Biomedical subjects

Petr Klán

Publications and source records attributed to Petr Klán.

16 recordsLinked to original sources

PubMed2006

2,5-Dimethylphenacyl carbamate: a photoremovable protecting group for amines and amino acids.

2,5-Dimethylphenacyl (DMP) carbamates (1a-c) released the corresponding free amines or amino acids in high chemical yields, albeit with quantum yields Phi of only 0.04-0.09, upon irradiation in either aprotic or protic solvents. The photoreaction proceeded principally from the triplet excited state via the E-photoenol. The lifetimes of the triplet enol and the E- and Z-enols in the ground state were determined by laser flash photolysis. The primary photoinitiated transformation liberated a carbamic acid derivative, which subsequently decarboxylated to the amino group-containing compound. Exhaustive irradiation of a DMP-protected aniline (1a) in acetonitrile did not provide aniline in quantitative chemical yields, because it was involved in reductive cleavage of the starting material as an electron donor, thereby decreasing the overall deprotection yield (86%). Phenylalanine methyl ester, liberated from 1c, was, however, obtained in excellent chemical yield (97%). It was also found that the carbamates, while thermally stable, released amines with higher quantum yields in acidic methanol solutions. The DMP chromophore is proposed as an excellent photoremovable protecting group for amino acids and, under specific conditions, for amines in organic synthesis and biochemistry.

Amines↗

PubMed2006

Photochemistry of 2-alkoxymethyl-5-methylphenacyl chloride and benzoate.

Irradiation of 2-(alkoxymethyl)-5-methyl-alpha-chloroacetophenones (1a-c) and 2-(methoxymethyl)-5-methylphenacyl benzoate (1d) in dry, nonnucleophilic solvents afforded 3-alkoxy-6-methylindan-1-ones (3a-c) in very high chemical yields. 3-Methylisobenzofuran-1(3H)-one (2) was, however, isolated as a major photoproduct in the presence of trace amounts of water. Quenching experiments and laser flash spectroscopy revealed that the indanone derivatives 3 are formed by 1,5-hydrogen migration from the lowest triplet excited state of the acetophenones 1 and cyclization of the resulting photoenols. In contrast, production of the lactone 2 in wet solvents was found to result from two consecutive photochemical transformations. The photoenols produced by photolysis of 1a-c add water as a nucleophile to form 2-acetyl-4-methylbenzaldehyde (4), which is further converted to 2 via a second, singlet state photoenolization process. Exhaustive photolysis of 1a in methanol produced the acetal 2-(dimethoxymethyl)-5-methylacetophenone (7a) as the exclusive product. The remarkable selectivity of these photoreactions may well be useful in synthetic organic chemistry.

Journal Article↗

PubMed2006

Enhanced protonation of cresol red in acidic aqueous solutions caused by freezing.

The protonation degree of cresol red (CR) in frozen aqueous solutions at 253 or 77 K, containing various acids (HF, HCl, HNO3, H2SO4, and p-toluenesulfonic acid), sodium hydroxide, NaCl, or NH4Cl, was examined using UV/Vis absorption spectroscopy. CR, a weak organic diacid, has been selected as a model system to study the acid-base interactions at the grain boundaries of ice. The multivariate curve resolution alternating least-squares method was used to determine the number and abundances of chemical species responsible for the overlaying absorption visible spectra measured. The results showed that the extent of CR protonation, enhanced in the solid state by 2-4 orders of magnitude in contrast to the liquid solution, is principally connected to an increase in the local concentration of acids. It was found that this enhancement was not very sensitive to either the freezing rate or the type of acid used and that CR apparently established an acid-base equilibrium prior to solidification. In addition, the presence of inorganic salts, such as NaCl or NH4Cl, is reported to cause a more efficient deprotonation of CR in the former case and an enhanced protonation in the latter case, being well explained by the theory of Bronshteyn and Chernov. CR thus served as an acid-base indicator at the grain boundaries of ice samples. Structural changes in the CR molecule induced by lowering the temperature and a presence of the constraining ice environment were studied by the absorption and 1H NMR spectroscopies. Cryospheric and atmospheric implications concerning the influence of acids and bases on composition and reactivity of ice or snow contaminants were examined.

Journal Article↗

PubMed2006

Chain mechanism in the photocleavage of phenacyl and pyridacyl esters in the presence of hydrogen donors.

