Gottfried Köhler

Excited state properties and deactivation pathways of 7-aminocoumarins

Solvent polarity effects on the photophysical properties and spectral shifts of some 7-aminocoumarins were investigated in a combined experimental and theoretical study. Solvatochromic shifts were found in accordance with Onsagers coatinuum model and excited state dipole moments were estimated and compared to semiempirically calculated values. In solvents of a dielectric constant D < 10 the fluorescence quantum yield is generally unity with the only exception of the primary 7-aminocoumarins C120 and C151 in alkane solution. Ground and/or excited state complexation with polar solvent molecules enhances fluorescence. The onset of a nonradiative decay pathway for flexible 7-aminocoumarins with an acceptor substituent on the α-pyrone ring at a solvent D ≈ 10 is in good agreement with a rotatory decay mechanism to a twisted intramolecular charge transfer (TICT) state. This internal quenching is temperature activated, the activation barrier and pre-exponential factor increasing with polarity. TICT formation is discussed on the basis of dielectric solvation effects on the energy-hypersurface describing the twist of the amino group.

G-protein-coupled glucocorticoid receptors on the pituitary cell membrane.

Rapid, nongenomic actions of glucocorticoids (GCs) have been well documented, but information about putative membrane receptors that mediate them is scarce. We used fluorescence correlation spectroscopy to search for membrane GC-binding on the mouse pituitary cell line AtT-20. A slowly diffusing fraction (τ3; diffusion constant 3×10-10 cm2 s-1) of fluorescein-labeled dexamethasone on the cell membrane corresponds to fluorescein-dexamethasone binding. Preincubation experiments were performed to test binding specificity: a 500-fold excess of unlabeled dexamethasone abolished subsequent fluorescein-dexamethasone membrane binding from 58±2 (control) to 8±1 (% of τ3, mean ± s.e.m.), the natural ligand corticosterone prevented it partially (29±2), while the sex steroids estradiol (56±4) and progesterone (50±4) and the GC-receptor antagonist RU486 (56±2) had no effect. Preincubation with pertussis toxin resulted in disappearance of the slowest diffusion component (11±4) suggesting association of the receptor with a G-protein. Varying the concentration of fluorescein-dexamethasone showed that membrane binding is highly cooperative with an apparent Kd of 180 nM and Bmax of 230 nM. Taken together, these results demonstrate high-affinity GC-binding on the cell membrane of AtT-20 cells with characteristics distinct from intracellular binding.

PATHWAYS FOR FORMATION OF HYDRATED ELECTRONS FROM EXCITED PHENOL AND RELATED COMPOUNDS

Abstract. The dependence of primary photophysical and photochemical processes, especially of electron photoejection, in phenol and related compounds in aqueous solution on excitation intensity and excitation energy is examined. Theoretical and experimental evidence is presented for the possibility of three pathways for electron ejection: (1) A monophotonic pathway via the fluorescent state, which most probably does not involve the lowest triplet state; (2) a monophotonic pathway requiring higher excitation energies, which takes place in competition with internal conversion to the fluorescent state; and (3) a consecutive biphotonic pathway in which the lowest triplet state absorbs the second photon, and which can become predominant at high intensities, e.g. under flash conditions. It is shown that this model reconciles apparently conflicting results published in the literature.

Interleukin 8 (IL-8) - a universal biomarker?

Many clinical conditions including various types of cancers are complex and generally require invasive, laborious, expensive and time-consuming investigations for their diagnosis, treatment and follow-up. There is therefore a general need for exploring non-invasive markers in clinical medicine. Interleukin 8 (IL-8) is currently being applied in various subspecialties of medicine either for the purpose of rapid diagnosis or as a predictor of prognosis. Nevertheless, there is need for large-scale studies to substantiate accuracy and outcome. This article will summarize current evidence suggesting that Interleukin 8 (IL-8) may serve as a useful biomarker.

Biophysical characterization of the entire bacterial surface layer protein SbsB and its two distinct functional domains.

The crystalline bacterial cell surface layer (S-layer) protein SbsB of Geobacillus stearothermophilus PV72/p2 was dissected into an N-terminal part defined by the three consecutive S-layer homologous motifs and the remaining large C-terminal part. Both parts of the mature protein were produced as separate recombinant proteins (rSbsB1–178 and rSbsB177–889) and compared with the full-length form rSbsB1–889 (rSbsB). Evidence for functional and structural integrity of the two truncated forms was provided by optical spectroscopic methods and electron microscopy. In particular, binding of the secondary cell wall polymer revealed a high affinity dissociation constant of 3 nm and could be assigned solely to the soluble rSbsB1–178, whereas rSbsB177–889 self-assembled into the same lattice as the full-length protein. Furthermore, thermal as well as guanidinium hydrochloride induced equilibrium unfolding profiles monitored by intrinsic fluorescence, and circular dichroism spectroscopy allowed characterization of rSbsB1–178 as an α-helical protein with a single cooperative unfolding transition yielding a ΔG value of 26.5 kJ mol–1. The C-terminal rSbsB177–889 could be characterized as a β-sheet protein with typical multidomain unfolding, which is partially less stable as stand-alone protein. In general, the truncated forms showed identical properties compared with the full-length rSbsB with respect to structure and function. Consequently, rSbsB is characterized by its two functionally and structurally separated parts, the specific secondary cell wall polymer binding rSbsB1–178 and the larger rSbsB177–889 responsible for formation of the crystalline array.

