Primary Carbonyl Research Articles

Anionic platinum carbonyls in zeolites: effect of Si/Al ratio and of zeolitic structure

Ship-in-bottle synthesis of anionic platinum carbonyls (Chini complexes) was studied in zeolites of various Si/Al ratio and types — NaLSX, NaX, NaY, dealuminated Y, NaEMT, NaM, NaBEA and MCM-41. The effect of decationation of Y zeolites was also examined. The increasing Si/Al ratio led to the increase of Pt nuclearity, the decationation of Y zeolite did not affect the final carbonyl formed. Recarbonylation of platinum after removal of carbonyl ligands by mild oxidation resulted in anionic carbonyl identical with the primary complex, while some changes were found in samples recarbonylated after decomposition of primary carbonyls via mild heating in vacuum. The carbonylation process was examined in situ by FTIR spectroscopy; the carbonyl complexes were decomposed by programmed heating with mass spectroscopic analysis of gases released.

NaX-encaged Pt carbonyls: reversible substitution of CO ligands by oxygen and ammonia. Evidence for a conservation of the polynuclear Pt skeleton

The carbonylation of platinum tetrammine ions in NaX zeolite to platinum anionic (Chini) carbonyl complexes ([Pt3(CO)6]n2−, n=2), oxidation of these complexes and their recarbonylation were studied using in situ FTIR and UV/Vis spectroscopy. The near-surface layers as well as the Pt state were examined by XPS, and normalized absorbance of white line of the Pt L3-edge, evaluated from EXAFS experiments, was monitored in the course of the formation and decomposition of anionic Pt carbonyl complexes. It follows that the positive charge of platinum of the starting complex [Pt(NH3)4]2+ in NaX is largely maintained during carbonylation as well as oxidation and recarbonylation. From the kinetics of recarbonylation of the oxidized complex, which is faster than the primary carbonylation by more than one order of magnitude, a preservation of the Pt skeleton of the primary carbonyl is deduced. Mass spectrometric analysis of the gases evolved during decomposition of the complexes formed by carbonylation, oxidation and recarbonylation, enabled two suggestions to be made: (i) the composition of the oxidized complex; and (ii) stoichiometries for the conversions in agreement with all experimental data.

OH Yields in the Gas-Phase Reactions of Ozone with Alkenes

Hydroxyl radical yields are reported for the gas-phase ozonolyses of a range of alkenes. 1,3,5-Trimethylbenzene was employed as an OH tracer, and the diminution in its concentration was used to calculate OH yields by both a simple analytical kinetic expression and a numerically integrated model. The following OH yields were obtained, relative to alkene consumed: ethene (0.14), propene (0.32), 2-methylpropene (0.60), 2,3-dimethyl-2-butene (0.89), isoprene (0.44), β-pinene (0.24), and α-pinene (0.83). A structure activity relationship (SAR) is presented for the estimation of OH yields based on structural moieties and reaction branching ratios. Reaction stoichiometries (Δ[alkene]/Δ[ozone]) are also reported, along with primary carbonyl yields measured in the presence and absence of excess SO2, both under “OH-free” conditions. Reaction stoichiometries are shown to be correlated with alkene OH yields, and the mechanistic implications of this observation are discussed. The fractional increase in primary carbon...

Product yields in the reactions of ozone with Z-but-2-ene, E-but-2-ene and 2-methylbut-2-ene

Preliminary results on some aspects of the mechanisms of the reactions of ozone with Z-but-2-ene, E-but-2-ene and 2-methylbut-2-ene are presented. Primary carbonyl product yields were measured under ‘OH-free’ conditions and OH yields were determined by monitoring the disappearance of 1,3,5-trimethylbenzene. Ethanal yields relative to alkene consumed were measured as 0.86±0.03, 0.85±0.07 and 0.66±0.02 for the reactions of ozone with Z-but-2-ene, E-but-2-ene and 2-methylbut-2-ene, respectively. For 2-methylbut-2-ene, the propanone yield was 0.35±0.03. Hydroxyl radical yields relative to alkene consumed were measured as 0.33±0.07, 0.54±0.11 and 0.81±0.16 for the reactions of ozone with Z-but-2-ene, E-but-2-ene and 2-methylbut-2-ene, respectively.