[reaction: see text] Excited phenacyl and 3-pyridacyl esters of benzoic acid react with an excess of aliphatic alcohols in a chain reaction process involving hydrogen transfer from the ketyl radical intermediates, leading to benzoic acid in addition to acetophenone and 3-acetylpyridine, respectively, as the byproducts. While the maximum quantum yields reached 4 in both cases, the 2- or 4-pyridacyl ester photoreduction proceeded with the efficiency below 100% under the same conditions. The investigation indicates that a radical coupling between ketyl radicals, both formed from the excited ester by hydrogen abstraction from an alcohol, is accompanied by the elimination of benzoic acid from the ester ketyl radical itself. A partitioning between two reactions was found to be remarkably sensitive to the chromophore nature, such as a position of the nitrogen atom in the pyridacyl moiety. The magnitude of a radical chain process is dependent on the efficiency of consecutive steps that produce free radicals capable of a subsequent ester reduction. The driving force of a possible electron transfer from the ketyl radicals to the ester has been excluded on the basis of cyclic voltametry measurements. The observed quantum yields of photoreduction were found to be diminished by formation of relatively long-lived light absorbing transients, coproducts obtained apparently by secondary photochemical reactions. Additionally, it is shown that basic additives such as pyridine can further increase the efficiency of the photoreduction by a factor of 4. A radical nature of the reduction mechanism was supported by finding a large kinetic chain length of an analogous reaction initiated by free radicals generated thermally yet again when phenacyl or 3-pyridacyl benzoate was used. Both phenacyl and pyridacyl chromophores are pronounced to be valuable as the photoremovable protecting groups when high quantum and chemical yields of carboxylic acid elimination are important, but higher concentrations of the hydrogen atom donors are not destructive for a reaction system or are experimentally impractical.

Benzoates↗

PubMed2005

Aggregation of methylene blue in frozen aqueous solutions studied by absorption spectroscopy.

The paper presents a qualitative as well as quantitative spectroscopic study of methylene blue (MB) aggregation that occurs upon freezing the aqueous solutions over a wide concentration range. The Gaussian curve analysis and the multivariate curve resolution-alternating least squares method were used to determine the number and concentration of chemical species responsible for the overlaying absorption visible spectra measured. The results show the extent of aggregation for the concentrations above 10(-7) mol L(-1), being dependent on the freezing rate and the initial concentration. While the local concentration of MB at the grain boundaries of polycrystalline ice increased by approximately 3 orders of magnitude upon fast freezing at 77 K compared to the liquid phase, the concentration raised at least by 6 orders of magnitude upon slow freezing at 243 K. Since enhancement of the local concentration of solutes plays an important role in (photo)chemical transformations in solid aqueous media, this work helps to understand how the initial conditions control the course of the process. The results are relevant in other interdisciplinary fields, such as environmental chemistry, cosmochemistry, or geochemistry.

Journal Article↗

PubMed2005

Photodecarbonylation of dibenzyl ketones and trapping of radical intermediates by copper(II) chloride in frozen aqueous solutions.

This paper presents a quantitative and qualitative study of the Norrish type I reaction of dibenzyl ketone (DBK) and 4-methyldibenzyl ketone (MeDBK), producing the benzyl radicals and consequently recombination products, in frozen aqueous solutions over a broad temperature range (-80 to 20 degrees C). This work extends previous research on the cage effects in various constrained media to provide information about the dynamics and reactivity of the photochemically generated intermediates at the grain boundaries of ice matrix. As the temperature of aqueous solutions decreases, the solute concentrations become high at layers covering ice crystals, causing efficient molecular segregation. The cage effect experiments have shown that diffusion of the benzyl radicals within such reaction aggregates is still remarkably efficient at temperatures below -50 degrees C, independently of the initial ketone concentration in the range of 10(-6)-10(-4) mol L(-1). In addition, the study of trapping the benzyl radicals formed in situ by CuCl2 was used as a qualitative probe of heterogeneous bimolecular reactions in the frozen aqueous matrix and on its surface. Molecules of both solutes were found to be segregated from the ice phase to the same location and underwent chemical reactions within diffusion and intermediates lifetimes limits. Understanding the fundamental physicochemical processes in ice is unquestionably important in related environmental or cosmochemical investigations.

Journal Article↗

PubMed2004

2,5-Dimethylphenacyl carbonates: a photoremovable protecting group for alcohols and phenols.

Synthesis and photochemistry of a photochemically removable protecting group for alcohols and phenols, based on the 2,5-dimethylphenacyl (DMP) chromophore, is described. DMP carbonates release the corresponding hydroxy group containing molecules in high isolated yields (>70%), with quantum yields Phi= 0.1-0.2 in methanol and Phi= 0.36-0.51 in cyclohexane. The reactions proceed predominantly by the triplet pathway via E-photoenols, the lifetimes of which of approximately 2 s or 3 ms in cyclohexane or methanol, respectively, were determined by laser flash photolysis.

