Nonradiative decay processes in benzene

We investigate the behavior of single vibronic level nonradiative decay rates in benzene and benzene-d₆. The effects of excitation in a promoting mode which undergoes frequency and geometry changes in the S₁ relaxation (to T₁ or S₀) are considered in detail. Calculated relative nonradiative decay rates are compared with experimental values and are used to assign triplet state vibrational frequencies to the ν_s, ν₁₀ and ν₁₆ vibrations. This comparison also indicates that none of these modes, nor the modes ν₁ and ν6, are likely to be the dominant promoting modes for the S₁ → T₁ decay. Some simple expressions are given which provide good estimates of the vibronic state dependence of the non-radiative decay rates. In conjuction with experimental decay rate data, these estimates can aid in guiding spectral assignments of vibronic bands. Simple but general theoretical criteria are derived which are useful in determining those vibrations which are poor (or good) accepting modes. Our previous theory is generalized to consider absolute nonradiative decay rates. The results are used to suggest a possible mechanism for the “channel three” decay process observed by Callomon . Although the numerical applications presented here are to benzene electronic relaxation processes, the theoretical developments also apply to and the calcultions illustrate general features of nonradiative decay in the statistical limit.     

Nonradiative decay mechanism of fluoren-9-ylidene malononitrile ambipolar derivatives

We report recent results on the nonradiative decay (NRD) of fluoren-9-ylidene malononitrile (FM) ambipolar derivatives (FMDs). 2,7- and 3,6-disubstituted FMDs present distinctive photophysics. Charge separation was found dominant for excited state relaxation. The radiative decay (RD) is sensitive to changes in temperature and solvent medium only for the case of 3,6-FMDs. Excited state deactivation of carbazole-containing 3,6-FMD (CPAFM36) was exclusively nonradiative in polar solvents with excited state lifetimes shorter than 10 ps. The charge separation/recombination mechanism of the corresponding FMDs is suggested to fall in the inverted Marcus region of electron transfer. Given the electron-withdrawing properties of the FM unit, its ambipolar derivatives are suggested as potential candidates for air-stable organic thin-film transistors and molecular organic photovoltaics.     

The guanine tautomer puzzle: quantum chemical investigation of ground and excited states

Combined density functional and multireference configuration interaction methods have been employed to explore the ground and low-lying electronically excited states of the most important tautomeric and rotameric forms of guanine with the purpose of resolving the conflicting assignments of IR-UV bands found in the literature. The calculations predict sharp 1(pi-->pi*) origin transitions for the RN1 rotamer of the 7H-amino-hydroxy species and the RN7 rotamer of the 9H-amino-hydroxy species. The other 9H-amino-hydroxy rotamer, RN1, undergoes ultrafast nonradiative decay and is thus missing in the UV spectra. Because of its very small Franck-Condon factor and the presence of a conical intersection close by, it appears questionable, whether the 1(pi-->pi*) origin transition of 9H-amino-oxo-guanine can be observed experimentally. Vibrational overlap is more favorable for the 1(pi-->pi*) origin transition of the 7H- amino-oxo form, but also this tautomer is predicted to undergo ultrafast nonradiative decay of the 1(pi-->pi*) population. The good agreement of calculated IR frequencies of the amino-oxo species with recent IR spectra in He droplets and their mismatch with peaks observed in IR-UV spectra indicate that none of the bands stem from 7H- or 9H-amino-oxo guanine. Instead, our results suggest that these bands originate from 7H-imino-oxo guanine tautomers. In the excited-state dynamics of the biologically relevant 9H-amino-oxo tautomer, a diffuse charge transfer state is predicted to play a significant role.     

Capillary electrochromatography of biologically relevant flavonoids

Flavonoids were separated utilizing CEC technique. Baseline separation of biologically relevant flavonoids was obtained using a 100 microm ID fused-silica capillary filled with 3 microm Silica-C18 material and an optimized mobile phase comprising of 20 mM Tris-HCl (pH 6.5), ACN and water at a ratio of 10/40/50 v/v/v. Separations were carried out at 25 kV and a column temperature of 25 degrees C. The influence of relevant parameters for the CEC separation, such as buffer concentration, pH, separation voltage, and ACN concentration, was investigated and optimized. Dependencies of the electroendoosmotic flow (EOF) on these parameters and effects on the resolution of the analytes were studied. During analyses the solvents used for dissolving the samples turned out to have significant effects on the separation of flavonoids. The optimized system was then successfully used for the separation of the flavonoids epicatechin, myricetin, quercetin, naringenin, and hesperetin. CEC turned out to be a useful complementary tool for the economic analysis of flavonoids in addition to common HPLC, muHPLC, and CE methodologies. This method can be used for real applications in phytomics.     

