Assessment of the partitioning of probes to membranes by spectroscopic titration

Pdots, a new type of nanoparticle, bind to mTHPC via their lipid modified surface and exhibit very high FRET efficiency between the core and the sensitizer

Pdots are a new type of nanoparticle which exhibit strong potential for future applications in biophysics and cell biology. They are composed of organic chromophoric polymers, whose surfaces can be modified with different amphiphilic polymers, such as PEGylated lipids to make them very stable as colloids in water. We demonstrate in this manuscript that the lipid nano-coating around the Pdot can bind very efficiently to amphiphilic molecules, such as photosensitizers e.g. meso-tetrahydroxyphenylchlorin (mTHPC). As a result the sensitizer is brought into very close contact with the cores of the Pdots, and resonance energy transfer from the core to the sensitizer is very efficient; in some cases it is close to 1. We show the spectroscopic properties of two types of Pdots; their sizes, which are in the 13-47 nm range, depend on the kind of polymer and the length of the PEGylated lipid chains that wrap it. We measured the efficiency of FRET by investigating the decrease in donor intensity or its lifetime upon binding with mTHPC. We also show the relative yields of singlet oxygen that are obtained via two pathways: by exciting the Pdots which transfer the energy to the attached sensitizer, or by exciting the sensitizer directly. This methodology could be used to enhance the use of a photosensitizer by employing both pathways in parallel.

Roles of hydrophilicities and hydrophobicities of dye and sacrificial electron donor on the photochemical pathway

Relative rates of the photosensitized production of singlet oxygen ((1)O(2)) and of superoxide (O(2) (•-)) were determined using different couples of dyes and sacrificial electron donors (SEDs) of either high or low hydrophobicities. Such rates were also measured in the absence and presence of single unilamellar vesicles (SUVs) with 9DMPC:1DMPA mol ratio composition. The dyes aluminum phthalocyanine tetrasulfonate (AlPcS(4)) and pheophorbide-a (PHEO) were used as hydrophilic and hydrophobic photosensitizers, respectively. Xanthine (X) and glutathione (GSH) were used as hydrophobic and hydrophilic SEDs, respectively. The presence of SUVs in the aqueous sample produces the physical separation or encounter of SEDs and photosensitizers according to their membrane binding constants. When both the SED and the photosensitizer are localized within the same phase, a strong decrease in the rate of (1)O(2) formation, united to a strong increase in the rate of O(2) (•-) formation, is observed, relative to when both of these species are localized in different phases. The lipid phase is always present in the biological milieu. Thus, the use of a hydrophobic couple of both dye and SED (as in the case of X and PHEO), as well as a hydrophilic couple of both dye and SED (as in the case of GSH and AlPcS4), should strongly favor the Type I mechanism over the Type II. Since only a small number of hydroxyl radicals are needed to initiate a chain reaction of phospholipid peroxidation, the latter could be more toxic to the tumor tissue than peroxidation by a much higher concentration of singlet oxygen molecules.

Photosensitization by the Near-IR-absorbing Photosensitizer Lutetium Texaphyrin: Spectroscopic, In Vitro and In Vivo Studies

The spectroscopic and biological properties of the new photosensitizer lutetium texaphyrin (Lu-Tex) were assessed in vitro and in vivo on a C26 colon carcinoma model, in comparison with hematoporphyrin (Hp), photofrin II (PII) and chlorin e 6( Chl ). Strong binding of Lu-Tex to lipid bilayer membranes was observed. The results of confocal fluorescence microscopy on C26 cells showed that Lu-Tex was localized in small vesicles in the cytoplasm, possibly in the lysosomes, while Chl and Hp were distributed in larger cytoplasmic vesicles attributed to mitochondria. Scanning electron microscopy and X-ray microanalysis revealed that photodynamic therapy with Lu-Tex induced only slight damage to the cell membrane, leading to a delayed cell response. Chl and Hp caused significant structural damage to the outer cell membrane, resulting in ionic imbalance and fast cell death. The in vitro quantitative assessment of the relative efficiency per absorbed photon of the sensitizers revealed that Lu-Tex was less effective than Chl and Hp . However, the results of our in vivo study showed that at the same light and drug doses the anti-tumor efficiency of the agents was in the following order: Lu-Tex > Chl > PII . The strong in vivo anti-tumor effect of Lu-Tex can be explained by its higher integrated absorption in the long-wavelength range.