Alcohols↗

PubMed2004

Toxicity increases in ice containing monochlorophenols upon photolysis: environmental consequences.

The toxic effects of photoproducts formed upon the photolysis of 2- and 4-chlorophenol (CP) frozen solutions in polycrystalline ice phase were determined with a bacterial luminescence test (Vibrio fisheri), and in vitro biomarker assay for dioxin-like effects (inductions of AhR-dependent luciferase in H4IIE-luc cells) and compared to the toxic effects of products of the same photoreaction in aquatic phase. Coupling photoproducts formed in ice samples (3'-chlorobiphenyl-2,4'-diol and 3-chlorobiphenyl-2,2'-diol from 2-CP photolysis and 5-chlorobiphenyl-2,4'-diol from 4-CP photolysis) were found to be more toxic to V. fisheri than parent CPs and elicited significant inductions of dioxin-like effects (the effective concentrations EC50 approximately 3 x 10(-5) mol L(-1) corresponded to known weaker ligands of AhR, such as nonplanar polychlorinated biphenyls or polycyclic aromatic hydrocarbons). To complete the picture, a photoproduct formed from 4-CP (5-chlorobiphenyl-2,4'-diol) was synthesized, and a detailed toxicity assessment with purified compound confirmed the results obtained with irradiated samples. Our findings support a recently proposed model according to which solar radiation can trigger the formation of new types of organic pollutants in polar ice or tropospheric ice cloud particles, presenting possibly greater risk to the environment than the parent compounds.

Animals↗

PubMed2004

Photodegradation of organic pollutants on the spruce needle wax surface under laboratory conditions.

The photochemistry of selected organic compounds, including common pollutants, on the paraffin (as a model matrix) and spruce wax surfaces was studied under laboratory conditions. Two model transformations were evaluated: (1) intramolecular rearrangements of valerophenone and 2-nitrobenzaldehyde, and (2) hydrogen abstraction between an excited benzophenone and the hydrocarbon paraffin/wax chains. The steric or polar influence of the solid matrix on conformational and translational motion, its optical properties, hydrogen abstraction probabilities, and consequences of the guest-molecule segregation are discussed in this work. Furthermore, the photochemical reactivity of some common anthropogenic pollutants, such as chlorinated biphenyls (4-chlorobiphenyl, 2,4-dichlorobiphenyl, and 4,4'-dichlorobiphenyl), 4-chlorophenol, and DDT, was evaluated. The surface of spruce wax is presented as probable reaction medium for photochemical transformations. Although the matrix presents certain restrictions for bimolecular reactions, common photodegradations should be generally feasible in nature. In addition, paraffin was found to be a suitable model matrix for the studies of possible photochemical transformations that can occur on natural plant surfaces.

Air Pollutants↗

PubMed2003

Environmental ice photochemistry: monochlorophenols.

Photolysis of 2- and 4-chlorophenol samples in water ice of the initial concentrations 10(-7) to 10(-2) mol L(-1) is reported. Major phototransformations appeared to be based on the coupling reactions due to chlorophenol aggregation at the grain boundaries of the polycrystalline state. The main products, chlorobiphenyldiols, belong to the family of phenolic halogenated compounds (such as hydroxylated polychlorobiphenyls) that are known xenobiotics found in nature. No photosolvolysis products, that is products from intermolecular reactions between organic and water molecules, were observed at temperatures below -10 degrees C. Raising the temperature to -5 degrees C caused a moderate photosolvolytic activity in the case of 4-chlorophenol (formation of hydroquinone), in contrast to 2-chlorophenol which was almost exclusively transformed into pyrocatechol. It is suggested that photosolvolysis above this temperature occurs in a liquid or quasi-liquid layer that covers the ice crystal surfaces. The results support our model in which significant amounts of some persistent, bioaccumulative, and toxic compounds may be generated by photochemistry of primary pollutants in cold ecosystems and in the upper atmosphere, and may be subsequently released to the environment.

Chlorophenols↗

PubMed2003

N-(1-naphthylacetyl)glycine phenacyl ester and phenacyl (1-naphthylacetoxy)acetate.

The structures of the bichromophoric compounds N-(1-naphthylacetyl)glycine phenacyl ester, C(22)H(19)NO(4), (I), and its oxygen analogue, phenacyl (1-naphthylacetoxy)acetate, C(22)H(18)O(5), (II), have been determined. The molecules of (I) are held together by intermolecular N-H...O hydrogen bonds between the carbonyl and N-H groups, while compound (II) does not show any hydrogen bonding in the crystal.

