Jacob Arnbjerg

Singlet Oxygen Sensor Green®: Photochemical Behavior in Solution and in a Mammalian Cell

The development of efficient and selective luminescent probes for reactive oxygen species, particularly for singlet molecular oxygen, is currently of great importance. In this study, the photochemical behavior of Singlet Oxygen Sensor Green® (SOSG), a commercially available fluorescent probe for singlet oxygen, was examined. Despite published claims to the contrary, the data presented herein indicate that SOSG can, in fact, be incorporated into a living mammalian cell. However, for a number of reasons, caution must be exercised when using SOSG. First, it is shown that the immediate product of the reaction between SOSG and singlet oxygen is, itself, an efficient singlet oxygen photosensitizer. Second, SOSG appears to efficiently bind to proteins which, in turn, can influence uptake by a cell as well as behavior in the cell. As such, incorrect use of SOSG can yield misleading data on yields of photosensitized singlet oxygen production, and can also lead to photooxygenation‐dependent adverse effects in the system being investigated.

Reaction of Singlet Oxygen with Tryptophan in Proteins: A Pronounced Effect of the Local Environment on the Reaction Rate

Singlet molecular oxygen, O(2)(a(1)Δ(g)), can influence many processes pertinent to the function of biological systems, including events that result in cell death. Many of these processes involve a reaction between singlet oxygen and a given amino acid in a protein. As a result, the behavior of that protein can change, either because of a structural alteration and/or a direct modification of an active site. Surprisingly, however, little is known about rate constants for reactions between singlet oxygen and amino acids when the latter are in a protein. In this report, we demonstrate using five separate proteins, each containing only a single tryptophan residue, that the rate constant for singlet oxygen reaction with tryptophan depends significantly on the position of this amino acid in the protein. Most importantly, the reaction rate constant depends not only on the accessibility of the tryptophan residue to oxygen, but also on factors that characterize the local molecular environment of the tryptophan in the protein. The fact that the local protein environment can either appreciably inhibit or accelerate the reaction of singlet oxygen with a given amino acid can have significant ramifications for singlet-oxygen-mediated events that perturb cell function.

Photophysics of Squaraine Dyes: Role of Charge-Transfer in Singlet Oxygen Production and Removal

The unique optical properties of squaraines render these molecules useful for applications that range from xerography to photodynamic therapy. In this regard, squaraines derived from the condensation of nitrogen-based heterocycles and squaric acid have many promising attributes. Key solution-phase photophysical properties of six such squaraines have been characterized in this study. One feature of these molecules is a pronounced absorption band in the region approximately 600-720 nm that has significant spectral overlap with the fluorescence band (i.e., the Stokes shift is small). As such, effects of emission/reabsorption yield unique excitation wavelength dependent phenomena that are manifested in quantum yields of both fluorescence and sensitized singlet oxygen production. Comparatively small squaraine-sensitized yields of singlet oxygen production and, independently, large rate constants for squaraine-induced deactivation of singlet oxygen are consistent with a model in which there is appreciable intra- and intermolecular charge-transfer in the squaraine and squaraine-oxygen encounter complex, respectively. The results reported herein should be useful in the further development of these compounds for a range of oxygen-dependent applications.

One- and Two-Photon Excitation of β-Carbolines in Aqueous Solution: pH-Dependent Spectroscopy, Photochemistry, and Photophysics

Beta-carboline (betaC) alkaloids are present in a wide range of biological systems and play a variety of significant photodependent roles. In this work, a study of the aqueous solution-phase photochemistry, photophysics, and spectroscopy of three important betaCs [norharmane (nHo), harmane (Ho), and harmine (Ha)] and two betaC derivatives [N-methylnorharmane (N-Me-nHo) and N-methylharmane (N-Me-Ho)] upon one- and two-photon excitation is presented. The results obtained depend significantly on pH, the ambient oxygen concentration, and the betaC substituent and provide unique insight into a variety of fundamental photophysical phenomena. The data reported herein should not only help to understand the roles played by betaC alkaloids in biological systems but should also help in the development of methods by which the photoinduced behavior of these important compounds can be controlled.

Temperature Effects on the Solvent-Dependent Deactivation of Singlet Oxygen

Singlet molecular oxygen, O(2)(a(1)Delta(g)), is an intermediate in a variety of oxygenation reactions. The reactivity of singlet oxygen in a given system is influenced, in part, by competitive solvent-dependent channels that deactivate singlet oxygen in a nonradiative process. It has long been considered that these deactivation channels depend only slightly on temperature. This conclusion has been incorporated into the accepted empirically derived model of electronic-to-vibrational energy transfer used to account for the effect of solvent on the lifetime of singlet oxygen, tau(Delta). The current study reveals that tau(Delta), in fact, can depend quite significantly on temperature in certain solvents (e.g., D(2)O and benzene-d(6)). These results can have practical ramifications in studies of singlet oxygen reactivity. From a fundamental perspective, these data indicate that aspects of the model for nonradiative deactivation of singlet oxygen need to be re-evaluated.

