FLUORESCENCE CHARACTERISTICS OF INDOLES IN NON-POLAR SOLVENTS: LIFETIMES, QUANTUM YIELDS AND POLARIZATION SPECTRA

Abstract— Fluorescence lifetimes, quantum yields and polarization spectra were measured for indole, 3-methylindole and 2,3-dimethylindole in non-polar solvents. The results indicate simultaneous emission from thermally equilibrated ¹L_a and ¹L_b levels, with ¹L_a→¹ A dominating the 2,3-dimethylindole emission, and ¹L_b→¹ A dominating the indole emission. These results are consistent with previous assignments of the 0-0 transitions in absorption for these compounds. Radiative rates are: ¹L_a→¹ A , 2·0 × 10⁸ S^-1 and ¹L_b→¹ A . 0·62 → 10⁸ S^-1. In addition, the temperature dependence of the excitation and emission spectra are presented, which show that aggregation occurs with these indoles in hydrocarbons below approximately - 110°C. Possible applications to tryptophyl emission in the hydrophobic interiors of proteins are briefly discussed.     

Synthetic Zinc and Magnesium Chlorin Aggregates as Models for Supramolecular Antenna Complexes in Chlorosomes of Green Photosynthetic Bacteria

Abstract— A comparison of the spectra of in vitro (3-hydroxymethyl-13¹-oxometallochlorin) and in vivo chlorosomal (bacterio-chlorophyll- c ) aggregates suggests a similar supramolecular structure for the artificial oligomers and the bacte-riochlorophyll- c aggregates in the extramembranous antenna complexes (chlorosomes) of green photosynthetic bacteria. Synthetic zinc and magnesium chlorins have been found to aggregate in 1 % (vol/vol) tetrahydrofuran and hexane solutions and in thin films to form oligomers with the Q_y absorption bands shifted to longer wavelengths by about 1900 (Zn chlorins) and 2100 cm⁻¹ (Mg) relative to the corresponding monomer bands. Visible absorption and circular dichroism spectra of various zinc chlorins establish that a central metal, a 3¹-hydroxy and a 13¹-keto group are functional prerequisites for the aggregation. Vibrational bands measured by IR spectroscopy of solid films reveal two characteristic structural features of the oligomers: (1) a five-coordinated metallochlorin macrocycle with an axial ligand (bands at 1500-1630 cm⁻¹), and (2) a hydrogen bond between the keto oxygen of one chlorin and the hydroxy group of a second chlorin, the oxygen of which is chelated to the metal atom of a third molecule, i.e . C=O…H-O…M (=Zn or Mg).     

POLARIZED UV-ABSORPTION SPECTRA OF RETINAL ISOMERS-I. MEASUREMENTS IN EXTREMELY THIN MONOCRYSTAL PLATELETS

Abstract Crystals of all- trans retinal and both different forms of 11- cis , 12-s- cis retinal were grown on quartz slides with faces (101), (001) and (101), respectively, forming thin platelets of less than 0.2 μm thickness. Polarized UV absorption spectra at room temperature were measured in the range from 20 to 43 × 10³ cm⁻¹ with a microscope-spectrophotometer. In this spectral range three diffuse absorption bands were observed for all crystal types at similar wave numbers. A main absorption band was found at 25–28 × 10³ cm⁻¹, and two further bands at 32–34 and 38–40 × 10³ cm⁻¹. In case of all- trans retinal the latter band is by far the weakest in this spectral range. Additionally, the crystal spectrum of all- trans retinal shows a shoulder at the low wavenumber side of the main band which cannot be resolved in the corresponding solution spectrum. In the crystal spectra of 11- cis , 12-s- cis retinal, however, only a strong dissymmetry is observed at this side of the main band.     