Synthesis of some biologically relevant beta-C-glycoconjugates

[reaction: see text] An esterification-RCM approach to a variety of biologically relevant beta-C-glycoconjugates is reported herein. A range of carboxylic acids were coupled with several different olefin alcohols 1 to provide esters 3. The esters were then converted to the final ring-closed product 6 in three steps in 49-60% overall yield. The formed compounds are biologically relevant and serve as stable carbohydrate mimics of the corresponding O-glycosides.     

Polythiol binding to biologically relevant metal ions

The coordination properties of three peptides with CXXC motif: Ac-GCASCDNCRACKK-NH(2), Ac-GCASCDNCRAAKK-NH(2) and Ac-GCASCDNARAAKK-NH(2) as donors of four, three and two thiol ligands for Ni(2+),Cd(2+), Zn(2+) and Bi(3+) were studied by potentiometric titrations, UV-Vis and CD spectra measurements. Since the stability of the complexes is closely connected with the amount of the metal-bound cysteine sulfurs, competition plots of the complexes of peptides with 2, 3 and 4 cysteines further prove the involvement of all thiols in the metal ion binding. Furthermore, the sulfur-bound zinc complexes appear to be much more stable than the sulfur-bound nickel ones. The stabilities of the studied complexes decreases in the series Bi(3+) ? Cd(2+) > Zn(2+) > Ni(2+).     

CVD graphene electrochemistry: biologically relevant molecules

We investigate the electrochemical properties of CVD grown graphene towards the detection of various biologically prevalent analytes including l-ascorbic acid (AA), dopamine hydrochloride (DA), β-nicotinamide adenine dinucleotide (NADH), uric acid (UA) and epinephrine (EP). We find that the observed electrochemical response of the CVD-graphene towards these select analytes does not originate from the graphene, however, from various other contributions including the presence of 'graphitic islands' on the surface of the CVD-graphene which dominate its electrochemistry. In the systems studied within, it appears at best, CVD-graphene acts akin to that of an edge plane pyrolytic graphite (EPPG) electrode constructed from highly ordered pyrolytic graphite. However, in other cases, the response of the CVD-graphene is worse than that of an EPPG electrode, which is likely due to the low O/C ratio.     

Green synthetic approaches for biologically relevant organic compounds

The present review aims to present some framework of the effective and diverse green methodologies in conventional and unconventional media including water, solar energy, ionic liquids, ultrasonication and bio-based catalysts which constitute an important goal in organic synthesis and can be used to strengthen conventional laboratory techniques.     

A supramolecular hydrogel that responds to biologically relevant stimuli

First demonstration of heat and pH-responsive hydrogel of SDS and a zwitterionic amphiphile, sodium N-(n-dodecyl-2-aminoethanoyl)-L-valinate with very low minimum gelation concentration.     

Next generation techniques in the high resolution spectroscopy of biologically relevant molecules

Recent advances in the technology of test and measurement equipment driven by the computer and telecommunications industries have made possible the development of a new broadband, Fourier-transform microwave spectrometer that operates on principles similar to FTNMR. This technique uses a high sample-rate arbitrary waveform generator to construct a phase-locked chirped microwave pulse that gives a linear frequency sweep over a wide frequency range in 1 μs. The chirped pulse efficiently polarizes the molecular sample at all frequencies lying within this band. The subsequent free induction decay of this polarization is measured with a high-speed digitizer and then fast Fourier-transformed to yield a broadband, frequency-resolved rotational spectrum, spanning up to 11.5 GHz and containing lines that are as narrow as 100 kHz. This new technique is called chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. The technique offers the potential to determine the structural and dynamical properties of very large molecules solely from fully resolved pure rotational spectra. FTMW double resonance techniques employing a low-resolution UV laser facilitate an easy assignment of overlapping spectra produced by different conformers in the sample. Of particular interest are the energy landscapes of conformationally flexible molecules of biological importance, including studies of their interaction with solvent and/or other weakly bound molecules. An example is provided from the authors' work on p-methoxyphenethylamine, a neurotransmitter, and its complexes with water.     