Silica-Coated Gold Nanorods with a Gold Overcoat: Controlling Optical Properties by Controlling the Dimensions of a Gold−Silica−Gold Layered Nanoparticle

Silica shells were directly coated onto surfactant-capped gold nanorods by a simple one-step method. The procedure required no intermediate coating of the gold nanorod prior to the formation of the smooth silica shell, the thickness of which could be accurately controlled over the range 60-150 nm. These silica-encased gold nanorods were then covered with a gold overcoat to yield nanoparticles with unique optical properties that varied with the thicknesses of both the silica layer and the gold overcoat. Using these bulk solution-phase techniques, homogeneous distributions of gold-silica-gold layered nanoparticles with a pronounced plasmon extinction band in the near-IR (i.e., approximately 900-1700 nm) are readily and reproducibly prepared. More specifically, when using a core gold nanorod whose dimensions yield a plasmon band in the visible region of the spectrum (e.g., approximately 685 nm), the effect of the gold overcoat is to produce a broad plasmon band that is red-shifted by as much as approximately 1000 nm. As such, these multilaminate particles should be of interest as a convenient tool to enhance weak near-IR radiative transitions (e.g., singlet oxygen, O(2)(a(1)Delta(g)), phosphorescence at 1270 nm).

Molecular Tuning of Phenylene-Vinylene Derivatives for Two-Photon Photosensitized Singlet Oxygen Production

Substituent-dependent features and properties of the sensitizer play an important role in the photosensitized production of singlet oxygen, O(2)(a(1)Delta(g)). In this work, we systematically examine the effect of molecular changes in the sensitizer on the efficiency of singlet oxygen production using, as the sensitizer, oligophenylene-vinylene derivatives designed to optimally absorb light in a nonlinear two-photon process. We demonstrate that one cannot always rely on rule-of-thumb guidelines when attempting to construct efficient two-photon singlet oxygen sensitizers. Rather, as a consequence of behavior that can deviate from the norm, a full investigation of the photophysical properties of the system is generally required. For example, it is acknowledged that the introduction of a ketone moiety to the sensitizer chromophore often results in more efficient production of singlet oxygen. However, we show here that the introduction of a carbonyl into a given phenylene-vinylene can, rather, have adverse effects on the yield of singlet oxygen produced. Using these molecules, we show that care must also be exercised when using qualitative symmetry-derived arguments to predict the relationship between one-and two-photon absorption spectra.

Single molecule atomic force microscopy studies of photosensitized singlet oxygen behavior on a DNA origami template.

DNA origami, the folding of a long single-stranded DNA sequence (scaffold strand) by hundreds of short synthetic oligonucleotides (staple strands) into parallel aligned helices, is a highly efficient method to form advanced self-assembled DNA-architectures. Since molecules and various materials can be conjugated to each of the short staple strands, the origami method offers a unique possibility of arranging molecules and materials in well-defined positions on a structured surface. Here we combine the action of light with AFM and DNA nanostructures to study the production of singlet oxygen from a single photosensitizer molecule conjugated to a selected DNA origami staple strand on an origami structure. We demonstrate a distance-dependent oxidation of organic moieties incorporated in specific positions on DNA origami by singlet oxygen produced from a single photosensitizer located at the center of each origami.

Metal-Enhanced 1270 nm Singlet Oxygen Phosphorescence†

The first excited state of molecular oxygen, singlet oxygen (O2(a Dg), or simply O2), is a reactive intermediate of importance in processes ranging from polymer degradation to cell death. The most unambiguous way to detect O2 is by its characteristic phosphorescence at 1270 nm. However, this emission is very weak because the overall deactivation of O2 is dominated by nonradiative pathways; typical phosphorescence yields (FP) are on the order of 10 !5 to 10. This can make optical detection of O2 difficult, particularly in spatially resolved experiments from small volumes. Thus, much would be gained if FP could be increased. Herein, we demonstrate that the 1270 nm radiative decay of O2 can be enhanced by coupling to localized surface plasmon resonances (LSPRs) in carefully designed gold nanostructures. O2 is commonly produced in a photosensitized process (Scheme 1) with FP expressed as a product of the O2 yield, FD, and the fraction of O2 molecules that decay radiatively, tDkr [Eq. (1)], where the O2 lifetime, tD= (knr+ kr), is

Influence of an intermolecular charge-transfer state on excited-state relaxation dynamics: solvent effect on the methylnaphthalene-oxygen system and its significance for singlet oxygen production.

The extent to which an intermolecular charge-transfer (CT) state can influence excited-state relaxation dynamics is examined for the system wherein 1-methylnaphthalene (MN) interacts with molecular oxygen. The MN-O2 system is ideally suited for such a study because excited states can be independently accessed by (i) irradiation into the discrete MN-O2 CT absorption band, (ii) direct irradiation of MN, and (iii) the photosensitized production of triplet state MN. Changing the solvent in which the MN-O2 system is dissolved influences the MN-dependent photoinduced production of singlet oxygen, O2(a1Delta(g)), which, in turn, yields information about fundamental concepts of state mixing. Results of experiments conducted in the polar solvent acetonitrile differ substantially from those obtained from the nonpolar solvent cyclohexane. The data reflect differences in the energy and behavior of the solvent-equilibrated MN-O2 CT state, CT(SE), and the extent to which this state couples to other states of the MN-O2 system. In particular, the data are consistent with a model where both the MN triplet state and the MN-O2 CT(SE) state are immediate precursors of O2(a1Delta(g)). Although the work reported herein is of direct and practical significance for the wide variety of systems in which O2(a1Delta(g)) can be produced upon irradiation, it also serves as an accessible model for a study of general issues pertinent to state mixing and the solvent-dependent dynamics of CT-mediated excited-state relaxation.