A review of recent literature on volume-conserving cis–trans photoisomerization reaction mechanism, including hula twist, is presented. Differences between substrates trapped in amorphous solids and chromophores that are protein bound are discussed.

We show by way of physical organic reasoning that the currently known photochemical results of the chromophore of photoactive yellow protein (PYP) are consistent with that expected of a least volume-demanding process for an anchored, tethered chromophore. The primary photoreaction, interestingly, does not appear to involve a hula-twist process. However, the latter might be involved during subsequent transition of dark intermediates. Absorption data of intermediates obtained from a microsecond time-resolved spectroscopic study of three PYP mutants (E46Q, T50V and R52Q) are consistent with the above analyses.

E/Z-photoisomerizations within molecular crystals are varied. Existing cases are summarized. They require crystal lattices that allow for long-range molecular movements in the nontopotactic solid-state mechanism. Reactivity and directionality can be foreseen on the basis of the crystal packing. The reacting crystal changes continuously by phase rebuilding, phase transformation and disintegration. Two possibilities for the chemical mechanism exist: (1) highly space-demanding (cooperative) double-bond rotations; and (2) space-conserving hula-twist (HT) motions while the substituents move within their planes and only one C—H unit undergoes out-of-plane translocation. If internal rotation cannot be reasonably modeled within the crystal lattice, HT remains the only choice, as in the case of trans-1,2-dibenzoylethene. Direct experimental proof is still lacking because the differences in the conformational outcome could not be assessed in the studied examples. Density functional theory calculations of cis-1,2-dibenzoylethene revealed very low differences in energy content of the helical (s-cis, s-cis)- and the almost orthogonal (s-cis, s-trans)-cis-conformers. The almost orthogonal (s-cis, s-trans)-cis-conformer that is found in the pure crystal is very similar to the calculated counterpart. It is suggested that more favorable initial conformers might be obtained by proper vinylic substitution. The stereochemical outcome of highly space-demanding thermal vinylic-bond rotations followed by cyclizations of conjugated bisallenes to give bismethylene cyclobutenes excludes the alternative HT mechanism (double-bond isomerization) in the present cases. But space-conserving HT might be a mechanistic alternative in less-substituted cases under photoexcitation. The stereochemical consequences are discussed.

Highly branched stilbene dendrimers were synthesized and their photochemical behavior was studied. Even the stilbene dendrimer with molecular weight over 6500 underwent trans–cis isomerization in the excited singlet state within the lifetime of 10 ns. The photoisomerization of C=C double bond of stilbene dendrimers in the excited state may proceed by a volume-conserving novel mechanism such as hula-twist rather than conventional 180° rotation around the C=C double bond based on fluorescence and isomerization experiments.

Ring-fused retinal analogs were designed to examine the hula-twist mode of the photoisomerization of the 9-cis retinylidene chromophore. Two 9-cis retinal analogs, the C11–C13 five-membered ring–fused and the C12–C14 five-membered ring–fused retinal derivatives, formed the pigments with opsin. The C11–C13 ring-fused analog was isomerized to a relaxed all-trans chromophore (λ_max > 400 nm) at even −269°C and the Schiff base was kept protonated at 0°C. The C12–C14 ring-fused analog was converted photochemically to a bathorhodopsin-like chromophore (λ_max = 583 nm) at −196°C, which was further converted to the deprotonated Schiff base at 0°C. The model-building study suggested that the analogs do not form pigments in the retinal-binding site of rhodopsin but form pigments with opsin structures, which have larger binding space generated by the movement of transmembrane helices. The molecular dynamics simulation of the isomerization of the analog chromophores provided a twisted C11–C12 double bond for the C12–C14 ring-fused analog and all relaxed double bonds with a highly twisted C10–C11 bond for the C11–C13 ring-fused analog. The structural model of the C11–C13 ring-fused analog chromophore showed a characteristic flip of the cyclohexenyl moiety toward transmembrane segments 3 and 4. The structural models suggested that hula twist is a primary process for the photoisomerization of the analog chromophores.

The potential role of “H–vinyl” conical intersections in a photochemical process known as the hula twist has been evaluated. The H–vinyl conical intersections of butadiene are explored with complete active space self-consistent field calculations, and proposals about the geometries involved in the hula-twist mechanism are discussed and contrasted with the conventional one-bond flip mechanism of cis–trans isomerization.