The effect of lipid composition, bilayer phase and temperature on the uptake of hematoporphyrin by liposomal membranes

In this study we investigated, spectroscopically, the binding of hematoporphyrin (HP) to non-charged lipid vesicles as a function of temperature and the molecular structure of the phospholipid. The temperature dependence of partitioning was employed to evaluate the thermodynamic parameters of the process. We studied the binding of HP to liposomes composed of different phospholipids: natural lecithin and three chemically defined phosphatidylcholines: dimiristoyl-phosphatidylcholine (DMPC), 1-palmitoyl-2-myristoyl-phosphatidylcholine (PMPC) and 1-stearoyl-2-myristoyl-phosphatidylcholine (SMPC), at different temperatures. The last three lipids differ only in the length of the fatty acid on 1 position of the glycerol backbone. Consequently, they have different phase transition temperatures and different order parameters. For SMPC, PMPC and DMPC, we checked the effect of temperatures above and below the phase transition while for lecithin, whose phase transition temperature is well below 0 °C, only temperatures above the phase transition could be tested. A very distinct effect of the phase transition on the binding constant was observed. Below this temperature a dramatic decrease in the binding was observed as the temperature was increased. Above the phase transition, the effect of temperature declined and the changes were minor compared to the changes observed when the bilayers undergo the solid-gel phase transition. Differences in HP binding to the various bilayers were attributed to the differences in the order parameters of DMPC, PMPC, SMPC and lecithin bilayers.

Lipid composition affects the rate of photosensitized dissipation of cross-membrane diffusion potential on liposomes

Hydrophobic or amphiphilic tetrapyrrole sensitizers are taken up by cells and are usually located in cellular lipid membranes. Singlet oxygen is photogenerated by the sensitizer, and it diffuses in the membrane and causes oxidative damage to membrane components. This damage can occur to membrane lipids and to membrane-localized proteins. Depolarization of the Nernst electric potential on cells' membranes has been observed in cellular photosensitization, but it was not established whether lipid oxidation is a relevant factor leading to abolishing the resting potential of cells' membranes and to their death. In this work, we studied the effect of liposomes' lipid composition on the kinetics of hematoporphyrin-photosensitized dissipation of K(+)-diffusion electric potential that was generated across the membranes. We employed an electrochromic voltage-sensitive spectroscopic probe that possesses a high fluorescence signal response to the potential. We found a correlation between the structure and unsaturation of lipids and the leakage of the membrane, following photosensitization. As the extent of nonconjugated unsaturation of the lipids is increased from 1 to 6 double bonds, the kinetics of depolarization become faster. We also found that the kinetics of depolarization is affected by the percentage of the unsaturated lipids in the liposome: as the fraction of the unsaturated lipids increases, the leakage through the membrane is enhanced. When liposomes are composed of a lipid mixture similar to that of natural membranes and photosensitization is being carried out under usual photodynamic therapy (PDT) conditions, photodamage to the lipids is not likely to cause enhanced permeability of ions through the membrane, which would have been a mechanism that leads to cell death.

