UV−vis spectra of 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP) have been studied in dilute (1.0
μM) aqueous solution at pH 7.0 as a function of added electrolytes, including phosphate buffer, methyl viologen
dichloride (MV), and propyl viologen sulfonate (PVS). At concentrations ≤1.0 μM and pH ≥ 7.0, TCPP is
considered to be tetraanionic and unaggregated. It shows a strong B (Soret) band at 414.2 nm that can be
decomposed into three Gaussians, and follows Beer's law with ε = 4.8 × 105 M-1 cm-1 at pH 7.0 in 5.0 mM
phosphate buffer. Deviations from Beer's law appear above 2 μM under the above conditions; linear regions
for Beer's law plots depend on the pH, temperature, and ionic strength. Even around 1 μM, the Soret absorption
strength is a very sensitive function of electrolyte concentration; for example, ε = 3.6 × 105 M-1 cm-1 at pH
7.0 in 62 mM phosphate buffer, and ε = 5.1 × 105 M-1 cm-1 in aqueous solution of minimal ionic strength
(0.1 mM NaOH, pH 10). In the titration of TCPP with MV, two types of experiments were performed, at low
ionic strength (which necessarily increased in the later stages of the titration) or at high (effectively constant)
ionic strength. In 5.0 mM phosphate buffer at pH 7.0 two isosbestic points are observed, at 418.4 nm (in the
range of 40−300 μM MV) and at 421.5 nm (15−90 mM MV), considered to represent the formation of 1:1
and 1:2 complexes (porphyrin−viologen). By nonlinear regression analysis, the complexation constants are
calculated to be K
1 = 3350 M-1 and K
2 = 68 M-1. Application of principal component analysis identifies the
spectra of both complexes and gives K
1 = 2200 M-1 and K
2 = 100 M-1. The complexation constants, the
spectra of the complexes, and the positions of the isosbestic points vary with the buffer concentration. At 62
mM phosphate buffer, K
1 = 600 M-1 and K
2 = 40 M-1 by nonlinear regression analysis (K
1 = 1200 M-1 and
K
2 = 140 M-1 by principal component analysis). 1H NMR spectra indicate that both TCPP and MV undergo
concentration-dependent resonance shifts that can be correlated with the calculated complexation constants.
Similar titration of TCPP with PVS also shows evidence for 1:1 and 1:2 complexes, but with lower equilibrium
constants compared to those from titration with MV. At high PVS concentrations (>25 mM) a blue-shifted
Soret band attributed to TCPP dimer is observed. The same spectrum is observed under a variety of other
conditions, including the presence of a cationic surfactant (4 μM CTAB), very high ionic strength (3 M
NaCl), or a polycationic complexing agent. These effects illustrate the variety of influences that affect the
porphyrin absorption spectrum in aqueous solution and offer cautions about the conditions of experimental
control and data analysis that are necessary to extract meaningful titration data.