Applications of X-ray absorption spectroscopy to biologically relevant metal-based chemistry

Recent developments in the understanding of the biosynthesis of the active site of the nitrogenase enzyme, the structure of the iron centre of [Fe]-hydrogenase and the structure and biomimetic chemistry of the [FeFe] hydrogenase H-cluster as deduced by application of X-ray spectroscopy are reviewed. The techniques central to this work include X-ray absorption spectroscopy either in the form of extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES) and nuclear resonant vibrational spectroscopy (NRVS). Examples of the advances in the understanding of the chemistry of the system through integration of a range of spectroscopic and computational techniques with X-ray spectroscopy are highlighted. The critical role played by ab initio calculation of structural and spectroscopic properties of transition-metal compounds using density functional theory (DFT) is illustrated both by the calculation of nuclear resonance vibrational spectroscopy (NRVS) spectra and the structures and spectra of intermediates through the catalytic reactions of hydrogenase model compounds.     

The interaction of platinum-based drugs with native biologically relevant proteins

This study focuses on the identification of the products that are formed upon binding of therapeutically relevant platinum complexes to proteins like β-lactoglobulin A (LGA), human serum albumin (HSA), or human hemoglobin (HB). The respective proteins were incubated with the platinum-based anticancer drugs cisplatin, carboplatin, and oxaliplatin. LGA was selected as the model protein in addition to the two most abundant blood proteins HSA and HB. In case of the model protein, the effect of free thiol groups on the affinity of cisplatin, carboplatin, and oxaliplatin was investigated by means of liquid chromatography electrospray ionization time-of-flight mass spectrometry (LC/ESI-ToF-MS). The reduced form of LGA, which contains four free thiol groups more than the native LGA, shows a much higher affinity to the platinum-based drugs. By means of liquid chromatography coupled to inductively coupled plasma mass spectrometry, the reaction behavior of the platinum-based drugs towards HSA and HB was investigated under different conditions considering the chloride concentration (4 or 100 mM) and the incubation time (24 and 48 h). In case of carboplatin, less than 6 % protein-bound platinum was detected. However, both cisplatin and oxaliplatin display a high affinity to the proteins investigated. Further information was obtained by means of LC/ESI-ToF-MS. In case of oxaliplatin, the complex [Pt(DACH)]²⁺ (DACH?=?C₆N₂H₁₄) was identified interacting with HSA and HB. For cisplatin, different results were observed for the two proteins. The complex [Pt(NH₃)₂Cl]⁺ interacted predominantly with HSA and [Pt(NH₃)₂]²⁺ with HB.

The decay mechanism of photoexcited guanine - a nonadiabatic dynamics study

Ab initio surface hopping dynamics calculations were performed for the biologically relevant tautomer of guanine in gas phase excited into the first ππ? state. The results show that the complete population of UV-excited molecules returns to the ground state following an exponential decay within ～220 fs. This value is in good agreement with the experimentally obtained decay times of 148 and 360 fs. No fraction of the population remains trapped in the excited states. The internal conversion occurs in the ππ? state at two related types of conical intersections strongly puckered at the C2 atom. Only a small population of about 5% following an alternative pathway via a nπ? state was found in the dynamics.     

Nonradiative decay dynamics of methyl-4-hydroxycinnamate and its hydrated complex revealed by picosecond pump-probe spectroscopy

The lifetimes of methyl 4-hydroxycinnamate (OMpCA) and its mono-hydrated complex (OMpCA-H(2)O) in the S(1) state have been measured by picosecond pump-probe spectroscopy in a supersonic beam. For OMpCA, the lifetime of the S(1)-S(0) origin is 8-9 ps. On the other hand, the lifetime of the OMpCA-H(2)O complex at the origin is 930 ps, which is ～100 times longer than that of OMpCA. Furthermore, in the complex the S(1) lifetime shows rapid decrease at an energy of ～200 cm(-1) above the origin and finally becomes as short as 9 ps at ～500 cm(-1). Theoretical calculations with a symmetry-adapted cluster-configuration interaction (SAC-CI) method suggest that the observed lifetime behavior of the two species is described by nonradiative decay dynamics involving trans → cis isomerization. That is both OMpCA and OMpCA-H(2)O in the S(1) state decay due to the trans → cis isomerization, and the large difference of the lifetimes between them is due to the difference of the isomerization potential energy curve. In OMpCA, the trans → cis isomerization occurs smoothly without a barrier on the S(1) surface, while in the OMpCA-H(2)O complex, there exists a barrier along the isomerization coordinate. The calculated barrier height of OMpCA-H(2)O is in good agreement with that observed experimentally.     

Synthesis of glycophostones: cyclic phosphonate analogues of biologically relevant sugars

Analogues of L-fucose, N-acetyl-D-glucosamine, N-acetyl-D-mannosamine, and N-acetyl neuraminic acid in which the anomeric carbon atom was replaced by a phosphonyl group (phostones or cyclic phosphonates) were synthesized by stereocontrolled methods relying on the Abramov reaction.