Photosensitized effects of Rose Bengal on structure and function of lens protein "alpha-crystallin"

The conformational changes of the bovine lens protein "alpha-crystallin" have been investigated in the presence of the photosensitizer Rose Bengal (RB), in the dark as well as after visible light irradiation. Absorption and fluorescence emission spectra of RB [5 x 10(-6) M] and Fourier transform-IR spectra of alpha-crystallin [5 mg mL(-1)] were significantly altered upon RB alpha-crystallin complex formation. RB was found to bind to alpha-crystallin in a molecular pocket characterized by a low polarity, with Trp most likely involved in this interaction. The binding constant (K(b)) has been estimated to be of the order of 2.5 (mg/mL)(-1). The intrinsic fluorescence of alpha-crystallin was quenched through both dynamic and static mechanisms. Light-induced photosensitized effects showed structural modifications in alpha-crystallin, including tertiary and secondary structure (an increase in unordered structure) alterations. Notwithstanding those photoinduced structural variations detected in alpha-crystallin when complexed with RB, the protein still retains its ability to play the role of chaperone for beta-crystallin.

Comparison of photodynamic efficacy of tetraarylporphyrin pegylated or encapsulated in liposomes: in vitro studies

Two photosensitizing systems: (1) tetrakis(4-hydroxyphenyl)porphyrin (p-THPP) encapsulated in sterically stabilized liposomes (SSL) and (2) p-THPP functionalized by covalent attachment of poly(ethylene glycol) (p-THPP-PEG(2000)) were studied in vitro. The dark and photo cytotoxicity of these systems were evaluated on two cell lines: HCT 116, a human colorectal carcinoma cell line, and DU 145, a prostate cancer cell line and compared with these determined for free p-THPP. It was demonstrated that both encapsulation in liposomes as well as attachment of PEG chain result in pronounced reduction of the dark cytotoxicity of the parent porphyrin. The liposomal formulation showed higher than p-THPP-PEG(2000) photocytotoxicity towards both cell lines used in the studies.

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.

Which physical and structural factors of liposome carriers control their drug-loading efficiency?

The correlation between structural and physical properties of lipid membrane and its drug-loading efficiency were studied. The properties of bilayer were altered by incorporation of several lipidic modifiers: cholesterol, oleic acid, methyl oleate, and pegylated lipid. By using the molecular probe technique it was demonstrated that the membrane properties, such as micropolarity, microviscosity and free volume were considerably changed by incorporation of the modifiers. The partitioning of two different porphyrins between the bulk aqueous phase and the modified liposomes was studied using the fluorescence methods, and liposome-binding constants were determined. It was found that cholesterol reduced the partitioning of both porphyrins into liposomal bilayer. On the contrary, the incorporation of methyl oleate and pegylated lipid causes a pronounced increase in the value of the binding constants of both porphyrins. It was concluded that the free volume rather than hydrophobicity of bilayer is a governing factor in the solute partitioning into lipid bilayers.

On the correlation between hydrophobicity, liposome binding and cellular uptake of porphyrin sensitizers

A crucial factor in choosing a porphyrin or analogous photosensitizer for photodynamic therapy (PDT) is its ability to incorporate into the cells. For hydrophobic compounds that partition passively into the cytoplasmic membrane, a partition coefficient between an organic solvent and water, P, is one factor that could be used to predict the molecule's ability to diffuse into biomembranes. We synthesized several porphyrins, modified with two, three or four meso-substituents and studied their spectroscopic and photophysical properties. The octanol-water partitioning coefficients, log P, were calculated as a parameter for hydrophobicity. We found these porphyrins to be very hydrophobic, with log P values in the range of 8.9-11.8. These were correlated with the binding constants of these porphyrins into liposomes, K(b), as well as to their uptake by cells. The correlation between the estimated log P and K(b) is nearly linear but negative, indicating, apparently, that there is lesser binding to liposomes with increased hydrophobicity. On the other hand, all of the studied porphyrins are taken up by cells, but there is no clear correlation between cellular uptake and the log P or K(b). Lipinski's pharmacological "rule of 5" predicts poor permeation of drugs into cells when log P is greater than five. This may be relevant for diffusional binding to liposomes, where aqueous aggregation can interfere strongly with cellular uptake. In such extreme conditions, neither liposome binding nor other rules seem to predict porphyrin behavior in vitro.