Reaction of Methylamine with Acetonylacetone and Its Application in Polymer Analysis

The reaction between methylamine and acetonylacetone is a very important one in pyrolysis polymer analysis. Polymers containing either primary amine functional groups or carbonyl functional groups separated by two methylene units can be derivatized by this reaction. The derivatized polymer undergoes a unique thermal degradation mechanism which favors qualitative and quantitative analysis by pyrolysis gas chromatography. In this study, the chromatographic and spectroscopice evidence about this reaction has been generated to verify this mechanism. The applications of this derivatization reaction in polymer analysis of (1) polyallylamine derivatized with acetonylacetone, (2) ethylene−carbon monoxide alternating copolymer derivatized with methylamine, and (3) ethylene−carbon monoxide, styrene−carbon monoxide alternating terpolymer derivatized with methylamine have been demonstrated.

Formation of hydrogen peroxide in the ozonolysis of isoprene and simple alkenes under humid conditions

The ozonolysis of isobutene and isoprene was performed in a 570 ℓ static reactor at 295 K and 730 Torr synthetic air in the presence and absence of water vapour, with the reactant concentration ranges of 1–6 ppmv. Products were analysed by a combination of FTIR spectroscopy, GC-FID, and HPLC. For both alkenes, the yields of H 2O 2 and the primary carbonyl products (acetone for isobutene, methacrolein and methylvinyl ketone for isoprene) increased under humid conditions. In the isoprene ozonolysis, the H 2O 2 yields relative to the O 3 conversion were, as determined from the initial rate of the formation, 1 and 9% for dry and humid conditions, respectively. The increase in its yield under the humid conditions was correlated with the sum of the increase in the yields of methacrolein and methylvinyl ketone (∼13%). This was explained by rapid decomposition of the transient α-hydroxy hydroperoxides formed in the reaction of H 2O with the two stabilised C 4 Criegee intermediates. Atmospheric relevance of the results is discussed.

The gas-phase reaction of alkenes with ozone: formation yields of carbonyls from biradicals in ozone–alkene–cyclohexane experiments

Data from ozone–alkene–cyclohexane experiments are analyzed with focus on carbonyls other than the primary carbonyls R 1COR 2 and R 3COR 4. Formation of these carbonyls is consistent with O 3+R 1R 2CCR 3R 4→ α(R 1COR 2+R 3R 4COO)+(1− α) (R 3COR 4+R 1R 2COO) followed by reactions of the biradicals R 1R 2COO and R 3R 4COO via an unsaturated hydroperoxide, e.g. for the biradical RCH 2CHOO, (RCH 2CHOO) ∗→(RCHCH(OOH)) ∗→(RCH(OH)CHO) ∗ (1), followed by (RCH(OH) CHO) ∗→RCH(OH)CHO and/or H 2+RC(O)CHO (2) or by (RCH(OH)CHO) ∗→HCO+RCHOH (3a) followed by RCHOH+O 2→HO 2+RCHO (3b). Formation yields of carbonyls for biradicals from 35 alkenes are presented as follows: RCHOO→ A(CND)+ B(MFC+CO+2HO 2) and R 1R 2COO→ A 1(CND 1)+ A 2(CND 2)+ B 1(MFC 1+HO 2)+ B 2(MFC 2+HO 2), where CND are the C n hydroxycarbonyls and/or α-dicarbonyls (reaction (2)), MFC are the monofunctional carbonyls (reaction (3)), and C n is the number of carbon atoms in the biradical. For 12 monosubstituted biradicals RCHOO, A=0.02–0.30 (average=0.14), B=0.04–0.23 (average=0.14) and A+ B=0.07–0.42 (average=0.24). For eight disubstituted biradicals R 1R 2COO, A=0.20–0.42 (average=0.29), B=0.25–0.58 (average 0.32) and A+ B=0.48–0.86 (average >0.61). Consistent with, and supportive of, the biradical→peroxide pathway that involves H-atom abstraction (reaction (1)), no carbonyls are observed to form when R= t-butyl. Reactions 2 and 3 are of comparable magnitude, with on the average B/ A=1.40 ( n=16) for RCHOO and 1.13 ( n=9) for R 1R 2COO. Values of A, B and A+ B are 2–3 times higher for R 1R 2COO than for RCHOO. Using data for alkenes that lead to the same biradical, formation yields of primary carbonyls for eigth alkenes are corrected for the contribution of biradical reactions. Formation yields of CO and HO 2, which may form in pathways other than reactions (1)–(3), are ⩾ B for monosubstituted biradicals for CO, and, for HO 2, ⩾ B for R 1R 2COO and ⩾(2× B) for RCHOO. For a given alkene, the numerical value of ( A+ B) is a lower limit for the biradical→unsaturated hydroperoxide pathway and conversely the numerical value of (1−( A+ B)) is an upper limit of all biradical reactions other than reaction (1).