POLARIZED UV-ABSORPTION SPECTRA OF RETINAL ISOMERS—II. ON THE ASSIGNMENT OF THE LOW AND HIGH ENERGY ABSORPTION BANDS

Abstract The polarized UV-absorption spectra of all- trans retinal and both crystal forms of 11- cis , 12-s- cis retinal (presented in the previous paper, Part I) are analyzed using Lowry-Hudson functions to describe the band profiles. The polarization ratios of the polarized bands is used to determine the direction of the corresponding transition moments. For all- trans retinal the polarization spectra show that the absorption between 23 and 36 X 10³ cm⁻¹ is caused by three overlapping bands labeled S, A and B. For 11- cis retinal the B-band is also clearly resolved whereas the S and A bands are separated with much less certainty than for all- trans retinal.
Comparing these bands with the excited state manifold resulting from semiempirical CI-calculations including double excitations, the S-band could be assigned to the ¹A_g⁻→¹A_g-* and the A-band to the ¹A_g⁻→¹B_u+* transition. However, no transition is found in this manifold which could positively be assigned to the B-band because the transitions predicted in this spectral region have negligible oscillator strengths. In all the crystal spectra a further band C is observed around 39 X 10³ cm⁻¹ which is particularly pronounced in the case of 11- cis retinal. For this band an assignment to the ¹A_g⁻→¹A_g+*-transition is proposed.     

Molecular Requirement of Chlorosomal Chlorophylls. Self-Organization of a Chlorophyll Derivative Possessing a Hydroxyl Group at Ring II

Aggregation of zinc 7¹-hydroxyl-13²-demethoxycarbonyl-pheophytin a (Zn-7¹-OH-Chl) was examined in relation to the structure and function of the self-aggregates of 3¹-OH-type chlorophylls (Chi) in chlorosomes of green photosynthetic bacteria. The Zn-7¹-OH-Chl aggregates yielded a Q_y absorption band at 712 nm with a 1.2-fold larger width (full width at half maximum, 500 cm⁻¹) than the monomer's (420 cm⁻¹). Infrared and NMR spectroscopies revealed that each molecule in the aggregate links together with simultaneous coordination (C7¹-OH…Zn) and hydrogen bonding (C7¹-OH … O=C13¹). A nonlinear alignment of the constituent molecules in the oligomeric structure was assumed. Despite the similar molecular linkages, linearly aligned Q_y, moments in the Zn-3¹-OH-Chl aggregate gave a chlorosome-like broader, more redshifted Q_y band (740 nm; 670 cm⁻¹, 2.1-fold larger than the monomer's). Because it is advantageous for efficient light harvesting and energy transfer to have several Q_y, spectral components, spread over a wide spectral range, that can act as the energy gradient, it is concluded that not only the intermolecular linkages but the linear locations of OH, C=0 and Mg in the molecule are crucial for photosynthetic antenna of the self-assembled chiorosomal Chl.     

Measurement of Tumor Vascular Damage in Mice with 99m Tc-MIBI Following Photodynamic Therapy

Abstract— The clinical perfusion agent ^99mTc-MIBI was used to monitor changes in tumor vascular perfusion (TVP) induced by Photofrin® (Pll)-mediated photodynamic therapy (PDT). BALB/c mice bearing an EMT-6 tumor on each hind thigh were given an intravenous injection of 1, 2 or 5 mg kg⁻¹ PII. Twenty-four hours later, one tumor was illuminated (600–650 run, 200 mW cm⁻² 400 J cm⁻²) while the other served as a control. At various time intervals after PDT (0, 2 and 24 h) mice received an intravenous injection of ^99mTc-hexakismethoxy(sobutyusonitri-le (MIBI) (0.18 MBq g⁻¹) and were sacrificed 2 min later. The light-treated and the untreated tumors were then dissected, the radioactivity was counted and the percentage of the injected dose per gram of tumor (%ID g⁻¹) was calculated as a measure of TVP. We observed that TVP is drug dose dependent, develops progressively with time post-PDT and is inversely related to PDT efficacy. Our data show that early tumor retention of ^99mMIBI is a simple method to assess TVP and vascular damage induced by PDT.     

