Photoreaction Process Research Articles

Interaction and photodynamic activity of cationic porphyrin derivatives bearing different patterns of charge distribution with GMP and DNA

The interaction of amphiphilic cationic porphyrins, containing different patterns of meso-substitution by 4-(3- N, N, N-trimethylammoniumpropoxy)phenyl (A) and 4-(trifluoromethyl)phenyl (B) groups, with guanosine 5′-monophosphate (GMP) and calf thymus DNA have been studied by optical methods in phosphate buffer solution. The properties of these synthetic porphyrins were compared with those of representative standard of anionic 5,10,15,20-tetra(4-sulphonatophenyl)porphyrin (TPPS 4 4−) and cationic 5,10,15,20-tetra(4- N, N, N-trimethylammonium phenyl)porphyrin (TMAP 4+). Stable complexes with GMP were found for cationic porphyrins, except for monocationic AB 3 +. The binding constant ( K GMP ∼ 10 4 M −1) follows the order: A 3B 3+ ∼ ABAB 2+ > A 4 4+ ∼ TMAP 4+. Also, interaction with DNA was observed for all evaluated cationic porphyrins. For these related cationic porphyrins, the binding constant ( K DNA ∼ 10 5 M −1) increases with the number of cationic charges. On the other hand, the photodynamic activity of porphyrins was analyzed in solution of GMP and DNA. Monocationic AB 3 + is a less effective sensitizer to oxidize GMP in comparison with the other cationic porphyrins, in agreement with the lack of detected interaction with this nucleotide. The electrophoretic analysis of DNA indicates that photocleavage takes place when the samples are exposed to photoexcited tricationic and tetracationic porphyrins. In the presence of sodium azide the DNA decomposition was diminished. Also, reduction in the DNA photocleavage was observed under anoxic condition, indicating that oxygen is essential for DNA photocleavage sensitized by these cationic porphyrins. In addition, an increase in DNA degradation was not observed in deuteriated water. Therefore, an important contribution of type I photoreaction processes could be occurring in the DNA photodamage sensitized by these cationic porphyrins. These results provide a better understanding of the characteristics needed for sensitizers to produce efficient DNA photocleavage.

Photodissociation of Hydrated Hydrogen Iodide Clusters

Using high-level ab initio calculations and excited state ab initio molecular dynamics simulations, we show that hydrated iodic acids release hydrogen radicals and/or hydrogen molecules as well as iodine radicals upon excitation. Its photoreaction process involving charge transfer to the solvent takes place in four steps: 1) hydration of the acid, 2) charge transfer to water upon excitation of hydrated acid, 3) detachment of the neutral iodine atom, and 4) detachment of the hydrogen radical. The iodine detachment process from excited hydrated hydro-iodic acids is exothermic and the detachment of hydrogen radicals from hydrated hydronium radicals is spontaneous if the initial kinetic energy of the cluster is high enough to get over the activation barrier of the detachment. The complete release of the radicals can be understood in terms of kinetics. This study shows how the hydrogen and halogen radicals are dissociated and released from their hydrated acids. Simple experiments corroborate our predicted mechanism for the release of hydrogen molecules from iodic acid in water by ultraviolet light.

Transitional and permanent regimes in the adiabatic Floquet approach to photodissociation processes

Adiabatic formalisms are widely used, within the context of Floquet theory, to study photoreactive processes and control processes involving laser fields. In numerous cases, however, the use of the adiabatic hypothesis seems to be rendered problematic by the presence of one or more sudden variations in the chosen adiabatic parameters. The present paper shows that in such cases the behaviour of the system can be described quite accurately by using successive groups of a few instantaneous Floquet eigenvectors which are connected to each other by non-adiabatic transition factors. In the optical potential approach the relevant Floquet eigenvectors are those issuing from some bound states or from discretized states of vibrational continua. The approach described here leads to fast integration techniques for the relevant dynamical systems; the case of the molecule H+2 subjected to short laser pulses is used as an illustrative example. The two types of state are found to play different roles. The bound-state eigenvectors, relevant to the permanent regime, induce the dissociative flux transport when the laser field envelope varies slowly and ensure globally the correct integrated quantum flux. The discretized pseudo-scattering states associated with the transitional regime are relevant for sudden variations of the laser field envelope and govern the time dependence of the asymptotic dissociative flux by temporarily increasing or decreasing the flux transported by the states in the first group of Floquet resonance states. A detailed study shows that the lifetimes of the pseudo-scattering eigenvectors (which are generally regarded as somewhat arbitrary in the complex absorbing potential model) do have important physical relevance.