Pegylated tetraarylporphyrin entrapped in liposomal membranes

A system of poly(ethylene glycol) bound tetraarylporphyrin entrapped in liposomal membranes was investigated. The interactions between the 5-(4-hydroxymethylphenyl)-10,15,20-tritolylporphyrin (Po) covalently attached to the poly(ethylene glycol) chain (PEG-Po), and phosphatidylcholine liposomes in the aqueous solution were studied. The adsorption of the investigated polymer to lipid vesicles was confirmed by measurements of dynamic light scattering and zeta potential. Experimental results demonstrate that the diameter of liposomes increased and the absolute value of the zeta potential decreased after addition of PEG-Po. The binding constants (K(b)) of Po chromophores to liposome in pH range from 5.2 to 9.0 were determined using fluorescence spectroscopy. The degree of binding was found to be pH-independent and the average value was 24.6 +/- 0.9 mg ml(-1). The acid-base properties of the porphyrin chromophores and their aggregation in an aqueous solution were also studied. pK values associated with imine-N protonation of the porphyrin core were found to be 2.59 and 0.68 at the ionic strength of 0.1 M. The equilibrium constant for dimerization, K(D), was found to be 5 x 10(3) M(-1).

Enhanced Acidity, Photophysical Properties and Liposome Binding of Perfluoroalkylated Phthalocyanines Lacking C-H Bonds

The acid-base, spectroscopic, photophysical and liposome-binding properties of the recently synthesized free base, 29H,31H,1,4,8,11,15,18,22,25-octafluoro-2,3,9,10,16,17,23, 24-octakisperfluoro(isopropyl) phthalocyanine, F64PcH2, are reported. The perfluoroalkylation of the phthalocyanine core renders the hydrogen atoms acidic, with a pK(a) = 6. The F64Pc(-2) dianion is detected already at pH 3, by singular-value decomposition analysis of electronic spectra. F64Pc(-2) generates 1O2 with quantum yields phi(delta) = 0.252 (in MeOH) and 0.019 in liposomes. Metallation of the Pc macrocycle to yield F64PcZn increases phi(delta) to 0.606 and 0.126 in MeOH and liposomes, respectively. Surprisingly, F64Pc(-2) (but not F64PcH2 or F64PcZn) binds strongly to liposomes, with a binding constant K(b) = 25 (mg/mL)(-1). The fully protonated F64PcH2, but not the zwitterionic F64Pc(-2), might favor hydrogen bonding, thus reducing its lipophilicity. Similarly, the Lewis acidity of Zn in F64PcZn, and thus its ability to bind water within a hydrophobic perfluoroalkyl pocket, is significantly enhanced by the fluorinated substituents.

Porphyrin depth in lipid bilayers as determined by iodide and parallax fluorescence quenching methods and its effect on photosensitizing efficiency

Photosensitization by porphyrins and other tetrapyrrole chromophores is used in biology and medicine to kill cells. This light-triggered generation of singlet oxygen is used to eradicate cancer cells in a process dubbed "photodynamic therapy," or PDT. Most photosensitizers are of amphiphilic character and they partition into cellular lipid membranes. The photodamage that they inflict to the host cell is mainly localized in membrane proteins. This photosensitized damage must occur in competition with the rapid diffusion of singlet oxygen through the lipid phase and its escape into the aqueous phase. In this article we show that the extent of damage can be modulated by employing modified hemato- and protoporphyrins, which have alkyl spacers of varying lengths between the tetrapyrrole ring and the carboxylate groups that are anchored at the lipid/water interface. The chromophore part of the molecule, and the point of generation of singlet oxygen, is thus located at a deeper position in the bilayer. The photosensitization efficiency was measured with 9,10-dimethylanthracene, a fluorescent chemical target for singlet oxygen. The vertical insertion of the sensitizers was assessed by two fluorescence-quenching techniques: by iodide ions that come from the aqueous phase; and by spin-probe-labeled phospholipids, that are incorporated into the bilayer, using the parallax method. These methods also show that temperature has a small effect on the depth when the membrane is in the liquid phase. However, when the bilayer undergoes a phase transition to the solid gel phase, the porphyrins are extruded toward the water interface as the temperature is lowered. These results, together with a previous publication in this journal, represent a unique and precedental case where the vertical location of a small molecule in a membrane has an effect on its membranal activity.