DOES LOW-INTENSITY He-Ne LASER RADIATION PRODUCE A PHOTOBIOLOGICAL GROWTH RESPONSE IN Escherichia coli?

Abstract A photobiological study was camed out on the bacterium Escherichia coli in order to determine whether stimulation of growth occurred after irradiation of an inoculum with coherent red light. No enhancement or inhibition of growth was observed for cultures of the bacterium following irradiation of inocula with a Helium-neon laser (continuous wave, λ= 632.8 nm) at irradiances of 7.7 × 10¹⁵ and 1.8 × 10¹⁶ photons cm⁻² s⁻¹ using fluences of 4.5 × 10^−-1 and 4.5 J cm⁻² at each irradiance. Bacterial growth in irradiated and control cultures was monitored during a growth period of ca 2 h using a viable count technique after inocula in the early exponential phase had been diluted with fresh growth medium. These results do not provide support for the work of Kam et al . (1983, Nuov. Cim . 2D, 1138–1144), and Tiphlova and Karu (1988, Photochem. Photobiol . 48 , 467–471), which appear to show substantial enhancement of E. coli growth under these conditions.     

Fast Redox Perturbation of Aqueous Solution by Photoexcitation of Pyranine

Abstract— Intense illumination (60-120 MW/cm²) of an oxygen-free aqueous solution of pyranine (8-hydroxypyrene-l,3,6-tri-sulfonate) by the third harmonic frequency of an Nd-Yag laser (355 nm) drives a two successive-photon oxidative process of the dye. The first photon excites the dye to its first electronic singlet state. The second photon interacts with the excited molecule, ejects an electron to the solution and deactivates the molecule to a ground state of the oxidized dye (φ⁺). The oxidized product, φ⁺, is an intensely colored compound (Λ_max= 445 nm, ε= 43 000 ± 1000 M ⁻¹ cm⁻¹) that reacts with a variety of electron donors like quinols, ascorbate and ferrous compounds. In the absence of added reductant, φ⁺ is stable, having a lifetime of -10 min. In acidic solutions the solvated electrons generated by the photochemical reaction react preferentially with H⁺. In alkaline solution the favored electron acceptor is the ground-state pyranine anion and a radical, φ, of the reduced dye is formed. The reduced product is well distinguished from the oxidized one, having its maximal absorption at 510 nm with e = 25 000 ± 2000 M^-l cm⁻¹. The oxidized radical can be reduced either by φ^- or by other electron donors. The apparent second-order rate constants of these reactions, which vary from 10⁶ up to 10⁹M⁻¹ s⁻¹, are slower than the rates of diffusion-controlled reactions. Thus the redox reactions are limited by an energy barrier for electron transfer within the encounter complex between the reactants.     

PHOTOPHYSICAL PROPERTIES OF NUCLEIC ACID COMPONENTS—1. THE PYRIMIDINES: THYMINE, URACIL, N, N-DIMETHYL DERIVATIVES AND THYMIDINE

Abstract— Low-temperature (and some room temperature) absorption and emission, fluorescence and phosphorescence, data including quantum yields and lifetimes have been obtained from the title pyrimidine bases as a function of the nature of the solvent environment. Modest vibrational resolution has been observed for the first time in the absorption spectra, particularly for thymine and uracil. The excitation spectra also show structure. The quantum yields of fluorescence (φ_F) and phosphorescence are independent of the excitation wavelength. Thymine, thymidine and uracil have profoundly different photophysical properties in polar-aprotic vs polar-protic solvents. The N, N-dimethyl substitution of thymine and uracil produces photophysical changes comparable to the solvent change for the unsubsti-tuted bases. The species involved in the emission processes is the keto (lactam) form. It is probable that ^1,3(n,π*) state(s) has(have) changed order relative to a lowest ¹(π,π*) state as a consequence of both the solvent change and N, N-dimethyl substitution. The lowest triplet state is assigned as ³(n π*). We propose that an important factor contributing to the previously reported excitation wavelength dependence of φ_F and φ_T1 (φ_isc) for nucleic-acid components is the equilibrium coexistence of H-bonded and non-H-bonded forms each having different photophysical properties. Consideration is given of the impact of the significantly different photophysical properties of nucleic-acid bases as a function of the nature of the solvent upon the photochemical properties.     