Decay processes of singlet excited phenol in solution

Abstract The yields of fluorescence at 295 and 77 K and of phosphorescence at 77 K of phenol and anisole were determined in solvents of different polarities (hydrocarbons, ether, ethanol). Also intersystem crossing yields are reported for phenol and anisole in cyclohexane solution at 295 K. The data show that a non-radiative process, different from intersystem crossing, competes with the decay of S 1 phenol in liquid hydrocarbons. The nature of this deactivation pathway is discussed.

Nonradiative deactivation and triplet states in donor-aryl-acceptor compounds (dialkylaminobenzonitriles)

Abstract Intersystem crossing rates and triplet state properties of TICT (twisted intramolecular charge transfer) model compounds (dimethylaminobenzonitrile — DMABN — and its methyl substituted derivatives, 3-cyanoindoline, 3-cyanobenzquinuclidine — CBQ — and benzonitrile) were studied by various experimental and theoretical methods (emission spectroscopy, transient absorption techniques, EPR spectroscopy and semiempirical quantum chemical calculations) with the aim to obtain a quantitative deactivation pattern of this class of compounds in solvents of different polarity. The lowest triplet state, populated after excitation, is generally, a non-charge-transfer state, i.e. the 3 A 1 of planar DMABN, irrespective of solvent polarity and compound flexibility, with the only exception of the rigidly perpendicular CBQ, which relaxes to the 3 A 1 state of the benzonitrile moiety. In non-polar solvents intersystem crossing increases with increasing charge transfer character of the excited singlet; in ethanol, the intersystem crossing rate is about the same for all compounds studied, with the exception of benzonitrile, and in general significantly lower than in a non-polar environment. From the mechanistic point of view, intersystem crossing from 1 TICT can be described as a thermal charge recombination process in the normal Marcus region. With increasing solvent polarity, the decreasing energy gap between these two states then entails a decreasing intersystem crossing rate and an increasing 1 TICT lifetime, as experimentally observed. In hydroxylic solvents, internal conversion to the ground state is the second dominant deactivation pathway of 1 TICT states; it shows a significant deuterium isotope effect.

Solvent effects on the fluorescence properties of anilines

Abstract Fluorescence spectra, quantum yields and lifetimes are comparatively studied for aniline, N,N -dimethylaniline (DMA) and some further alkylsubstituted and rigid aniline derivatives in various solvents. A good correlation of Stokes shift with macroscopic solvent parameters is found for DMA but not for aniline. General and specific solvent effects are distinguished by studying the functional dependence of various spectroscopic and photophysical parameters on the polar co-solvent concentration in binary solvent mixtures. Excited state complexation of primary and secondary amines with alcohols is demonstrated by the double-exponential decay of the monomer fluorescence and the grow-in of exciplex emission found in aniline—ethanol— n -hexane ternary systems. Exciplex formation is attributed to solvent—donor hydrogen bonding and primarily causes enhanced intersystem crossing compared with that in the free molecule, an effect also evoked by N -methylation. Non-specific long-range interactions give rise to a general reduction in the non-radiative deactivation rate, with the exception of o,o′ -dimethyl-substituted anilines, for which an increase in this rate was found as the bulk becomes polar.

Excitation energy dependence of the quantum yields of fluorescence and electron formation from aqueous phenol by means of the heavy atom effect

Abstract The formation of hydrated electrons and H atoms from phenol in neutral aqueous solutions has been studied by excitation in its first and second excited singlet state. The quantum yields of both H and show a strong increase at higher excitation energies. However, this increase cannot fully account for the corresponding decrease of the fluorescence quantum yield. Using Cs+ as a heavy atom quencher, it could be shown that there are at least two pathways for electron formation, one in competition to internal conversion to the fluorescing state and another occurring after population of the fluorescing state, but most likely not from the triplet state.

Emissive Properties and Intramolecular Charge Transfer of Pyrazoloquinoline Derivatives

The fluorescence properties of several pyrazoloquinoline derivatives were performed by stationary as well as by time-resolved spectrosopy. Non-donor-substituted compounds show a high quantum yield; transition dipole moments of absorption and fluorescence as well as experimental and calculated lifetimes are in excellent agreement. The donor-substituted compound DMA-DPPQ exhibits a charge transfer fluorescence in polar solvents. Additionally, dual fluorescence appears in polar protic solvents. The nature of the charge transfer state is discussed with respect to the the TICT model.