Interaction of dicarboxylic metalloporphyrins with liposomes. The effect of pH on membrane binding revisited

The acid-base properties of Zn-hematoporphyrin IX (ZnHP) and Zn-mesoporphyrin IX (ZnMP) and the effect of pH on their binding to liposomes have been studied. The ionization constants for the two carboxylate groups of ZnHP were calculated by principal component analysis and are 5.7 +/- 0.1 and 6.9 +/- 0.05. The neutral species and the mono- and dianionic forms all bind to liposomes, but a strong pH effect on the binding constant was observed for both the investigated compounds. We also observed a decrease in the binding of the two anionic species when the membranes carried a negative charge. These results indicate that the porphyrins partition into the membrane with their carboxylic moieties near the lipid-water interface so that their deprotonation, leading to a charged molecule, does not prevent the insertion of the tetrapyrrole ring into the lipid environment of neutral liposomes.

Do liposome-binding constants of porphyrins correlate with their measured and predicted partitioning between octanol and water?

We tested correlations between lipophilicity parameters and the partitioning of sensitizers into membranes. For this purpose we investigated 17 porphyrins and two chlorins having various chemical structures. Some of these compounds possess an amphiphilic structure (including hematoporphyrin, deuteroporphyrin, mesoporphyrin, chlorin e6 and more). The others are very symmetrical sensitizers [meso-tetra(N-methyl-4-pyridyl)porphyrin, tetra-benzoporphyrin, coproporphyrin I dihydrochloride (CP), meso-tetra(4-carboxyphenyl)porphyrin (TCP) and meso-tetra(m-hydroxyphenyl)chlorin]. Our investigation also included two series of hematoporphyrins and protoporphyrins with varying lengths of alkylcarboxylate side groups. The partitioning of these compounds between the bulk aqueous phase and liposomes was studied by fluorescence methods, and a liposome-binding constant, Kb, was obtained. It was found that CP and TCP do not incorporate into the lipid phase at pH 7.3. An n-octanol-water partition coefficient (log P) and a distribution coefficient (log D) were predicted with a modeling software. The values of log D were also obtained experimentally. We found that for the studied molecules, the predicted log D correlated well with the measured values. The values of log D as well as log P, in turn, did not correlate nicely, for the whole group of studied compounds, with the binding constants to liposomes. However, in the case of porphyrins that share a similar structure, the Kb showed good linear correlation with both log P and log D. For the series of hematoporphyrins and protoporphyrins with different lengths of alkylcarboxyl groups, it was shown that prolongation of this group caused an increase in the lipophilicity and the liposome-binding constant. This effect is more pronounced for the proto- than for the hematoporphyrin series. The results highlight the possible use, as well as limitations, of lipophilicity parameters for the prediction of membrane binding.

The depth of porphyrin in a membrane and the membrane's physical properties affect the photosensitizing efficiency

Photosensitized biological processes, as applied in photodynamic therapy, are based on light-triggered generation of molecular singlet oxygen by a membrane-residing sensitizer. Most of the sensitizers currently used are hydrophobic or amphiphilic porphyrins and their analogs. The possible activity of the short-lived singlet oxygen is limited to the time it is diffusing in the membrane, before it emerges into the aqueous environment. In this paper we demonstrate the enhancement of the photosensitization process that is obtained by newly synthesized protoporphyrin derivatives, which insert their tetrapyrrole chromophore deeper into the lipid bilayer of liposomes. The insertion was measured by fluorescence quenching by iodide and the photosensitization efficiency was measured with 9,10-dimethylanthracene, a fluorescent chemical target for singlet oxygen. We also show that when the bilayer undergoes a melting phase transition, or when it is fluidized by benzyl alcohol, the sensitization efficiency decreases because of the enhanced diffusion of singlet oxygen. The addition of cholesterol or of dimyristoyl phosphatydilcholine to the bilayer moves the porphyrin deeper into the bilayer; however, the ensuing effect on the sensitization efficiency is different in these two cases. These results could possibly define an additional criterion for the choice and design of hydrophobic, membrane-bound photosensitizers.