The synthesis of novel coumarin dyes and the study of their photoreaction properties

A series of novel coumarin derivatives linked with an N, N-dimethylaniline group via cyclopentanonediene on position 4 of the coumarin ring were synthesized and their two-photon absorbing (TPA) properties were investigated by femtosecond laser pulses. Compared to commercial coumarin dyes, novel compounds presented greatly enhanced TPA cross-sections around 100–400 GM. The photoreaction kinetic study showed that these dyes had fast electron transfer with commercial coinitiators, such as hexaarylbisimidazoles or iodonium salt. The dye/coinitiator combination can be used as highly efficient photoinitiating component in photopolymerization materials. In addition, a self-accelerating phenomenon was observed in the photoreaction process of the dye/iodonium salt.

Photooxidative Degradation of Dye Pollutants Accumulated in Self‐Assembled Natural Polyelectrolyte Microshells under Visible Radiation

A novel route to facilitate the degradation of dye pollutants, a class of well-known recalcitrant organic pollutants, is reported. This new approach is based on a natural polyelectrolyte microshell that was preformed by the alternate adsorption of the anionic alginate sodium (ALG) and the cationic chitosan (CHI) onto weakly cross-linked melamine formaldehyde (MF) colloidal particles, and the subsequent sacrifice of MF templates in 0.1 M HCl. The as-prepared microshells could accumulate rhodamine B (RhB) and fluorescein (Flu) efficiently in water under ordinary conditions by means of a simple mixing process. The photodegradation of the accumulated RhB and Flu was examined in the presence of Fe3+ and H2O2 under visible radiation. The accumulated RhB and Flu are rapidly degraded and the assembled shells maintain their intact spherical shape throughout the photoreaction process. Results of recycling degradation experiments and the photochemical behavior of the shells, as demonstrated by confocal laser scanning microscopy (CLSM), UV-visible spectroscopy, and scanning force microscopy (SFM), further suggest that the constructed shells may be used as environmentally friendly microcontainers for the elimination of dyes in wastewater.

Photodynamic activity of a new sensitizer derived from porphyrin-C 60 dyad and its biological consequences in a human carcinoma cell line

The photokilling activity of a porphyrin-C 60 (P-C 60) dyad was evaluated on a Hep-2 human larynx-carcinoma cell line. This study represents the first evaluation of a dyad, with high capacity to form a photoinduced charge-separated state, to act as agent to inactivate cells by photodynamic therapy (PDT). Cell treatment was carried out with 1 μM P-C 60 incorporated into liposomal vesicles. No dark cytotoxicity was observed using 1 μM P-C 60 concentration and during long incubation time (24 h). The uptake of sensitizer into Hep-2 was studied at different times of incubation. Under these conditions, a value of 1.5 nmol/10 6 cells was found after 4 h of incubation showing practically no change even after 24 h. The cell survival after irradiation of the cells with visible light was dependent upon light exposure level. A high photocytotoxic effect was observed for P-C 60, which inactivated 80% of the cells after 54 J/cm 2 of irradiation. Moreover, the dyad kept a high photoactivity even under argon atmosphere. Thus, depending on the microenviroment where the sensitizer is localized, this compound could produce a biological photodamage through either a 1O 2-mediated photoreaction process or a free radical mechanism under low oxygen concentration. The mechanism of cell death was analyzed by Hoechst-33258, toluidine blue staining, TUNEL and DNA fragmentation. Cell cultures treated for 24 h with P-C 60 and irradiated with a dose of 54 J/cm 2 showed a great amount of apoptotic cells (58%). Moreover, changes in cell morphology were analyzed using fluorescence microscopy with Hoechst-33258 under low oxygen concentration. Under this anaerobic condition, necrotic cellular death predominated on apoptotic pathway. There were more apoptotic cells under air irradiation condition than under argon irradiation condition. To determine the apoptotic pathway, caspase-3 activation was studied by caspase-3 activity detection kits. The last results showed that P-C 60 induced apoptosis by caspase-3-dependent pathway. These results indicated that molecular dyad, which can form a photoinduced charge-separated state, is a promising model for phototherapeutic agents and they have potential application in cell inactivation by PDT.

