Electrostatic core shielding in dendritic polyglutamic porphyrins.

Polyglutamic dendritic porphyrins of the general formula H2PophGlu(N)OR (H2Porph = free-base meso-tetra-4-carboxyphenylporphyrin (H2TCPP), Glu=dendrimer layer composed of L-glutamates, N= 1-3: dendrimer generation number, R = terminal group (All, H)) were synthesized and characterized with NMR and MALDI-TOF mass spectroscopy. The free-acid terminated compounds were found to be highly soluble in water, with both their absorption and fluorescence spectra dependent on pH. The value of the porphyrin mono-protonation constant, measured by fluorescence rationing, increased monotonously in the studied series of dendrimers (pK3=6.31. 6.70, and 6.98, for N=1, 2, 3, respectively). For the largest dendrimer, H2PorphGlu(3)OH, pK3 was found shifted by almost two pH units relative to the non-modified H2Porph. The second protonation constant (K4) was much less affected by the dendritic substituents. At pH values less than 3.5 there were noticeable changes in fluorescence intensity and quantum yield even for the highly soluble H2PorphGlu(3)OH. This suggests that interactions between individual dendritic molecules in solution are favored by full protonation of the peripheral glutamic carboxyls. The "dendrimer-protected" porphyrins are convenient fluorescent pH sensors in the biological pH range.