Understanding the self-assembly of meso-tetra(4-sulfonatophenyl)porphyrin (H$_{2}$TPPS$^{4-})$ in aqueous solutions

The aggregation of \textit{meso}-tetra(4-sulfonatophenyl)porphyrin (H2TPPS4-) in phosphate solutions was investigated as a function of pH, concentration, time, ionic strength, and solution preparation (either from dilution of a freshly prepared 2 mM stock or by direct preparation of $\mu $M solution concentrations) using a combination of complementary analytical techniques. UV-Vis and fluorescence spectroscopy indicated the formation of staggered, side-by-side (J-type) assemblies. Their size and self-associative behavior was determined using analytical ultracentrifugation and small angle X-ray scattering. Our results indicate that in neutral and basic solutions of H$_{2}$TPPS$^{4-}$, porphyrin dimers and trimers are formed at $\mu $M concentrations and in the absence of NaCl to screen any ionic interactions. At these low concentrations and pH 4, the protonated H$_{4}$TPPS$^{2-}$ species self-assembles leading to the formation of particularly stable aggregates bearing 25 $\pm $ 3 macrocycles. At higher concentrations, these structures further organize or re-organize into tubular, rod-like shapes of various lengths which were imaged by cryogenic and freeze-fracture transmission electron microscopy. Micron-scale fibrillar aggregates were obtained even at $\mu $M concentrations at pH 4 when prepared from dilution of a 2 mM stock solution and/or upon addition of NaCl.