tetra-o-tolylporphyrin (R = Me, R' = H, Fig. 1B) by visual comparison of the areas of the six types of methyl group resonances [11]. According to other studies on porphyrins (see references [3][4][5] and [12][13][14][15][16][17][18][19] for a representative list of examples), the existence of atropisomers is usually revealed in NMR spectroscopy by multiplets with rather broad peaks mainly due to extensive overlap of signals.To clearly illustrate atropisomerism, we discuss the wellresolved 250 MHz 1 H NMR spectrum of meso-tetrakis(3-sulfonatomesityl)porphyrin tetrasodium salt (Fig. 3), a porphyrin prepared (note 1) as a potential antiviral agent [20] Illustrating atropisomerism in the porphyrin series using NMR spectroscopy -the mesityl ring introduced in the meso positions of the porphyrin has methyl substituents in both ortho positions of the phenyl group, which greatly increases the activatio...