Vertical gradients of delta15N and delta18O in soil atmospheric N2O--temporal dynamics in a sandy soil

The greenhouse gas nitrous oxide (N(2)O) can be both formed and consumed by microbial processes in the soil. As these processes fractionate strongly in favour of (14)N and (16)O, delta(15)N and delta(18)O gradients of N(2)O in the soil profile may elucidate patterns of N(2)O formation, consumption or emission to the atmosphere. We present the first in situ data of such gradients over time for a mesic typic Haplaquod seeded with potatoes (Solanum tuberosum L.). On two adjacent fields in 2002 and 2003, topsoil N(2)O fluxes were measured and the soil atmosphere was regularly sampled for N(2)O concentrations, delta(15)N and delta(18)O signatures of N(2)O at depths of 18, 48 and 90 cm during approximately 400 days. During the entire sampling period, the N(2)O concentrations were the highest and the delta(15)N signatures the lowest in the subsoil (48 or 90 cm depth) as compared with the topsoil, indicating production of N(2)O in the subsoil. For delta(15)N, differences greater than 30 per thousand between topsoil and subsoil on the same date were regularly observed. The highest N(2)O concentration of 100385 microL m(-3) at 90 cm depth on 1 July 2003, was preceded by the lowest delta(15)N value of -43.5 per thousand one week earlier. This was followed by a 150-day general decrease of N(2)O concentrations at 90 cm depth to 1723 microL m(-3) and a simultaneous enrichment of delta(15)N to +7.1 per thousand, mostly without a significant topsoil flux. There was a negative logarithmic relationship between N(2)O concentration at 90 cm depth and its delta(15)N signature. This relationship indicated a delta(15)N signature of -40 to -45 per thousand during the production of N(2)O in the subsoil, and a subsequent enrichment during the consumption of N(2)O. We conclude that the isotopic signature of the N(2)O topsoil flux is the result of various processes of consumption and production at different depths in the soil profile. It is therefore not a reliable estimator for the overall delta(15)N signature of N(2)O in the soil atmosphere, nor for indirect losses of N(2)O to the environment. Therefore, these findings will pose a further challenge to ongoing efforts to draw up a global isotopic budget for N(2)O.     

LIGHT DOSIMETRY FOR PHOTODYNAMIC THERAPY BY WHOLE BLADDER WALL IRRADIATION

Abstract In Photodynamic Therapy (PDT) there is a need for accurate quantitative light dosimetry. This has become particularly apparent in the treatment of superficial bladder cancer, either by focal or by whole bladder wall irradiation. We have studied this problem using a spherical model of the bladder, consisting of two concentric thin-walled plastic spheres, 8 and 10 cm in diameter. The inner sphere was filled with water or with a light-scattering medium. The space between the spheres was filled with an optically tissue equivalent liquid. An isotropic light source was placed at the center of the spheres. Light energy fluence rates (light "dose rates") during PDT of the bladder simulated in this manner, were measured using a specially developed miniature light detector and were also calculated using a mathematical model. These data were confirmed by measurements in vivo (dog bladder). In the case of focal irradiation at a wavelength of 630 nm, the ratio (R) between the true light fluence rate at the bladder surface and the fluence rate due to the primary light beam is somewhat larger than 1, depending on the diameter of the primary beam. The maximum ratio is 2, for a beam diameter of several centimeters. In the case of whole bladder wall PDT, R is larger than 5. This is due to the strong scattering of (red) light by tissue and indicates that the intensity of the primary beam, which is usually reported, is not a good measure of the true fluence rate for whole bladder wall PDT. When the light source is moved away from the center of the spheres, the rate of change of the fluence rate at the bladder wall is more than a factor of 2 larger when the bladder cavity is filled with a light-scattering suspension, as compared with water. The use of a light-scattering medium may therefore not be advantageous to achieve a uniform light distribution across the bladder wall.     

Metathesis reactions for the synthesis of ring-fused carbazoles

[reactions: see text] The metathesis reaction is used as a key step for the synthesis of the indolo[2,3-a]carbazole core of rebeccamycin 13 and the sulfur analog of furostifoline 21. Using the same methodology for the attempted synthesis of furostifoline, we unexpectedly formed tert-butyl-2a-methyl-1,2,2a,10c-tetrahydro-6H-cyclobuta[c]furo[3,2-a]carbazole-6-carboxylate 26 from the unstable diene, tert-butyl 2-(2-isopropenyl-3-furyl)-3-vinyl-1H-indole-1-carboxylate 25, presumably via a spontaneous pi8 electrocyclization reaction.     

MNDO Calculations on [n]metacyclophanes

MNDO calculations on [n]metacyclophanes and a variety of related compounds are reported. An analysis of the strain, imposed by the oligomethylene bridge, and its distribution over the distorted benzene ring and the oligomethylene bridge is presented. The effect of strain and bending of the benzene ring on aromatic delocalization is investigated by a comparison of ΔH^o_f of the bent benzene systems with that of a correspondingly bent, but localized 1,3,5-cyclohexatriene. The results indicate that, even in the case of the highly bent [4]metacyclophane ( 1a ), localization is unfavorable by about 10 kcal/mol.     

