Immobilized Porphyrins as Versatile Stationary Phases in Liquid Chromatography

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Abstract
The preparation, characterization and potential liquid chromatographic applications of various porphyrin-silica stationary phases are reviewed. These new phases are synthesized by covalently linking unsymmetrical tetraphenylporphyrin (TPP) derivatives (monocarboxyl- or monohydroxyl-), as well as native protoporphyrin IX (ProP) to appropriately derivatized silica support matrices. The porphyrinsilicas may be further modified by metallation with a wide range of central metal ions (e.g., Cu2+, Zn2+, Ni2+, Fe+3, In3+, Sn4+, etc.) by refluxing in the presence of metal ion salts. Columns packed with either metallated or unmetallated materials exhibit exceptionally high shape selectivity in the separation of polycyclic aromatic hydrocarbons (PAHs) and fullerenes owing to strong π-π interactions between such solutes and the immobilized porphyrin structures. Columns packed with metallated porphyrins, particularly In(III) and Sn(IV), may also be used for separation of anions, including aromatic carboxylates and sulfonates, via selective ligation reactions with the metal centers. Similar coordination chemistry can be exploited to achieve selective peptide separations through a combination of specific metal ion affinity reactions of certain amino acids (histidine, tryptophan) with given metal ion centers (i.e., Fe+3, Cu+2) and concomitant π-π interactions of aromatic amino acids with the immobilized conjugated macrocycle. In the area of peptide/protein separations, the metalloporphyrin-silicas may offer an attractive alternative to current immobilized metal ion affinity phases (IMAC), because of their exceptionally strong metal ion binding constants. This allows the columns to provide reproducible analytical and preparative separations without potential for metal ion contamination of the purified materials.