High-level ab initio calculations show that the singlet photochemical cis–trans isomerization of organic molecules under isolated conditions can occur according to two distinct mechanisms. These mechanisms are characterized by the different structures of the conical intersection funnels controlling photoproduct formation. In nonpolar (e.g. hydrocarbon) polyenes the lowest-lying funnel corresponds to a (CH)₃ kink with both double and adjacent single bonds twisted, which may initiate hula-twist (HT) isomerization. On the other hand, in polar conjugated systems such as protonated Schiff bases (PSB) the funnel shows a structure with just one twisted double bond. The ground-state relaxation paths departing from the funnels indicate that the HT motion may take place in nonpolar conjugated systems but also that the single-bond twist may be turned back, whereas in free conjugated polar molecules such as PSB a one-bond flip mechanism dominates from the beginning. The available experimental evidence either supports these predictions or is at least consistent with them.

In this article we have examined the very low-temperature photochemistry of three acyclic 1,3-dienes. We have used high-temperature deposition techniques combined with matrix isolation to create samples enriched with the thermally meta-stable s-cis form. This technique has allowed us to examine the separate photochemistry of the s-cis and s-trans conformers. Our results suggest the presence and the absence of barriers on the excited-state surface. In particular, we have found that the electrocyclic closure and s-cis–s-trans photochemical isomerization stops at 15 K for 2,3-dimethyl-1,3-butadiene-d₁₀. The closure occurs at higher temperatures in solution but is slowed by a deuterium isotope effect. The s-trans conformer of EE-2,4-hexadiene shows almost no photoreactivity in a matrix under 254 nm irradiation, but the s-cis conformer is rapidly converted to ZE-2,4-hexadiene (ZE-HXD). The photoreactivity of ZE-HXD is similar in that there is a relatively quick conversion of the s-cis conformer under these conditions, with only a very slow conversion of the s-trans to photoproducts.

The OGG1 proteins are DNA N-glycosylases–apurinic-apyrimidinic lyases that are responsible for the removal of 8-oxo-7,8-dihydroguanine (8-oxoG) base in DNA. The human enzyme (hOGG1) is a monomer of 345 amino acids containing 10 buried tryptophan (Trp) residues that are very sensitive to UVB irradiation. The photolysis quantum yield of these Trp residues is about 0.3 and 0.1 in argon- and air-saturated solutions, respectively. Matrix-assisted laser desorption-ionization–time-of-flight mass spectrometry shows that several cleavage sites are identical under aerobic and anaerobic photolysis of Trp residues; one of them includes the active site. Western blots and polyacrylamide gel electrophoresis indicate that fragments of high molecular size are also formed. In addition to common photochemical paths with argon-saturated solutions, specific reactions occur in air-saturated solutions of hOGG1. The photolysis rate is inhibited by more than 50% on binding of hOGG1 to a 34mer oligonucleotide containing a single 8-oxoG–C base pair. Binding to the oligonucleotide with 8-oxoG–C induced a 20% quenching of the hOGG1 fluorescence, suggesting interaction of nucleic acid bases with the Trp residue(s) responsible for the photolysis. Using 2,6-diamino-4-hydroxy-5-N-methylformamidopyrimidine (Me-FapyG) and 8-oxoG as substrates, it is shown that protein photolysis induces photoinactivation of the DNA N-glycosylase activities. The excision of 8-oxoG is more affected than that of Me-FapyG at the same dose of UVB irradiation under both air and argon conditions. Besides the role of Trp residues, the possible involvement of Cys 253 in the photoinactivation process of hOGG1 is discussed.