EXCITATION ENERGY TRANSFER IN THE CRYPTOPHYTES. FLUORESCENCE EXCITATION SPECTRA AND PICOSECOND TIME-RESOLVED EMISSION SPECTRA OF INTACT ALGAE AT 77 K

Excitation spectra of chlorophyll- a (Chl- a ) fluorescence in intact cells of Cryptomonas ovata, Chroomonas pauciplastida and Chroomonas salina were determined at 77 K. For all species the excitation spectra for emission from Chl- a associated with photosystem II (PSII) showed increased contributions by a carotenoid (493 nm) and phycobiliproteins, and decreased contributions by carotenoid (417 nm, 505 nm) and Chl- a (445 nm) as compared to excitation spectra for emission from Chl- a associated with photosystem I (PSI). Excitation spectra of C. salina and C. ovata showed an increased contribution by Chl- c ² to PSII Chl- a fluorescence emission. In all three species the absorbance band positions of Chl- a , as determined from the excitation spectra, were similar to those previously described in green plants. green algae and phycobilisome-containing organisms. Time-resolved 77 K fluorescence emission spectra of C. ovata and C. salina showed successive emission from both phycoerythrin and Chl- c ², PSII Chl- a , and PSI Chl- a. C. pauciplastida showed successive emission from phycocyanin, PSII Chl- a , and PSI Chl- a. Spectral red-shifts with time were observed for the phycobiliprotein peaks in all three species. The fluorescence decay of phycoerythrin in C. ovata and C. salina was faster than that of phycocyanin in C. pauciplastida. The results are discussed in relation to the organization of the antenna pigments of PSII and PSI in the cryptophyte algae.     

LIGHT-INDUCED FOURIER TRANSFORM INFRARED SPECTROSCOPIC INVESTIGATIONS OF THE INTERMEDIARY ELECTRON ACCEPTOR REDUTION IN BACTERIAL PHOTOSYNTHESIS

Abstract— Molecular changes associated with the light-induced reduction of the intermediary electron acceptor I (bacteriopheophytin, BPh) in bacterial photosynthesis were studied by means of Fourier transform infrared (FTIR) difference spectroscopy. Chromatophore membranes and reconstituted reaction centers (RCs) of Rhodopseudomonas viridis were prereduced with sodium dithionite and illuminated in order to trap photochemically the state I⁻. Fourier transform infrared spectra of these samples were recorded before, during and after illumination, with an accuracy better than 10⁻³ absorbance units. Difference spectra of I⁻ in chromatophores and in RCs closely correspond to each other. In the carbonyl stretching frequency region between 1640 and 1750 cm⁻¹, bands are tentatively attributed to a shift (from 1713 to 1683 cm⁻¹) of a keto carbonyl group, a change of an acetyl carbonyl grou at 1656 cm⁻¹ and a decrease in absorbance strength of ester carbonyl groups (at 1746 and 1732 cm⁻P) after reduction of I. These groups likely belong to the BPh molecule, although at least one of the ester carbonyls could be assigned to an amino acid side chain. The absence of strong bands in the amide I and amide II region excludes large protein conformational changes associated with I reduction.     

