Conformational Analysis of Swallowtail Motifs in Porphyrins

Aqueous solubilization of porphyrins, often accomplished with appended polar aryl groups, can also be achieved with symmetrically branched alkyl (i.e., swallowtail) groups terminated with polar moieties. Here, carboxylic acids are employed as termini (versus prior phosphono- or phosphoester termini) in designs of trans-AB-porphyrins bearing a single swallowtail group (A) or trans-A₂-porphyrins bearing two swallowtail groups. Variable-temperature ¹H NMR studies (-60 to 97 °C) revealed that the 4-heptanedioic acid group at the meso-position of the free base porphyrin rotates with rate constant 5 s^-1 (310 K) and Arrhenius energy barrier E_a = 11.5 kcal/mol, whereas an isopropyl group undergoes rotation ∼1000-times faster (k = 5770 s^-1). The interconversion is sufficiently fast that conformational diastereomers, as when two such swallowtail groups are present in a trans-A₂-porphyrin, would not be isolable at room temperature (Class I atropisomers). DFT calculations (4 porphyrins containing the isopropyl or 4-heptanedioic acid groups) showed that the lowest energy conformer contains the swallowtail C-H unit in the plane of the porphyrin. The presence of one or two 4-heptanedioic acid moieties imparted solubility of the porphyrin in phosphate-buffered saline (PBS). The results suggest applications in the life sciences where compact, aqueous-soluble porphyrins are desired.