A 12-porphyrin system: Syntheses of peptide porphyrins with multiple histidines and the aggregation behavior in the presence of hemin

Article abstract of DOI:10.1246/bcsj.72.1351

We succeeded in combining multiple-histidine peptides with porphyrins; that is to say, we synthesized three peptide porphyrins consisting of α₄- meso-tetrakis(o-aminophenyl)porphyrin, α₄-H₂TAPP, and amphiphilic peptide, α₄(PepA₁₈)(n)(AG)(4-n)-H₂TAPP (n = 1, 3, and 4; PepA₁₈ = EEALEKHEKALEKHEKAG), in the liquid phase. These compounds were designed to construct multiple porphyrin systems; in other words, each peptide attached on the H₂TAPP was designed to contain two histidines. The stability of the porphyrins, (AG)_4-H₂TAPP and (AG)₁(Boc-AG)₃-H₂TAPP, as well as the coupling conditions between the porphyrin fragments and the 16-residue peptide, EEALEKHEKALEKHEK, to give α₄-(PepA₁₈)(n)(AG)(4-n)-H₂TAPP (n = 1, 3, and 4) were studied in detail with respect to the temperature, solvents, coupling reagents, additives, and amines. This search revealed that the combination of DMF/benzotriazol-1-yl-oxytris(pyrrolidino)phosphonium hexafluorophosphate (PyBOP)/N,N-diisopropylethylamine is suitable for the purpose. The search also clarified that it is possible to synthesize α₄- (PepA₁₈)₄-H₂TAPP and α₄-(PepA₁₈)₃(AG)₁-H₂TAPP selectively by choosing the coupling additives 1-hydroxy-7-azabenzotriazole (HOAt) and 1- hydroxybenzotriazole (HOBt, respectively. All of the compounds showed 40% helicity in a solution (phospate buffer, pH = 7.0) containing 2,2,2- trifluoroethanol (TFE). The UV-vis and circular-dichroism spectrophotometric titrations of Fe(III) protoporphyrin IX chloride, B, with α₄(PepA₁₈)₄- ZnTAPP, A, indicated that up to three equivalents of B were incorporated into A in a buffer solution containing 15% TFE. Sedimentation-equilibrium ultracentrifugation experiments showed that A is a dimer in the solution, and that this dimer is transformed to a trimer when B is incorporated into A. These results suggest that A achieves a 12-porphyrin system consisting of different kinds of metalloporphyrin.