INVESTIGATION OF THE ALLOMERIZATION REACTION OF CHLOROPHYLL a: USE OF DIODE ARRAY HPLC, MASS SPECTROMETRY AND CHEMOMETRIC FACTOR ANALYSIS FOR THE DETECTION OF EARLY PRODUCTS

Abstract –The products of chlorophyll allomerization in methanol were isolated and analyzed by open column sucrose chromatography, liquid chromatography mass spectrometry (LCMS) and DAD-HPLC (diode-array high-performance liquid chromatography). Four main bands were found with molecular ions of (a) 908, (b) 938, (c) 938 and (d) 938, consistent with the structures (a) 13²-hydroxy-chlorophyll a (II), (b) and (c) Mg(II)-3¹,3²-didehydro-15¹-hydroxy-15¹-methoxy-rhodochlorin-15 acetic acid δ-lactone 15²-methyl 17³-phytyl ester and its epimer (III) and (d) Mg(II)-3¹,3²-didehydro-rhodochlorin-15-glyoxylic acid 13¹,15²-dimethyl 17³-phytyl ester (IV), evidence enhanced by UV/visible spectroscopy, chromatographic coelutions and chemometrics. Chlorophyll a was degraded both in the dark and light, under O₂ and N². DAD-HPLC of the resultant degradation mixtures were analyzed using the chemometric heuristic-evolving latent projection method for resolution. Ultraviolet/visible spectra of II and III are reproducibly extracted from the mixtures after a short degradation time, whereas III and IV are the dominant compounds after longer degradation times. Changes in relative elution order of IV using open column chromatography and reverse-phase HPLC are established. A possible allomerization pathway is proposed.     

FLUORESCENCE OF AZINES

Abstract— A comparison of the transient absorption spectra from the photolysis of disulfides in solution suggests that C-S bond breakage is a common primary photolytic process. This process becomes more important as the resulting carbon centered radical is stabilized by increasing alkyl substitution or resonance interaction with an aromatic system. The perthiyl radical product is characterized by λ_max∽380 nm,ε₃₈₀∽1700 M ⁻¹ cm⁻¹ and decays by second order kinetics with k ₂∽3.7×10⁸ M ⁻¹ s⁻¹ in water.
In the presence of O₂, the photolysis of disulfides which produce the thiyl radical give transient absorptions in the 500–600 nm region. Possible identities of these transients are discussed.     

LIGHT INDUCED FLUORESCENCE SPECTRAL CHANGES IN NATIVE PHYTOCHROME FROM Secale cereale L. AT LIQUID NITROGEN TEMPERATURE

Abstract— Fluorescence spectra of native rye phytochrome were determined under different light conditions at liquid nitrogen temperature. Fluorescence spectrum of the red-light-absorbing form (Pr) had a major peak at about 685 nm (14 600 cm⁻¹) and a broad sub-peak at about 515 nm (19 400 cm⁻¹). The peak height at 685 nm was reduced by irradiation with monochromatic light of 640 nm, and a new peak became obvious at about 702 nm (14250 cm⁻¹). This spectral change was almost completely reversed by subsequent irradiation with 700-nm light. Fluorescence spectrum of the photoequilibrium mixture of Pr and far-red-light absorbing form under continuous red light showed a sharp peak at about 685 nm having a peak height ca. 12% of Pr, and a broad sub-peak at about 508 nm (19 700 cm⁻¹). Light of 730 nm did not reduce the peak height at about 685 nm but induced a new shoulder at about 699 nm (14300 cm⁻¹). Monochromatic light of 640 and 700 nm given following the light of 730 nm could not reverse the spectral change at 699 nm induced by the irradiation with 730-nm light. Fluorescence spectrum of Pr in partially degraded phytochrome was similar to that in native phytochrome but the peak position in the red region was shifted by about 5 nm (100 cm⁻¹) to the blue.     