Acetates↗

PubMed2003

Temperature dependent photochemical cleavage of 2,5-dimethylphenacyl esters.

The study of the temperature-sensitive photochemical release of a carboxylic acid from 2,5-dimethylphenacyl ester is reported. Quantum yields of the benzoate ester degradation in benzene increased from 0.22 at room temperature to 0.28 at 50 degrees C whereas a more significant increase (nearly by a factor of 3) was observed in methanol and ethanol, reaching a high reaction efficiency (0.25) typically found in non-polar solvents. The reaction proceeds predominantly via the triplet pathway and the E-photoenol in the whole temperature range in methanol solution. A higher quantum efficiency in heated methanol is explained by enhancing the E-photoenol population. This picture was partially confirmed by the quantum chemical calculations. The 2,5-dimethylphenacyl chromophore is proposed as an efficient photoremovable protecting group for carboxylic acids in solutions under conventional or microwave-assisted heating for applications in organic synthesis, such as the solid-phase synthesis.

Chemistry, Organic↗

PubMed2003

Comparison of the effects of UV, H2O2/UV and gamma-irradiation processes on frozen and liquid water solutions of monochlorophenols.

The effects of UV irradiation, both in the presence and absence of hydrogen peroxide, as well as of gamma irradiation on 2- and 4-chlorophenol in a solid water ice matrix have been studied and compared to those effects known to occur in aqueous solutions. While UV photolysis (>280 nm) of monochlorophenols leads to efficient coupling reactions in ice and photosolvolysis products in liquid water; hydroxylation to chlorobenzenediols is the main pathway in the presence of H2O2 in both phases. The results show that the solute molecules accumulate in a layer surrounding the ice crystal walls during the freezing process, where they then react. The radiation chemistry of chlorophenol ice samples involves preferential coupling reactions at -78 degrees C rather than reactions with the OH radicals produced by cleavage of water molecules under the conditions employed (1 kGy h(-1)). The apparent similarities between the chemistry in the UV/H2O2-treated liquid and solid, and y-irradiated liquid and solid samples are discussed. It is suggested that the reactions of OH radicals within polycrystalline ice or snow are important natural processes that should be considered in environmental, ice-core or astrophysical research.

Journal Article↗

PubMed2002

Ice (photo)chemistry. Ice as a medium for long-term (photo)chemical transformations--environmental implications.

This review accounts for the current knowledge about the distribution, accumulation, and chemical/photochemical transformations of persistent, bioaccumulative, and toxic compounds (PBTs) in water ice, especially in the connection with polar regions and atmospheric cloud particles. (Photo)reactions on/in ice are discussed in terms of photochemistry, photobiology, paleochemistry, as well as astrophysics. Authors propose a model, in which a significant amount of some PBTs are generated by (photo)chemistry of primary pollutants in ice, which may subsequently be released to the environment. It is argued that ice photochemistry might play an important role in the chemical transformations in cold ecosystems and in the upper atmosphere, particularly now when the ozone layer is partially depleted.

Air Pollutants↗

PubMed2002

2,5-dimethylphenacyl esters: a photoremovable protecting group for phosphates and sulfonic acids.

2,5-Dimethylphenacyl phosphoric and sulfonic esters release the corresponding acids upon irradiation in nearly quantitative isolated yields, with quantum yields phi = 0.71 and 0.68 in methanol, 0.09 and 0.19 in benzene. In methanol solution the reactions proceed predominantly via the (Z)-photoenol, the lifetimes of which (20 and 25 micros) were determined by laser flash photolysis. The chromophore is proposed as an excellent photoremovable protecting group for use in organic synthesis and biochemistry.

Acetophenones↗

PubMed2002

Temperature-sensitive photochemical aromatic substitution on 4-nitroanisole.

A temperature-sensitive photochemical nucleophilic aromatic substitution on 4-nitroanisole by a hydroxide ion in homogeneous solutions, in a two-phase system under phase-transfer catalysis conditions, and in the microwave field is reported. It was found that reaction regioselectivity dramatically changes with temperature in the region of -20 to 196 degrees C. The quantum yield of the 4-methoxyphenol formation was found to be temperature independent, in contrast to that of the 4-nitrophenol formation, suggesting that there is a temperature dependent process occurring after the partitioning between replacement of the nitro group and the methoxy group has taken place. The reaction was also investigated by using quantum chemical calculations. A technique for microwave-assisted photochemical synthesis is proposed as an efficient and practical tool for organic synthesis.

Journal Article↗