The photolysis (>300 nm) of ochratoxin A (OTA, N-{[(3R)-5-chloro-8-hydroxy-3-methyl-1-oxo-7-isochromanyl]carbonyl}-3-phenyl-L-alanine, 1) in the presence of excess (2 and 12 molar equiv) cysteine (CySH) has been investigated and found to yield sulfur adducts 5 and 6 that are characterized by liquid chromatography–mass spectrometry and ¹H-NMR spectroscopy. The adduct 5 was ascribed to the Michael addition conjugate resulting from covalent attachment of CySH to the ochratoxin quinone (4) generated by photooxidation of OTA. This species was also formed by photolysis of a synthetic sample of the hydroquinone of OTA (ochratoxin hydroquinone, 3) in the presence of 12 equiv L-CySH. The conjugate 5 derived from photolysis of 3 with L-CySH was used for ¹H-NMR analysis. The sulfur adduct 6 was the major species detected from covalent attachment of CySH to photoactivated OTA, and it resulted from direct displacement of the OTA Cl atom by CySH. The implications of the cysteinyl adducts to the in vivo toxicity of OTA are discussed, with particular emphasis given to conjugate 5, as products from the photooxidative pathway may be of relevance to the nephrotoxic properties of OTA.

Peroxidizability of fatty acids in the air is roughly proportional to the number of double bonds, but in vivo peroxidation proceeds in a more complex manner. Here, we compared the effects of dietary and topically applied oils enriched with linoleic acid (LA, 18:2n-6) or alpha-linolenic acid (ALA, 18:3n-3) on UV-induced skin injury in a strain of hairless mice. The UVB-induced erythema score was significantly lower in mice with topically applied creams containing LA and ALA than in mice with the basal cream; no significant increase in the score was detected in the ALA group compared with the LA group. However, dietary ALA inhibited the increase in erythema score after UVB irradiation compared with LA. The peroxidizability index of the skin total lipids was significantly higher, but UVB-induced prostaglandin E2 (PGE2) production was significantly lower in the group fed an ALA-rich diet compared with the group fed an LA-rich diet. The levels of thiobarbituric acid–reactive substances, estimated in the presence of butylated hydroxytoluene in the assay mixture, were not affected by UVB treatment or by the dietary fatty acids, but the severity of the skin lesion was associated with PGE2 levels. These results indicate that the type of fatty acids, n-6 or n-3, is critical for the suppression of UVB-induced skin lesion when the skin fatty acids are modified by dietary manipulation. Anti-inflammatory activity of dietary flaxseed oil with relatively high ALA and low LA contents was demonstrated in UVB-irradiated hairless mice.

It is well known that UV radiation contributes to the development of skin cancer. Exposure to solar radiation is predominantly responsible for the high incidence rate of skin cancer, but there are also indications that sunbeds are involved. The aim of the present investigation was to determine the UV emission spectra of sunbeds. It included the most common sunbed models, which cover more than 50% of the Swiss market. The UV emission spectra of sunbeds have special characteristics and are different from the sun spectrum, which can be seen in high-resolution spectral measurements. Sunbed emission spectra are similar to the sun spectrum in the UVB (280–320 nm) range but reach values 10 to 15 times higher in the UVA (320–400 nm) range. An average erythema-effective irradiance of 0.33 W/m² was determined for sunbeds. This corresponds to a UV index of 13, which is significantly higher than the UV index of 8.5 of the high summer sun at noon at intermediate latitudes. The measurements were spread over the whole effective area of the sunbeds, and an inhomogeneous distribution of the irradiances with variations of up to 30% from the average value was found.

We have developed a UV monitor with polycrystalline (poly–)gallium nitride (GaN) UV sensors and evaluated its performance from the viewpoint of its effectiveness for use with photosensitive dermatosis patients. The poly-GaN UV sensor is sensitive to UV light from 280 to 410 nm even without optical filters. The UV monitor is a portable self-data-acquisition instrument with a minimum detection level (defined as average UV intensity over 290 to 400 nm) of 2 μW/cm² and can store UV dose data for 128 days. It allows easy measurement of four orders of magnitude of ambient UV intensity and dose from indoor light to direct solar radiation in summer. Trial use of the UV monitor by five xeroderma pigmentosum patients started in June 2000 and was carried out for 1 year. It was demonstrated that the UV monitor was useful in improving their quality of life.