PHOTOBLEACHING OF A CYANINE DYE IN SOLUTION AND IN MEMBRANES

Abstract— N,N'-bis(2-ethyl-1,3-dioxolane)-kryptocyanine (EDKC), a lipophilic dye with a delocalized positive charge, photosensitizes cells to visible irradiation. In phosphate-buffered saline (PBS), EDKC absorbs maximally at 700 nm (ε= 1.2 × 10⁵ M⁻¹ cm⁻¹) and in methanol, the absorption maximum is at 706 nm (ε= 2.3 × 10⁵ M⁻¹ cm⁻¹). EDKC partitions from PBS into small unilamellar liposomes prepared from saturated phospholipids and into membranes prepared from red blood cells (RBC) and binds to human serum albumin (HSA). The EDKC fluorescence maximum red shifts from 713 nm in PBS to 720–725 nm in liposomes and RBC membranes and the fluorescence intensity is enhanced by factors of 14–35 compared to PBS (φ= 0.0046). EDKC is thermally unstable in PBS (T_1/2= 2 h at 1.3 × 10⁻⁵ M EDKC), but stable in methanol. In liposomes and RBC membranes, EDKC is 10 times more stable than in PBS, indicating that it is only partially exposed to the aqueous phase. Quenching of EDKC fluorescence in liposomes and RBC membranes by trinitrobenzene sulfonate also indicates that EDKC is not buried within the membranes. Photodecomposition of EDKC was oxygen-dependent and occurred with a low quantum yield (6.4 × 10⁻⁴ in PBS). Singlet oxygen was not detected upon irradiation of EDKC in membranes or with HSA since the self-sensitized oxidation of EDKC occurred at the same rate in D₂O as in H₂O and was not quenched by sodium azide or histidine.     

ULTRASTRUCTURAL STUDIES ON THE MECHANISM OF THE PHOTODYNAMIC THERAPY OF TUMORS

Abstract Balb/c mice bearing a transplanted MS-2 fibrosarcoma were injected with 2.5 mg kg ¹ of either tetra(4-sulfonatophenyl/porphine (TPPS) in phosphate-buffered saline or 0.5 mg kg⁻¹ of Zn²⁺-phthalocyanine (Zn-Pc) incorporated into unilamellar liposomes of dipalmitoyl-phosphatidylcholine. Chromatographic studies showed that TPPS is mainly transported in the serum by globulins and albumin, while Zn-Pc is specifically bound by lipoproteins. Exposure of the injected mice to red light (300 J cm⁻²) caused extensive tumor necrosis. The ultrastructural analysis of tumor specimens taken from mice at 15 h after PDT showed that TPPS photoinduces a preferential necrosis of the neoplastic cells, while Zn-Pc causes severe photodamage to both the vascular system and the neoplastic cells. The different modes of tumor photosensitization by TPPS and Zn-Pc are discussed on the basis of the transport mechanism of the two dyes.     

FLUORESCENCE OF THYMINE IN AQUEOUS SOLUTION AT 300° K

Abstract— –Fluorescence of thymine in neutral aqueous solution at room temperature has been detected using the multiscaling operation of a multichannel analyzer. The emission maximum (2.96 μm^-1) and 0-0 transition energy (3.37-3.45 μm^-1) are close to those determined at liquid nitrogen temperature in mixed solvents. The quantum efficiency of fluorescence excited at 3.77 μm^-1 is calculated to be 1.04 × 10^-4.
The corrected relative excitation spectrum shows significant differences from the absorption spectrum when both are determined under identical conditions of concentration and spectral bandwidth on the same instrument. The quantum yield of fluorescence decreases about 2-fold as the energy of excitation is increased beyond the 0-0' transition and follows the relation 1/φ°α E _excit..
This behavior is discussed in terms of (a) n π* and ππ* states, (b) emission from a minor tautomer and (c) kinetics of competing deactivation processes.     