Liposome binding constants and singlet oxygen quantum yields of hypericin, tetrahydroxy helianthrone and their derivatives: studies in organic solutions and in liposomes

The spectroscopy and photophysics of several hypericin and helianthrone derivatives were studied in methanol and when bound to liposomes. The singlet oxygen quantum yields (phi(delta)) were measured indirectly relative to Rose Bengal and hematoporphyrin IX, employing 9,10-dimethylanthracene as a singlet oxygen trap. Hypericin was found to have a phi(delta) of 0.39+/-0.01 in methanol, and 0.35+/-0.05 in lecithin vesicles, in agreement with literature values. A heavy atom effect was evident upon bromination, resulting in phi(delta) for tetrabromohypericin of 0.72+/-0.02, presumably due to enhanced intersystem crossing. Elimination of the anionic hydroxyls by methylation also enhanced phi(delta) to 0.81+/-0.01. Conversely, addition of anionic sulfate groups drastically reduced phi(delta) resulting in phi(delta)'s of 0.12+/-0.01, 0.052+/-0.003 and 0.40+/-0.01 for hypericin disulfonate, hypericin tetrasulfonate and hexamethyl hypericin tetrasulfonate, respectively. The non-sulfonated helianthrones exhibited low phi(delta)'s in solution. The liposome binding constants, Kb, were measured using a spectroscopic assay. Except for hexamethyl hypericin, all non-sulfonated compounds bound well with Kb's ranging from 15.5+/-0.1 to 48.7+/-3.9 (mg/ml)(-1). None of the tetrasulfonated compounds bound, however the hypericin disulfonate had a Kb of 4.1+/-0.2 (mg/ml)(-1). The phi(delta)'s of the compounds capable of binding were measured and, in the case of the hypericin derivatives, were found not to vary dramatically from those in the free state. Liposome-bound helianthrone and dimethyl tetrahydroxy helianthrone both exhibited high phi(delta)'s, i.e. >0.5. The variations in binding constant and sensitization efficiencies are explained in conjunction with the molecular structure. The relevance of the above data to photodynamic therapy is briefly discussed.

Synthesis and investigation of glycosylated mono- and diarylporphyrins for photodynamic therapy

The synthesis of a diaryl substituted porphyrin bearing a galactosyl and a cholesteryloxy substituent and of a galactosyl substituted monoaryl porphyrin is described. The spectroscopic and aggregation properties of both compounds were investigated. The galactosyl substituted monoaryl porphyrin (12) was efficiently incorporated into liposomes and lipoproteins whereas the diaryl porphyrin showed no interaction with these lipids. Furthermore the binding constants of compound 12 to HDL, LDL, VLDL, and PE and DMPC liposomes were estimated.

Interaction of the fluorescent probe RH421 with ribulose-1,5-bisphosphate carboxylase/oxygenase and with Na+,K(+)-ATPase membrane fragments

