Biomedical subjects

Jean-Pierre Aycard

Publications and source records attributed to Jean-Pierre Aycard.

10 recordsLinked to original sources

PubMed2006

UV photolysis of C4N2 in water ices: new possible route of synthesis of ammonium bicarbonate and ammonium formate.

Laboratory experiments involving ultraviolet (UV) irradiation of dicyanoacetylene (C(4)N(2)) trapped in water ice at 10 K have been conducted and monitored by infrared spectroscopy (FTIR). By the support of isotopic experiments and theoretical calculations, the irradiation of a DCA/H(2)O ice mixture at lambda > 230 nm has been found to be a possible source of NH(4)(+)HCO(3)(-) (ammonium bicarbonate) and NH(4)(+)HCOO(-) (ammonium formate). These latter compounds can arise from a proton-transfer reaction between H(2)O and the CN radical, which is issued from photolyzed C(4)N(2).

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PubMed2005

Matrix isolation Fourier transform infrared study of photodecomposition of formimidic acid.

The UV isomerization of formamide (HCONH2) trapped in xenon, nitrogen, argon, and neon cryogenic matrices has been monitored by Fourier transform infrared (FT-IR) spectroscopy. Formamide monomer is the only species present in the matrices after deposition; when UV-selective irradiation was carried out at 240 nm, the n --> pi transition allowed us to observe the formation of several isomers of formimidic acid [H(OH)C=NH]. On these latter species, we carried out selective IR irradiation of their OH stretching mode and compared the experimental and theoretical (B3LYP/6-311+G(2d,2p)) sets of bands. This study allowed us to characterize for the first time all the isomers of formimidic acid. We have then studied the vacuum UV photodecomposition (lambda > 160 nm) of this molecule at 10 K in argon and xenon matrices. Several primary photoproducts such as HCN.H2O, HNC.H2O, and HNCO.H2 complexes, yielded by dehydration and dehydrogenation processes, were characterized.

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PubMed2005

Photoreactivity of cyanoacetylene trapped in water ice: an infrared, isotopic and theoretical study.

Photoreactivity of cyanoacetylene with water was successively studied in cryogenic matrixes and in the solid phase at lambda > 120 nm. These studies were performed using FTIR spectroscopy, isotopic experiments and DFT calculations at the B3LYP/6-31G level of theory. The photolysis of cyanoacetylene complexed with water in an argon cryogenic matrix led to the formation of two products. The first one corresponds to the cyanoketene and the second to the HCN:C2O complex. Trapped in water ice and submitted to UV photolysis, the cyanoacetylene molecule shows great photoreactivity. Indeed, besides the cyanoketene and cyanhydric acid, we characterized and identified the formation of other compounds issued from the addition of water to the CC triple bond of cyanoacetylene.

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PubMed2005

Vacuum ultraviolet (VUV) photodecomposition of urea isolated in cryogenic matrix: first detection of isourea.

Vacuum ultraviolet (VUV) irradiation at wavelengths of lambda > 160 nm of urea-h4 (NH2CONH2) and urea-d4 (ND2COND2) has been monitored by Fourier transform infrared spectroscopy in argon and xenon matrixes. Several primary photoproducts, such as HNCO:NH3 (isocyanic acid:ammonia), CO:N2H4 (carbon monoxide:hydrazine) molecular complexes, and isourea (H2N(OH)C=NH), which is reported for the first time, were characterized. The assignment of complexes was achieved by co-depositing the pairs of respective species, whereas the isourea identification was based on the comparison between the experimental and theoretical (B3LYP) infrared spectra. Isourea is found in the argon matrix in its most stable (s-Z)-(E) configuration. It is an intermediate in the VUV decomposition process; its dehydration leads to the NH2CN:H2O complex. In the xenon matrix, the photochemistry of urea yields the HNCO:NH3 complex as a major product, whereas the CO:N2H4 complex is observed in trace amounts. The observed differences between the argon and xenon matrixes suggest the crossing between S1 and T1 potential surfaces of urea to be responsible for the formation of the HNCO:NH3 complex. A comparison is also performed with other carboxamides, such as formamide (HCONH2) or acetamide (CH3CONH2).

Acetamides↗

PubMed2005

Breaking of ketene bonds during HCl addition to trimethylsilylketene.

IR spectroscopy is coupled with the matrix isolation technique to study the reaction of trimethylsilylketene with HCl. From 50 K trimethylsilylketene reacts with hydrogen chloride, leading to the cleavage of the Si-C bond and the formation of trimethylsilyl chloride and acetyl chloride, through intermediate trimethylsilylacetyl chloride which was identified. A reaction profile for this result is proposed based on a theoretical study carried out at the DFT level.

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PubMed2005

Interstellar ice surface site modification induced by dicyanoacetylene adsorption.