Porphyrin‐fullerene C60 Dyads with High Ability to Form Photoinduced Charge‐separated State as Novel Sensitizers for Photodynamic Therapy¶

ABSTRACTThe photodynamic activities of a porphyrin‐C60 dyad (P‐C60) and its metal complex with Zn(II) (ZnP‐C60) were compared with 5‐(4‐acetamidophenyl)‐10,15,20‐tris(4‐methoxyphenyl)‐porphyrin (P), both in homogeneous medium‐bearing photooxidizable substrates and in vitro on the Hep‐2‐human‐larynx‐carcinoma cell line. This study represents the first evaluation of dyads, with a high capacity to form a photoinduced charge‐separated state, to act as agents to inactivate cells by photodynamic therapy (PDT). Absorption and fluorescence spectroscopic studies were performed in toluene and N,N‐dimethylformamide (DMF). The emission of the porphyrin moiety in the dyads is strongly quenched by the attached fullerene C60 moiety. The singlet molecular oxygen, O2(1Δg), productions (φΔ) were determined using 9,10‐dimethylanthracene (DMA). The values of φΔ were strongly dependent on the solvent's polarity. Comparable φΔ values were found for dyads and P in toluene, while O2(1Δg) production was significantly diminished for the dyads in DMF. In more polar solvent, the stabilization of charge‐transfer state takes place, decreasing the efficiency of porphyrin triplet‐state formation. Also, both dyads photosensitize the decomposition of L‐tryptophan in DMF. In biological medium, no dark cytotoxicity was observed using sensitizer concentrations ≤1 μM and 24 h of incubation. The uptake of sensitizers into Hep‐2 was studied using 1 μM of sensitizer and different times of incubation. Under these conditions, a value of ∼1.5 nmol/106 cells was found between 4 and 24 h of incubation. The cell survival after irradiation of the cells with visible light was dependent upon light‐exposure level. A higher photocytotoxic effect was observed for P‐C60, which inactivates 80% of cells after 15 min of irradiation. Moreover, both dyads keep a high photoactivity even under argon atmosphere. Thus, depending on the microenvironment where the sensitizer is localized, these compounds could produce biological photodamage through either an O2(1Δg)‐mediated photoreaction process or a free‐radicals mechanism under low oxygen concentration. These results show that molecular dyads, which can form a photoinduced charge‐separated state, are a promising model for phototherapeutic agents, with potential applications in cell inactivation by PDT.

Dihydrophenanthrene-Type Intermediates during Photoreaction of trans-4‘-Benzyl-5-styrylfuran

[structure: see text] Photoreaction of trans-4'-benzyl-5-styrylfuran (trans-BSF) has been studied by the 355-nm laser flash photolysis (LFP) in CH2Cl2 using a Nd3+:YAG laser (30 ps, 5 mJ pulse(-1) or 5 ns, 30 mJ pulse(-1)). Transient fluorescence and absorption spectra assigned to the singlet excited trans-BSF were observed during the 30-ps LFP, whereas a transient absorption spectrum with two peaks at 400 and 510 nm, assigned to the trans-fused dihydrophenanthrene (DHP)-type intermediate (DP1), was observed during the 5-ns LFP. It is clearly suggested that a two-photon absorption process is involved in the formation of DP1. The first photoreaction is the photoisomerization of trans-BSF, which occurs to give cis-BSF. The second photoreaction process is photocyclization of cis-BSF, which occurs to give DP1 decaying with the half lifetime (tau1/2) of 2.8-4.0 micros to produce another DHP-type intermediate (DP2) with an absorption peak at 400 nm in the absence of O2, through [1,9]-hydrogen shift. DP2 decayed with tau1/2 > 500 micros to give the product through aromatization. In O2-saturated CH2Cl2, DP1 decayed with tau1/2 = 250 ns to give a radical intermediate (X) with two peaks at 410 and 510 nm, through hydrogen abstraction of DP1 by O2. X decayed with tau1/2 = 150 micros to give the product through successive hydrogen abstraction.