Halogenated squaraine dyes are characterized by long wavelength absorption (>600 nm) and high triplet yields and therefore represent new types of photosensitizers that could be useful for photodynamic therapy. We have analyzed the cytotoxicity and genotoxicity of the bromo derivative 1, the iodo derivative 2 and the corresponding nonhalogenated dye 3 in the absence and presence of visible light. At concentrations of 1–2 μM, 1 and 2 reduced the cloning efficiency of AS52 Chinese hamster ovary cells to less than 1% under conditions that were well tolerated in the dark. Similarly, the proliferation of L5178Y mouse lymphoma cells was inhibited by photoexcited 1 and 2 with high selectivity. The squaraine 3 was much less efficient. Both 1 and 2 induced only few mutations in the gpt locus of the AS52 cells in the presence of light and were not mutagenic in the dark. No mutagenicity with and without irradiation was observed in Salmonella typhimurium TA100 and TA2638. However, both 1 and 2 plus light increased the frequency of micronuclei in AS52 cells. The results indicate that halogenated squaraines exhibit photobiological properties in vitro that are favorable for photodynamic therapeutical applications.

Protoporphyrin IX dimethyl ester (PME), a dimethyl esterification of protoporphyrin IX (PpIX), exhibits higher intracellular uptake into NPC/CNE2 cells, a poorly differentiated human nasopharyngeal carcinoma, than does PpIX. Phototoxicity studies reveal PME to be a more potent photosensitizer than is PpIX, at the early and late incubation time points. Correlating phototoxicity with subcellular localization indicates that PME is a more potent photosensitizer when its primary target of photodamage is mitochondria. Also, additional targeting of lysosome enhances phototoxicity.

A device that divides light into eight, four or two beams of equivalent power with only minor total power loss was designed, built and tested. The apparatus accepts light from a 200 μm diameter, 0.16 numerical aperture, silica–silica multimode optical fiber connected to one of several laser light sources for photodynamic therapy (PDT) of cancer. The incorporation of a variable iris diaphragm into the optical couplers allows the power of the beams to be independently set. Each of the beams can be coupled to a 400 or 600 μm diameter optical fiber to deliver the therapeutic light to the patient. This device is used in our institute for PDT of patients with either numerous small malignant tumors or single tumors with large surface area.

Amphiphilic sensitizers self-associate in aqueous environments and form aggregated species that exhibit no or only negligible photodynamic activity. However, amphiphilic photosensitizers number among the most potent agents of photodynamic therapy. The processes by which these sensitizers are internalized into tumor cells have yet to be fully elucidated and thus remain the subject of debate. In this study the uptake of photosensitizer aggregates into tumor cells was examined directly using subcellular time-resolved fluorescence spectroscopy with a high temporal resolution (20–30 ps) and high sensitivity (time-correlated single-photon counting). The investigations were performed on selected sensitizers that exhibit short fluorescence decay times (<50 ps) in aggregated form. Derivatives of pyropheophorbide-a ether and chlorin e6 with varying lipophilicity were used for the study. The characteristic fluorescence decay times and spectroscopic features of the sensitizer aggregates measured in aqueous solution also could be observed in A431 human endothelial carcinoma cells administered with these photosensitizers. This shows that tumor cells can internalize sensitizers in aggregated form. Uptake of aggregates and their monomerization inside cells were demonstrated directly for the first time by means of fluorescence lifetime imaging with a high temporal resolution. Internalization of the aggregates seems to be endocytosis mediated. The degree of their monomerization in tumor cells is strongly influenced by the lipophilicity of the compounds.

Photodynamic therapy (PDT) may trigger apoptosis or necrosis in cancer cells. Several steps in the induction and execution of apoptosis require high amounts of adenosine-5′-triphosphate (ATP). Because the mitochondrial membrane potential (Δψ) decreases early in apoptosis, we raised the question about the mechanisms of maintaining a sufficiently high ATP level. We therefore monitored Δψ and the intracellular ATP level of apoptotic human epidermoid carcinoma cells (A431) after photodynamic treatment with aluminum (III) phthalocyanine tetrasulfonate. A maximum of caspase-3–like activity and nuclear fragmentation was found at fluences of about 4 J cm^–2. Under these conditions apoptotic cells reduced Δψ rapidly, while the ATP level remained high for 4–6 h after treatment for cells supplied with glucose. To analyze the contribution of glycolysis to the energy supply during apoptosis, experiments were carried out with cells deprived of glucose. These cells showed a rapid drop of ATP content and neither caspase activation nor nuclear fragmentation could be detected. We conclude that the use of glucose as a source of ATP is obligatory for the execution of PDT-induced apoptosis.