Characterization of the
Self-Assembly of <i>meso</i>-Tetra(4-sulfonatophenyl)porphyrin
(H<sub>2</sub>TPPS<sup>4–</sup>) in Aqueous Solutions
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Abstract
The aggregation of <i>meso</i>-tetra(4-sulfonatophenyl)porphyrin
(H<sub>2</sub>TPPS<sup>4–</sup>) 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 μM 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 were determined using analytical ultracentrifugation and
small-angle X-ray scattering. Our results indicate that in neutral
and basic solutions of H<sub>2</sub>TPPS<sup>4–</sup>, porphyrin
dimers and trimers are formed at micromolar concentrations and in
the absence of NaCl to screen any ionic interactions. At these low
concentrations and pH 4, the protonated H<sub>4</sub>TPPS<sup>2–</sup> species self-assembles, leading to the formation of particularly
stable aggregates bearing 25 ± 3 macrocycles. At higher concentrations,
these structures further organize or reorganize 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 micromolar concentrations at pH 4 when prepared
from dilution of a 2 mM stock solution, upon addition of NaCl, or
both