POSSIBILITY OF MOLECULAR-SWITCH WITH CONTROLLED HYDROGEN BOND: UTILITY OF COMBINATION OF 2,2′-BIIMIDAZOLE AND REDOX-ACTIVE LIGAND

A new inorganic molecule [ Co(Hbim) ( C 6 H 4 O 2)( NH 3)2]2 that can be used as a new optically durable molecular switch was theoretically designed in the framework of density functional theory. Three stable minima, belonging to 1 A g , 5 A 1, and 9 A g states, were found in the complex. Theoretically predicted infrared spectra of the complexes showed that a strong peak of NH stretching vibration is observed at 2690, 2120, and 2770 cm -1 in the 1 A g , 5 A 1, and 9 A g states, respectively. The apparent red shift of the NH stretching vibration band in the 5 A 1 state make it possible to distinguish the electronic state from others (1 A g and 9 A g ). This means that the complex can be used as a molecular level switch whose memory can be stably read by IR light without any photoreaction process; namely, without memory degradation.

Porphyrin-fullerene C60 dyads with high ability to form photoinduced charge-separated state as novel sensitizers for photodynamic therapy

The photodynamic activities of a porphyrin-C60 dyad (P-C60) and its metal complex with Zn(II) (ZnP-C60) were compared with 5-(4-acetamidophenyl)-10,15,20-tris(4-methoxyphenyl)porphyrin (P), both in homogeneous medium-bearing photooxidizable substrates and in vitro on the Hep-2-human-larynx-carcinoma cell line. This study represents the first evaluation of dyads, with a high capacity to form a photoinduced charge-separated state, to act as agents to inactivate cells by photodynamic therapy (PDT). Absorption and fluorescence spectroscopic studies were performed in toluene and N,N-dimethylformamide (DMF). The emission of the porphyrin moiety in the dyads is strongly quenched by the attached fullerene C60 moiety. The singlet molecular oxygen, O2(1delta(g)), productions (phi(delta)) were determined using 9,10-dimethylanthracene (DMA). The values of phi(delta) were strongly dependent on the solvent's polarity. Comparable phi(delta) values were found for dyads and P in toluene, while O2(1delta(g)) production was significantly diminished for the dyads in DMF. In more polar solvent, the stabilization of charge-transfer state takes place, decreasing the efficiency of porphyrin triplet-state formation. Also, both dyads photosensitize the decomposition of L-tryptophan in DMF. In biological medium, no dark cytotoxicity was observed using sensitizer concentrations < or = 1 microM and 24 h of incubation. The uptake of sensitizers into Hep-2 was studied using 1 microM of sensitizer and different times of incubation. Under these conditions, a value of approximately 1.5 nmol/10(6) cells was found between 4 and 24 h of incubation. The cell survival after irradiation of the cells with visible light was dependent upon light-exposure level. A higher photocytotoxic effect was observed for P-C60, which inactivates 80% of cells after 15 min of irradiation. Moreover, both dyads keep a high photoactivity even under argon atmosphere. Thus, depending on the microenvironment where the sensitizer is localized, these compounds could produce biological photodamage through either an O2(1delta(g))-mediated photoreaction process or a free-radicals mechanism under low oxygen concentration. These results show that molecular dyads, which can form a photoinduced charge-separated state, are a promising model for phototherapeutic agents, with potential applications in cell inactivation by PDT.

Time development of a persistent hole burning in multiple-scattering media

We investigated the time development of persistent hole burning in photoreactive and multiple-scattering media. In this hole burning, a writing laser beam recodes the volume speckle pattern inside the medium through the photoreactive process, and then the luminescence intensity excited by a reading beam is measured as a function of the frequency or wave vector difference between the writing and reading beams. The time development reflects the statistics of the intensity fluctuations in the speckle inside the medium. One of the striking results is that the maximum hole depth is approximately 0.26 . We distinguish three types of samples according to the geometric configurations and the photoreactive processes and discuss the hole shape, hole width, and the depth during time development. Hole burning experiments were also performed using a spiropyran derivative photochromic dye. The experimental results showed good agreement with the theory.

Constrained Adiabatic Trajectory Method

The nonadiabatic temporal evolution which is associated with inelastic collisions and photoreactive processes typically produces a final state distribution that differs markedly from the initial state distribution. Nevertheless an adiabatic formalism is often used in a zeroth-order description of the processes; for time-periodic perturbations the Floquet theory has been used within an adiabatic framework to provide a compact dynamical theory which requires a basis composed of only a small number of Floquet eigenstates. The use of the generalized Floquet theory or of the concept of a super-adiabatic basis allows the adiabatic approach to be further extended to handle systems with quasi-periodic Hamiltonians. The present work proposes a new approach, in which the time duration of the interaction is artificially prolonged and special absorbing boundary conditions are introduced asymptotically over the lengthened time interval in such a way as to force the adiabaticity of the process. The method involves what can be thought of as time-dependent optical potentials. Some trial applications to semiclassical inelastic collisions and to photodissociation effects have shown that the use of the new technique permits a description of the dynamical processes which is so economical that the use of a single generalized Floquet eigenvector will suffice. The main technical feature of this constrained adiabatic trajectory method is that it converts the problem of solving the TDSE with an explicitly time-dependent potential into that of solving a static complex eigenvalue problem.

