Highly Non-Planar Dendritic Porphyrin for pH Sensing: Observation of Porphyrin Monocation

Abstract

Metal-free porphyrin-dendrimers provide a convenient platform for construction of membraneimpermeable ratiometric probes for pH measurements in compartmentalized biological systems. In all previously reported molecules, electrostatic stabilization (shielding) of the core porphyrin by peripheral negative charges (carboxylates) was required to shift the intrinsically low porphyrin protonation pK a 's into the physiological pH range (pH 6-8). However, binding of metal cations (e.g. K + , Na + , Ca 2+ , Mg 2+ ) by the carboxylate groups on the dendrimer could affect the protonation behavior of such probes in biological environments. Here we present a dendritic pH nanoprobe based on a highly non-planar tetraaryltetracyclohexenoporphyrin (Ar 4 TCHP), whose intrinsic protonation pK a 's are significantly higher than those of regular tetraarylporphyrins, thereby eliminating the need for electrostatic core shielding. The porphyrin was modified with eight Newkome-type dendrons and PEGylated at the periphery, rendering a neutral water-soluble probe (TCHpH), suitable for measurements in the physiological pH range. The protonation of TCHpH could be followed by absorption (e.g. ε Soret (dication)~270,000 M −1 cm −1 ) or by fluorescence. Unlike most tetraarylporphyrins, TCHpH is protonated in two distinct steps (pK a 's 7.8 and 6.0). In the region between the pK a 's, an intermediate species with a well-defined spectroscopic signature, presumably a TCHpH monocation, could be observed in the mixture. The performance of TCHpH was evaluated by pH gradient measurements in large unilamellar vesicles. The probe was retained inside the vesicles and did not pass through and/or interact with vesicle membranes, proving useful for quantification of proton transport across phospholipid bilayers. To interpret the protonation behavior of TCHpH we developed a model relating structural changes on the porphyrin macrocycle upon protonation to its basicity. The model was validated by DFT calculations performed on a planar and non-planar porphyrin, making it possible to rationalize higher protonation pK a 's of non-planar porphyrins as well as the easier observation of their monocations.

Isolation and Characterization of a Monoprotonated Hydroporphyrin

“…[28,29] In contrast to our structure, the conjugate base in one of the others (2-anthracenyl sulfonate) hydrogen bonds to the pyrrole-NH instead of the protonated pyrrole. of the B-ring and the adjacent A-and C-rings are 19.5(2) and 24.6(2)°, respectively, whereas rings containing N1, N3 and N4 are almost coplanar [4.9(2) to 9.4(2)°].…”

Structural Modulation of Chromic Response: Effects of Binding‐Site Blocking in a Conjugated Calix[4]pyrrole Chromophore

“…The diprotonated form is stabilized by binding of two counteranions (on the top and on the bottom) at the central NH groups and by a tautomeric process . The excellent quality of the data fit also indicates the low stability of any mono‐ and triprotonated OxP species, which are essentially absent due to the ease of further protonation (analogous with tetraphenylporphyrin monocation). On the basis of this analysis the chemical structures including the changes in conjugation of the individual species are given in Figure e.…”

Thermodynamics and Photodynamics of a Monoprotonated Porphyrin Directly Stabilized by Hydrogen Bonding with Polar Protic Solvents

“…Thep rotonation affords saddle-distorted diprotonated porphyrins (H 4 P 2 + )u nder strongly acidic conditions as shown in Scheme 1. [46,[57][58][59][60][61][62][63][64][65] Saddle-distorted H 2 Ps have been reported to form weak hydrogen bonds with polar protic solvents such as methanol (MeOH)n ot only in crystals [66][67][68] but in solutions as well. [36] The reason why H 3 P + is hard to detect in the protonation processi st hat the protonation of H 2 Pg ives much higher basicity which is much highert han that of H 2 P( K 1 !…”