Spectroscopic Detection of Tetracationic Porphyrin H-Aggregation on Polyanionic Matrix of Inorganic Polyphosphate

Abstract

Self-assembly of tetracationic porphyrin TMPyP(4+) onto polyanionic matrix of inorganic polyphosphate (PPS) in aqueous solutions has been studied in a wide range of molar phosphate-to-dye ratios using techniques of polarized fluorescence, absorption, resonance Raman spectroscopy and static light scattering. The binding of TMPyP(4+) to PPS is characterized by the binding constant of 3 x 10(5) M(-1) and the cooperativity parameter of about 150. The fluorescence quenching of the bound TMPyP(4+) evidences the stacking of the porphyrine chromophores. Under the stoichiometric binding ratio TMPyP(4+) forms extended continuous face-to-face aggregates (so-called H-aggregates) which manifest themselves by a blue shift (12 nm) and a large hypochromisity (51%) of the Soret absorption band. Each face-to-face TMPyP(4+) stack is formed with participation of four PPS chains. Formation of such columnar aggregates is promoted by the ability of PPS chains to take a helix conformation where negative charges are arranged along two oppositely situated rows with intercharge distance of 0.36 nm which corresponds to the thickness of the porphyrin pi-electronic system. The ability of each PPS strand to be template for formation of two porphyrin stacks results in the integration of the adjacent stacks into higher-order aggregates which dimension was estimated from the fluorescence polarization data.

Spectroscopic Studies on Binding of Porphyrin-Phenazine Conjugate to Four-Stranded Poly(G)

“…In this way, 5-fold fluorescence quenching with simultaneous increase of p can be explained by formation of porphyrin stacks on polyanionic matrix of poly(G) as a result of neutralization of positive charges localized on methylpyridil groups of porphyrins by negatively charged phosphate groups of poly(G). The deviation of the titration curve from a linear form which was observed earlier for pure electrostatic binding of TMPyP 3+ to inorganic polyphosphate (PPS) [38] confirms that dissociation of the heterodimers occurs upon binding process. The symmetrically ascending part of titration curve registered for the ImPzn moiety at 562 nm in the same P/D range obviously indicate the reduction of energy transfer from phenazine part to porphyrin as a result of the dimer dissociation.…”

“…5), that corresponds to twice as high polymer concentration than that (P/D≈3) registered for individual TMPyP 3+ dye bound to poly(G) and PPS. Its magnitude, (I/I 0 ) min =0.2, is equal to that observed for TMPyP 3+ stacks on PPS matrix [38].…”

“…200,000 M −1 cm −1 ) and four less intense Q-bands centered at 522, 561, 583.5 and 642.5 nm [29]. Its shape is similar to the spectrum of tetracationic TMPyP4 porphyrin [38], however small red shift (ca. 3 nm) of all bands is observed.…”

“…In particular, it is observed for TMPyP4 porphyrin and its derivatives, whose fluorescence spectra in water represent single structureless bands, which splits in non-polar solvents [43], upon the dimer or stack formation [38], and upon intercalation to DNA [44], when overlapping between the π-electronic systems of neighbouring chromophores or chromophore and bases is occurred. Since at the concentration of 10 μM, the TMPyP 3+ porphyrin is mainly in monomeric form, it can be assumed that the aforementioned spectral transformations, as well as FRET quenching of ImPzn fluorescence most likely indicate the formation of internal heterodimer where π-staking interaction between TMPyP 3+ and ImPzn chromophores is occurred [29].…”

“…Fluorescent measurements were carried out on laboratory spectrofluorimeter based on the DFS-12 monochromator (LOMO, 350-800 nm range, dispersion 5 Å/mm) by the method of photon counting [38]. The fluorescence was excited by the stabilized linearly polarized radiation of a halogen lamp, at λ exc =440 and 530 nm.…”