Effect of the fluorinated groups on nematic liquid crystal alignment on monomer crosslinked film

It was found in this work that photosensitive monomers, bisphenol A dicinnamate ester and hexafluorobiphenol a dicinnamate ester were crosslinked under irradiation of linearly polarized ultraviolet light. The exposed films induced homogeneous and homeotropic alignment of liquid crystals (LC), respectively. We verified through experiments that it was fluorinated groups that caused the generation of LC homeotropic alignment on the crosslinked film. Photoreaction process was revealed by Fourier transform infrared spectra. There was no clear morphological anisotropy on these aligned films observed through atomic force microscope analysis. The surface energies were measured and homeotropic alignment reason was discussed in this work.

A trial for in situ quantitative TEM–EELS measurement related to the photoreaction process ofTiO2 films

Ultraviolet light was used to irradiate hydrocarbons onTiO2 thin film in high-resolution transmission electron microscopy (TEM). The hydrocarbon wasdecomposed by photocatalysis of titanium oxide. Reaction processes were studied by usingan in situ TEM observation system which was newly constructed for the present study. Formeasuring photocatalytic activities, electron energy loss spectroscopy (EELS)was used. We developed a new TEM method for observing crystal structures ofTiO2 and quantifying photocatalytic activities concurrently. We succeededin measuring the photodecomposition activity of hydrocarbons onTiO2 films in a submicron area. The spatial resolutions of the present method werecompared with those of other methods for measuring photocatalytic activity.

Photosensitized degradation of dyes in polyoxometalate solutions versus TiO2 dispersions under visible-light irradiation: mechanistic implications.

This article examines the photoxidation of a dye (rhodamine-B, RhB) by visible-light irradiation in the presence of a polyoxometalate (12-tungstosilicic acid, H(4)SiW(12)O(40)), and compares it with the analogous process in the presence of TiO(2). The photoreaction processes were examined by UV-visible spectroscopy, fluorescence spectroscopy, high-performance liquid chromatography (HPLC), liquid chromatography/mass spectral techniques (LC-MS), and total organic carbon (TOC) assays in order to identify the intermediates produced. Formation of oxygen species, such as H(2)O(2) and O(2)*-, was also investigated to clarify the details of the reaction pathway. With the use of SiW(12)O(40)(4-) ions as the photocatalyst, the photoreaction leads mainly to N-dealkylation of the chromophore skeleton. In contrast, cleavage of the whole conjugated chromophore structure predominates in the presence of TiO(2). Strong O(2)*-/HO(2)*- ESR signals were detected in the TiO(2) dispersions, whereas only weak ESR signals for the O(2)*- radical ion were seen in the SiW(12)O(40)(4-) solutions during the irradiation period. Experimental results imply that reduction of O(2) occurs by different pathways in the two photocatalytic systems.

Reactions of Vinyl Ethers and Application to Photoreactive Process

Polymeric materials consisting of an acidic polymer, vinyl ether crosslinker and photoacid generator were found to give positive-working photoresists. An interesting characteristics of the system may be what any polymers having acidic groups as the molecular structure, can be converted to photopolymer only adding vinyl ether crosslinker and PAG. The review will report the examples of the conversion of polymers such as cresol novolak resin, p-hydroxy polystyrene, poly(acrylic acid), and poly(amic acid) to the photosensitive polymers. Among the three components composing the photosensitive polymer, the polymer decides the main characters such as developing agent, optical and mechanical properties. The crosslinker governs the solubility contrast between the polymer layers of the exposed or the unexposed portion. PAG is most important component for this type of photosensitive polymer, because it governs the sensitivity as well as the spectral sensitivity. The photosensitive polymer composed of the poly(amic-acid), divinyl ether crosslinker and PAG gave the patterns, which are very strong against the heat treatment. In the field of nanolithographic patterning, very short wavelength light is used to transfer the patterns of nanometer resolution. For this purpose, the photoresists have to be transparent at 193nm light. Alicyclic compounds containing vinyl ether groups prepared as crosslinker were found to be transparent at 193nm. Several polymers have been synthesized for 193nm lithography. One of those polymers and the alicyclic compound were combined to prepare the photoresist for 193nm lithography. The performance of the photoresist will be also reviewed.