Ab initio Calculation of the charge distribution and the ligand field splitting in the tetrahedral halo complexes CuCl 4 2− 4 and NiCl 4 2−

The charge distribution and the ligand field splitting in the tetrachloro complexes CuCl ₄ ^2– and NiCl ₄ ^2– have been investigated by means of the restricted Hartree-Fock method. A rather large basis set of contracted Gaussian type orbitals has been employed. The charge distributions have been analysed by means of Mulliken population analyses. The ligand field splitting 10Dq has been compared with literature results known for the octahedral cluster NiF ₆ ^4– occurring in KNiF₃. A detailed analysis has been carried out for CuCl ₄ ^2– . From calculations on a selected number of states of NiCl ₄ ^2– the Racah parameters B and C have been obtained.     

An isomerization-ring-closing metathesis strategy for the synthesis of substituted benzofurans

Twelve substituted benzofurans were synthesized from their corresponding substituted 1-allyl-2-allyloxybenzenes using ruthenium-mediated C- and O-allyl isomerization followed by ring-closing metathesis.     

A transient product of DNA alkylation can be stabilized by binding localization

A 9-aminoacridine conjugate of a silyl-protected bis(acetoxymethyl)phenol (bisQMP) was synthesized and evaluated as an inducible cross-linking agent of DNA to test our ability to harness the chemistry of reactive quinone methide intermediates (QM). The acridine component was chosen for its ability to delivery an appendage to the major groove of DNA, and the silyl-protected component was chosen for its ability to generate two quinone methide equivalents in tandem upon addition of fluoride. This design created competition between reaction of (1) the 2-amino group of guanine that reacts irreversibly to form a stable QM adduct and (2) the more nucleophilic N7 group of guanine that reacts more efficiently but reversibly to form a labile QM adduct. This lability was apparently compensated by co-localization of the N7 group and QM in the major groove since the N7 adduct appeared to dominate the profile of products formed by duplex DNA. The controlling influence of acridine was also expressed in the sensitivity of the conjugate to ionic strength. High salt concentration inhibited covalent reaction just as it inhibits intercalation of the cationic acridine. As expected for QM formation, the presence of fluoride was indeed necessary for initiating reaction, and no direct benzylic substitution was observed. The conjugate also cross-linked DNA with high efficiency, forming one cross-link for every four alkylation events. Both alkylation and cross-linking products formed by duplex DNA were labile to hot piperidine treatment which led to approximately 40% strand scission and approximately 50% reversion to a material with an electrophoretic mobility equivalent to the parent DNA. All guanines exhibited at least some reactivity including those which were recalcitrant to cross-linking by an oligonucleotide-bisQMP conjugate designed for triplex formation [Zhou, G.; Pande, P.; Johnson, A. E.; Rokita, S. E. Bioorg. Med. Chem. 2001, 9, 2347-2354].     

Kinetics and mechanism of urethane formation catalyzed by organotin compounds. II. The reaction of phenyl isocyanate with methanol in DMF and cyclohexane under the action of dibutyltin diacetate

The kinetics of the dibutyltin diacetate (DBTDA)-catalyzed reaction of phenyl isocyanate with methanol in DMF and cyclohexane have been studied by monitoring the rate of change of the absorbance of the reaction mixture at 281.6 and 280.9 nm, respectively. Our results indicate that the mechanism of the reaction is independent of the nature of the (inert) solvent. The subsequent reaction steps are (i) complexation of methanol to DBTDA, (ii) dissociation of the complex into a proton and an anion of composition [(n-C₄H₉)₂Sn(OCOCH₃)₂(OCH₃)]^?, (iii) insertion of the isocyanate into the tin—alkoxy bond (the rate-determining step), and (iv) methanolysis of the thus-formed urethane precursor with simultaneous regeneration of the anion. The rate of the catalyzed reaction increases on goin from coordinating to noncoordinating solvents. The following sequence in order of increasing rate constant was observed: DMF < dibutyl ether < cyclohexane. This is because in coordinating solvents, DBTDA and methanol, apart from forming a complex with each other, also form complexes with the solvent.     

Epidermal cis-urocanic acid levels correlate with lower specific cellular immune responses after hepatitis B vaccination of ultraviolet B-exposed humans

Urocanic acid (UCA) is a major UV-absorbing chromophore in the epidermis and has been suggested to act as one of the initiators of UV-induced immunosuppression. cis-UCA, the isomer from UCA that is formed upon UV exposure, has been shown to impair some cellular immune responses. cis-UCA levels were determined in a study in which the influence of ultraviolet B (UVB) exposure on immune responses after hepatitis B vaccination in human volunteers was established. A significant increase in cis-UCA levels was found in the skin of UVB-exposed volunteers compared with controls. cis-UCA levels, calculated as the percentage of the total UCA amount, in UVB-exposed volunteers correlated significantly with the cumulative UVB dose received in 5 consecutive days, i.e. the higher the UVB dose (J/m2), the higher the cis-UCA levels (until a cis-UCA plateau was reached in the so-called photostationary state). Correlations between skin cis-UCA levels and immune responses were determined, and they revealed no statistically significant correlations among lymphocyte proliferation responses after either mitogenic stimulation or stimulation with recall antigens. No correlation was found between cis-UCA levels and hepatitis B-specific antibody titers. However, we found a statistically significant negative correlation between cis-UCA levels and hepatitis B-specific lymphocyte proliferation responses when volunteers were irradiated with UVB before hepatitis B vaccination. In other words, volunteers with high cis-UCA levels caused by UVB exposure showed lower cellular immune responses against hepatitis B antigen after hepatitis B vaccination.