Dicyanoacetylene adsorbed on amorphous ice water at 10 K presents an interaction with the dangling H site and induces a s(4) adsorption site formation due to the restructuring of the ice bulk. Warming up the sample provokes the dicyanoacetylene desorption from the H(2)O ice film, which could be due to the beginning of the ice crystallization process. The desorption activation energy measured by temperature-programmed desorption (E(d) = 42 +/- 5 kJ x mol(-1)) is in good agreement with that calculated (E(d) = 46 kJ x mol(-1)) and gives evidence of a hydrogen-bonded adsorbed state on amorphous ice films.

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PubMed2005

Carbodiimide production from cyanamide by UV irradiation and thermal reaction on amorphous water ice.

Cyanamide (NH(2)CN), an interstellar molecule, is a relevant molecule in prebiotic chemistry, because it can be converted into urea in liquid water. Carbodiimide (HNCNH), the most stable cyanamide isomer, is able to assemble amino acids into peptides. In this work, using FTIR spectroscopy, we show that carbodiimide can be formed from cyanamide at low temperature (10 K), by a photochemical process in argon matrix, in water matrix, or in solid film. We also report experimental evidence about the carbodiimide formation when cyanamide is condensed at low temperature (50-140 K) on an amorphous water ice surface, or when it is trapped in the water ice. The water ice acts as a catalyst. This isomerization reaction occurs at low temperature (T < 100 K), which agrees with those expected in the interstellar clouds composed of dust grains in which water is the most predominant compound. Finally, the hydrolysis reaction of cyanamide or carbodiimide leading to urea or isourea formation is not observed under our experimental conditions.

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PubMed2004

Experimental study of water-ice catalyzed thermal isomerization of cyanamide into carbodiimide: implication for prebiotic chemistry.

Cyanamide (NH2CN) is a molecule of interstellar interest which can be implied in prebiotic chemistry. We showed, by FTIR spectroscopy, that cyanamide can be isomerized in carbodiimide (HNCNH), another interstellar relevant molecule, by a reaction involving the amorphous water-ice surface as catalyst. This isomerization occurs at low temperature (T < 100 K) which agrees quite well with that expected in the interstellar clouds composed of dust grains in which water is the most predominant constituent.

Carbodiimides↗

PubMed1998

Photolysis of Matrix-Isolated Acryloyl Chloride: 1,3 Chlorine Migration and Further Evolutions.

Photolysis, at lambda >/= 310 nm (DeltaE < 387 kJ mol(-1)), of acryloyl chloride 1 isolated in argon matrixes at 10 K yields 3-chloro-1,2-propenone 4 through 1,3-chlorine migration. There is no evidence of cyclopropenone or propadienone formation. 4 is also synthesized by irradiation of 3-chloropropanoyl chloride (lambda >/= 230 nm) isolated in argon matrix at 10 K. Identification is performed by comparison of experimental FT-IR spectrum with calculated ones (ab initio calculations at the 6-31G level). Irradiation of 1 at lambda >/= 230 nm induces the photolysis of 4 which breaks into CO and the postulated transient 2-chloroethylidene 5 and/or into propadienone 2 complexed by HCl. The transient 5 collapses to form ground-state vinyl chloride 6 by 1,2 hydrogen migration. In the next step, 2 loses CO to form a new transient assumed to be vinylidene 7 which yields ethyne by intramolecular isomerization process and vinyl chloride by intermolecular reaction with HCl trapped in the same cage. CO, HCl, ethyne, and vinyl chloride are the final reaction products. Modeling of the 1,3 chlorine migration process from 1 using ab initio calculations at the MP2/6-31G level is performed in the ground state (S(0)) and the first singlet excited state (S(1)). The reaction energy value for an S(1) (509 kJ/mol) state process is higher than for an S(0) process (207.2 kJ/mol), these theoretical results suggesting the reaction take place in the ground state.

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PubMed1996

Photolysis of Matrix-Isolated 4-R-1,2,4-triazoline-3,5-diones: Identification of Aziridine-2,3-dione Transients.

Matrix-isolated 4-methyl-1,2,4-triazolinedione 1a and 4-phenyl-1,2,4-triazolinedione 1b were photolyzed at lambda >/= 335 nm and lambda >/= 310 nm, respectively. The reactions induced by photolysis were monitored by FT-IR spectroscopy. The isocyanates 2a and 2b are always the more abundant products with carbon monoxide. Methyl- and phenylaziridine-2,3-diones (3a and 3b) were detected as minor, but well-identified reaction products. The IR experimental absorption bands were assigned by comparison with literature data and with simulated infrared spectra obtained by ab initio calculation at the 6-31G level. Stable at the matrix temperature (10 K), 3a and 3b photolyzed to isocyanates and CO when irradiated at lambda >/= 230 nm. Irradiation of 2b at this wavelength induces its decomposition. The kinetic data show that the rate constant process 1a --> 2a is faster than the 1a --> 3a process (3.29 x 10(-)(3 )and 2.35 x 10(-)(4)min(-)(1) respectively).

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