A Novel β-CD−Hemin Complex Photocatalyst for Efficient Degradation of Organic Pollutants at Neutral pHs under Visible Irradiation

A novel photooxidative system for degrading toxic organic pollutants, including organic dyes and small molecular compounds such as Rhodamine B (RhB) and 2,4-dichlorophenol (DCP), is reported. A metal−organic complex (β-cyclodextrin−hemin, CDH), synthesized by combining β-CD with the bioactive small molecule hemin, can efficiently activate hydrogen peroxide to oxidize and mineralize the target compounds in aqueous media at neutral pHs under ambient conditions of temperature and pressure and with visible light irradiation (λ > 450 nm). The RhB and DCP were mineralized at yields of 72% and 85% at ratios of catalyst to substrate of 1:200 and 1:1000, respectively. Meanwhile, the catalyst exhibited excellent stability. The photoreaction processes were examined by UV/vis spectroscopy, high-performance liquid chromatography, ion chromatography, IR, spin-trapping electron paramagnetic resonance, and total organic carbon measurements. A possible photooxidative mechanism for the degradation of organic pollutants in ...

Influence of multilayer nanostructures on photoisomerization and photoorientation of azobenzene

Based on the layer-by-layer electrostatic deposition method, we have realized alternating multilayer systems of polyelectrolytes with an internal order via using membrane-forming polyionenes that contain rigid azobenzene chromophores linked to flexible alkylene spacers. The photoisomerization and photoreorientation of azobenzene moieties in the multilayer assemblies were investigated in dependence on nanostructures that were formed with varying spacer length of the polyionene. In combination with spectroscopic behavior, both molecular photoreaction and light-induced reorientation processes are used as probes to characterize order and dynamics of the multilayer assemblies. Increasing the spacer length from 6 to 10, 12 carbons results in a strong aggregation of polyionene in solution that again gives rise to a dense packing of the chromophores in the multilayer assemblies on a solid substrate. The supramolecular order influences kinetics of trans– cis photo-isomerization and reorientation of the azobenzene moieties under the action of linearly polarized light. In particular, the different irradiation conditions (wavelength, power density, polarization direction) have a significant influence on the photo-induced anisotropy.

Primary Photoreaction Processes in Vision.

This article reviews the primary reaction processes in rhodopsin, a light-sensor protein in our vision. Rhodopsin has an 11-cis retinal as the chromophore, which binds covalently with a lysine residue through a protonated Schiff base linkage. Picosecond time-resolved spectroscopy of 11-cis locked rhodopsin analogs revealed that the cis-trans isomerization of the chromophore is the primary reaction in rhodopsin. Then, generation of femtosecond laser pulses in the 1990s made it possible to follow the process of isomerization in real time. Formation of photorhodopsin within 200 fs was observed by a transient absorption (pump-probe) experiment, which also revealed that the photoisomerization in rhodopsin is a vibrationally coherent process. Femtosecond fluorescence spectroscopy directly captured excited-state dynamics of rhodopsin, so that both coherent reaction process and unreacted excited state were observed. Faster photoreaction of the chromophore in rhodopsin than that in solution implies that the protein environment facilitates the efficient isomerization process.

Photo-induced formation of dissolved gaseous mercury in coastal and offshore seawater of the Mediterranean basin

The first measurements on the daily trend of dissolved gaseous mercury (DGM) concentration determined in coastal and offshore waters of the Mediterranean basin are reported. Marked daily behaviour tracking solar radiation has been observed at the coastal sampling station with DGM values ranging from 11.0 to 38.9 pg/l. Contrary to these observations the DGM values in offshore water samples (11.9–20.0 pg/l) were independent of the sampling time, thus identifying the absence of higher levels during the hours of maximum insolation. The availability of Hg 2+ substrate necessary for the photo-reaction processes of DGM formation has been evaluated by measuring the reactive mercury concentration. In offshore waters the lower DGM concentrations are attributable to the substrate as a limiting factor. The highest concentration of DOC measured in coastal seawater with respect to the offshore one could moreover enhance the reaction rates of DGM production through the formation of inorganic mercury complexes and weaker organic associations.