Physicochemical Studies of Demetalation of Light-harvesting Bacteriochlorophyll Isomers Purified from Green Sulfur Photosynthetic Bacteria

Demetalation kinetics of bacteriochlorophylls (BChls) c, d and e from green sulfur photosynthetic bacteria were studied under weakly acidic conditions. Demetalation rate constants of BChl e possessing a formyl group at the 7-position were significantly smaller than those of BChls c and d , which had a methyl group at this position. The activation energy of demetalation of 3¹ R -8,12-diethyl([E,E])-BChl e was 1.5-times larger than that of 3¹ R -[E,E]-BChl c . ¹⁵N-labeled 3¹ R -[E,E]-BChls c and e were purified from cells of green sulfur bacteria grown in a medium containing ¹⁵NH₄Cl, and their ¹⁵N NMR spectra were measured. The chemical shifts of N₂₁, N₂₂ and N₂₃ atoms of 3¹ R -[E,E]-BChl e were lower-field shifted than those of 3¹ R -[E,E]-BChl c , respectively, and especially the difference in chemical shifts of N₂₂ was significantly large. These results suggest that the electron-withdrawing formyl group at the 7-position of BChl e affected an electronic state of the chlorin macrocycle and caused BChl e to be more tolerant for removal of the central magnesium compared with BChls c and d .     

ACTINOMETRY IN MONOCHROMATIC FLASH PHOTOLYSIS: THE EXTINCTION COEFFICIENT OF TRIPLET BENZOPHENONE AND QUANTUM YIELD OF TRIPLET ZINC TETRAPHENYL PORPHYRIN

Abstract— The extinction coefficient ε_T, of triplet benzophenone in benzene has been directly determined by absolute measurements of absorbed energy and triplet absorbance, Δ D ⁰_T, under demonstrably linear conditions where incident excitation energy, E ₀, and ground state absorbance, A ₀, are both extrapolated to zero. The result, 7220 ± 320 M ^-1 cm^-1 at 530 nm, validates and slightly corrects many measurements relative to benzophenone of triplet extinction coefficients made by the energy transfer technique, and of triplet yields obtained by the comparative method.
As E ₀ and A ₀ both decrease, Δ D ⁰_T becomes proportional to their product. In this situation, the ratio R = (1/ A ₀)(dΔ D ⁰_T/d E ₀) = (ε_T - ε_G)φ_T. Measurements of R , referred to benzophenone, give (ε_T - ε_G)φ_T for any substance, without necessity for absolute energy calibration.
Both absolute and relative laser flash measurements on zinc tetraphenyl porphyrin (ε_T - ε_G at 470 nm = 7.3 × 10⁴ M ^-1 cm^-1) give φ_T= 0.83 ± 0.04.     

PHOTODYNAMIC THERAPY OF B16 PIGMENTED MELANOMA WITH LIPOSOME-DELIVERED Si(IV)-NAPHTHALOCYANINE

The possibility of extending photodynamic therapy to the treatment of highly pigmented neoplastic lesions was tested by using Si(IV)-naphthalocyanine (SiNc) as a tumor-localizing agent. Si(IV)-naphthalocyanine displays intense absorbance at 776 nm (ɛ= 5 × 10⁵ M⁻¹ cm⁻¹), where melanin absorption becomes weaker. As an experimental model we selected B16 pigmented melanoma subcutaneously transplanted to C57BL mice. Upon injection of 0.5 or 1 mg kg⁻¹ of liposome-incorporated SiNc, maximal accumulation of the photosensitizer in the tumor was observed at 24 h with recoveries of 0.35 and 0.57 μg g⁻¹, respectively. However, the tumor targeting by SiNc shows essentially no selectivity, since the photosensitizer concentrations in the skin (peritumoral tissue) were very similar to those found in the tumor at all postinjection times examined by us. Irradiation of SiNc-loaded melanoma with 776 nm light from a diode laser at 24 h postinjection induces tumor necrosis and delay of tumor growth. The effect appears to be of purely photochemical nature at dose rates up to 260 mW cm⁻²; at higher dose rates, thermal effects are likely to become important.