Fluorescence titrations have shown that the voltage-sensitive probe RH421 interacts with the water-soluble protein ribulose-1,5-bisphosphate carboxylase/oxygenase and with Na+,K(+)-ATPase membrane fragments. The probe exhibits significantly different fluorescence excitation spectra in pure lipid and pure protein environments. Experiments with a range of polyamino acids showed interactions of the probe with tyrosine, lysine and arginine residues. At saturating RH421 concentrations (> or = microM) the probe quenches 60-75% of the total tryptophan fluorescence of the Na+,K(+)-ATPase preparation. Inhibition of the hydrolytic activity of the Na+,K(+)-ATPase occurs at RH421 concentrations in the micromolar range. This may be due to a probe-induced change in membrane fluidity. The sensitivity of the probe towards conformational changes of the Na+,K(+)-ATPase decreases hyperbolically as one increases the probe concentration. The decrease in sensitivity correlates well with association of the probe in the vicinity of membrane protein, as measured by tryptophan quenching. These results have important practical consequences for the application of RH421 as a voltage indicator in membrane preparations. Based on these and previously reported results, the fluorescent response of RH421 to the ATP-induced conformational change of the Na+,K+-ATPase is consistent with either a redistribution of dye from the liquid-crystalline lipid matrix into the vicinity of membrane protein or a reorganisation of the lipids surrounding the protein into a more rigid structure caused by the conformational change of the protein.

Spectroscopy and photosensitization of sapphyrins in solutions and biological membranes

A spectroscopic and photophysical study of three new sapphyrin molecules is presented. The sapphyrin backbone that was derivatized to make them water soluble possesses an absorption band around 700 nm, a desired property for biological photosensitization. We studied the absorption and fluorescence spectra, from which evidence for aggregation in solvents of different polarities was obtained. The extent of aggregation is correlated with the nature of the attached moiety. The absolute quantum yields of singlet oxygen production were measured, with 1,3-diphenyl isobenzofuran as a model target, and were 0.13-0.18 in ethanol. The binding constants to liposomes and to cells were determined spectroscopically and were found to correspond to the hydrophobicities of the compounds, with an additional effect, ascribed to the sugar moiety, which was found in the case of one of the sapphyrins. The efficiency of photodamage to Staphylococcus aureus by sapphyrins and hematoporphyrin was equivalent, on the basis of cells killed per microgram of sensitizer in the incubation mixture.

Charge- and pH-dependent binding sites for dibucaine in ionic micelles: a fluorescence study

Binding of micromolar concentrations of the local anesthetic dibucaine to micelles of cationic, zwitterionic and anionic detergents was studied using the fluorescence emission of dibucaine. Difference in quantum yields for charged and neutral dibucaine allowed to obtain shifts of pKa values due to binding. Estimates for the electrostatic potential affecting the tertiary amine of dibucaine were obtained from the pKa shifts. Change of fluorescence emission upon binding allowed to obtain the binding constants of both charged and neutral dibucaine to the micelles. The binding constant for the neutral form is essentially independent of micelle charge and of specific differences in detergent structure. Consistency between the ratio of neutral to cationic dibucaine binding constants and the measured pKa shift was tested. For LPC micelles complete agreement was found. For CTAC, however, the ratio of binding constants does not explain the pKa shift. The discrepancy between the results is used to estimate the errors involved upon neglecting non-coulombic electrostatic interactions of drugs to charged membrane surfaces. Fluorescence quenching with sodium iodide and nitroxide stearic acid derivatives allowed a depth profiling of the drug in the micelles.

Electric depolarization of photosensitized cells: lipid vs. protein alterations

We have monitored several photosensitized reactions in proteins, liposomes and cells under similar conditions. We found that the depolarization of K(+)-diffusion potential of liposomes or the leakage of an entrapped molecule, calcein, progress at a much slower rate than the photosensitized damage to proteins and the photosensitized killing of bacterial and leukemic cells. X-ray microanalysis revealed that upon light exposure of HP-treated leukemic cells and bacteria, they totally lost their cellular potassium. We deduce that the direct photosensitized oxidation of lipid components cannot cause the depolarization of cells, which in turn could be responsible for their death. A photosensitized damage to protein sites in the cell, probably in the membrane, is a more likely reason for the depolarization, the loss of potassium ions and cell death that is caused in light-activated photodynamic processes.