The Effect of pH on the Topography of Porphyrins in Lipid Membranes¶

Biocompatible and flexible graphene oxide/upconversion nanoparticle hybrid film for optical pH sensing

Free-standing optical hybrid film which is composed of positively-charged polyethylenimine-coated NaYF4:Yb,Er nanoparticles and negatively-charged graphene oxide (GO) has been developed to measure pH based on the pH-dependent luminescence quenching effect caused by GO. The isothermal titration calorimetry analyses indicate that the interaction between GO and NaYF4:Yb,Er nanoparticles becomes stronger with increasing pH, leading to a more significant fluorescence quenching of NaYF4:Yb,Er nanoparticles at high pH values. The excellent mechanical properties of the hybrid film endow the thin-film pH sensor with better repeatability and higher stability during the measurements. Quantitatively, the upconversion luminescence intensity of the hybrid film exhibits a linear trend over the pH range of 5.00-8.00. Because of excitation with a 980 nm laser, as expected, the hybrid film sensor is also sensitive to the urine measurements with reduced background absorption. In addition to its good biocompatibility, our free-standing hybrid film sensor would be a promising candidate for biological, medical, and pharmaceutical applications.

Self-assembling properties of porphyrinic photosensitizers and their effect on membrane interactions probed by NMR spectroscopy

Aggregation and membrane penetration of porphyrinic photosensitizers play crucial roles for their efficacy in photodynamic therapy. The current study was aimed at comparing the aggregation behavior of selected photosensitizers and correlating it with membrane affinity. Self-assembling properties of 15 amphiphilic free-base chlorin and porphyrin derivatives bearing carboxylate substituents were studied in phosphate buffered saline (PBS) by (1)H NMR spectroscopy, making use of ring current induced aggregation shifts. All compounds exhibited aggregation in PBS to a different degree with dimers or oligomers showing slow aggregate growth over time. Aggregate structures were proposed on the basis of temperature dependent chemical shift changes. All chlorin compounds revealed similar aggregation maps with their hydrophobic sides overlapping and their carboxylate groups protruding toward the exterior. In contrast, for the porphyrin compounds, the carboxylate groups were located in overlapping regions. Membrane interactions were probed using 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayer vesicles and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) micelles as models. The chlorin derivatives had higher membrane affinity and were all monomerized by DHPC micelles as opposed to the porphyrin compounds. The observed differences were attributed to the different aggregate structures proposed for the chlorin and porphyrin derivatives. Free accessibility of the carboxylate groups seemed to promote initial surface interaction with phospholipid bilayers and micelles.

pH-dependent distribution of chlorin e6 derivatives across phospholipid bilayers probed by NMR spectroscopy

The pH-dependent membrane adsorption and distribution of three chlorin derivatives, chlorin e6 (CE), rhodin G7 (RG), and monoaspartyl-chlorin e6 (MACE), in the physiological pH range (pH 6-8) were probed by NMR spectroscopy. Unilamellar vesicles consisting of dioleoyl-phosphatidyl-choline (DOPC) were used as membrane models. The chlorin derivatives were characterized with respect to their aggregation behavior, the pK(a) values of individual carboxylate groups, the extent of membrane adsorption, and their flip-flop rates across the bilayer membrane for pH 6-8. External membrane adsorption was found to be lower for RG than for CE and MACE. Both electrostatic interactions and the extent of aggregation seemed to be the main determinants of membrane adsorption. Rate constants for chlorin transfer across the membrane were found to correlate strongly with the pH of the surrounding medium, in particular, for CE and RG. In acidic solution, CE and RG transfer across the membrane was strongly accelerated, and in basic solution, all compounds were retained, mostly in the outer monolayer. In contrast, MACE flip-flop across the membrane remained very low even at pH 6. The protonation of ionizable groups is suggested to be a major determinant of chlorin transfer rates across the bilayer. pK(a) values of CE and RG were found to be between 6 and 8, and two of the carboxylate groups in MACE had pK(a) values below 6. For CE and RG, the kinetic profiles at acidic pH indicated that the initial fast membrane distribution was followed by secondary steps that are discussed in this article.

The binding of analogs of porphyrins and chlorins with elongated side chains to albumin

In previous studies, we demonstrated that elongation of side chains of several sensitizers endowed them with higher affinity for artificial and natural membranes and caused their deeper localization in membranes. In the present study, we employed eight hematoporphyrin and protoporphyrin analogs and four groups containing three chlorin analogs each, all synthesized with variable numbers of methylenes in their alkyl carboxylic chains. We show that these tetrapyrroles' affinity for bovine serum albumin (BSA) and their localization in the binding site are also modulated by chain lengths. The binding constants of the hematoporphyrins and protoporphyrins to BSA increased as the number of methylenes was increased. The binding of the chlorins depended on the substitution at the meso position opposite to the chains. The quenching of the sensitizers' florescence by external iodide ions decreased as the side chains became longer, indicating to deeper insertion of the molecules into the BSA binding pocket. To corroborate this conclusion, we studied the efficiency of photodamage caused to tryptophan in BSA upon illumination of the bound sensitizers. The efficiency was found to depend on the side-chain lengths of the photosensitizer. We conclude that the protein site that hosts these sensitizers accommodates different analogs at positions that differ slightly from each other. These differences are manifested in the ease of access of iodide from the external aqueous phase, and in the proximity of the photosensitizers to the tryptophan. In the course of this study, we developed the kinetic equations that have to be employed when the sensitizer itself is being destroyed.

Radiation-Inducedin VitroPhototoxic Potential of Some Fluoroquinolones

Photosensitizing drugs that can damage cellular biomolecules is a matter of concern. Lomefloxacin, norfloxacin, ofloxacin, and enoxacin (broad-spectrum antibiotics of fluoroquinolone group) are used for the treatment of Gram-positive and Gram-negative bacterial infections. Phototoxicity and possible mechanism of their action was assessed under the exposure of ambient levels of UV-A, UV-B, and sunlight at a concentration generally used in the treatment of various diseases. Singlet oxygen (1O2), superoxide anion radical (O2.-) generation, DNA damage, and lipid peroxidation in human blood were studied. All the fluoroquinolones tested in this study produced 1O2 and O2.- under exposure to UV-A, UV-B, and sunlight depending on the concentrations (0 to 60 microg/mL) of the drugs. Enoxacin showed a higher yield of 1O2 and O2.- than other drugs. These materials also degraded deoxyguanosine and induced lipid peroxidation in vitro under exposure to UV-A, UV-B, and sunlight (depending on the dose of radiation). The formation of the reactive oxygen species (ROS) by the photoexcited drugs may be considered as a possible mechanism of their action.