Products of the Gas-Phase Reactions of O3 with a Series of Methyl-Substituted Ethenes

Alkenes are important constituents of ambient air in polluted urban areas and are highly reactive in the atmosphere, reacting with OH radicals, NO3 radicals, and O3. The products of the gas-phase reactions of O3 with propene, cis-2-butene, trans-2-butene, 2-methylpropene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene were investigated, using gas chromatography with flame ionization detection and in situ Fourier transform infrared absorption spectroscopy, in the presence of sufficient cyclohexane to scavenge >90% (and generally >95%) of the OH radicals formed in these reac tions. The “primary” carbonyl products identified and quantified from these O3 reactions were HCHO and CH3CHO from propene; HCHO and CH3C(O)CH3 from 2-methylpropene; CH3CHO from cis- and trans-2-butene; CH3CHO and CH3C(O)CH3 from 2-methyl-2-butene; and CH3C(O)CH3 from 2,3-dimethyl-2-butene. The formation yields of these primary carbonyls were in the range 0.98−1.19 apart from the 2-methylpropene reaction where the decomposition of the [(CH3)2ĊOȮ]* biradical leads to additional formation of HCHO. In addition to the quantification of the HCHO, CH3CHO, and CH3C(O)CH3 formation yields from these reactions, CH3OH, CH2CO, CO, CO2, and CH4 were observed and quantified by FT-IR absorption spectroscopy

Gas Phase Reaction of Alkenes with Ozone: Formation Yields of Primary Carbonyls and Biradicals

The gas phase reaction of ozone with alkenes plays a major role in tropospheric chemistry including urban air quality. The reaction of trans-2,2-dimethyl-3-hexene and 2,4-dimethyl-2-pentene with ozone has been studied at ambient temperature and p = 1 atm of air (RH = 55% ± 10%) with sufficient cyclohexane added to scavenge the hydroxyl radical. Carbonyl products have been identified, and their formation yields are reported. The results are compared to those previously obtained for 35 other alkenes under the same conditions. For these alkenes, the sums of the formation yields of the primary carbonyls are close to the value of 1.0 that is consistent with the following reaction mechanism: O3 + R1R2CCR3R4 → α(R1COR2 + R3R4COO) + (1 − α)(R1R2COO + R3COR4), where R1COR2 and R3COR4 are the primary carbonyls and R1R2COO and R3R4COO are the corresponding biradicals. For 26 nonsymmetrical alkenes, the coefficients α range from 0.28 to 0.82 and indicate preferential formation of the more substituted biradicals and of the biradicals that bear the less bulky substituents. The results are directly relevant to the atmospheric chemistry of alkenes and to their role in oxidant and aerosol formation.

A kinetic and product study of the gas-phase reaction of ozone with vinylcyclohexane and methylene cyclohexane

The gas-phase reaction of ozone with vinylcyclohexane and methylene cyclohexane has been investigated at ambient T and p=1 atm of air in the presence of sufficient cyclo-hexane or 2-propanol added to scavenge OH. The reaction rate constants, in units of 10−18 cm3 molecule−1 s−1, are 7.52±0.97 for vinylcyclohexane (T=292±2 K) and 10.6±1.9 for methylene cyclohexane (T=293±2 K). Carbonyl reaction products were cyclohexyl meth-anal (0.62±0.03) and formaldehyde (0.47±0.04) from vinylcyclohexane and cyclohexanone (0.55±0.10) and formaldehyde (0.60±0.05) from methylene cyclohexane, where the yields given in parentheses are expressed as carbonyl formed, ppb/reacted ozone, ppb. The sum of the yields of the primary carbonyls is close to the value of 1.0 that is consistent with the simple mechanisms: O3+cyclo(C6H11)−CH(DOUBLEBOND)CH2→α(HCHO+cyclo(C6H11)CHOO)+(1−α)(HCHOO+cyclo(C6H11)CHO) for vinylcyclohexane and O3+(CH2)5C(DOUBLEBOND)CH2→α(HCHO +(CH2)5COO)+(1−α)(HCHOO+(CH2)5C(DOUBLEBOND)O) for methylene cyclohexane. The coefficients α are 0.43±0.10 for vinylcyclohexane and 0.52±0.05 for methylene cyclohexane, i.e., (formaldehyde+the substituted biradical) and (HCHOO+cyclohexyl methanal or cyclo-hexanone) are formed in ca. equal yields. Reaction rate constants, carbonyl yields, and reaction mechanisms are compared to those for alkene structural homologues. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 855–860, 1997

Carbonyl products of the gas-phase reaction of ozone with 1-alkenes

Carbonyl products have been identified and their formation yields measured in experiments involving the gas-phase reaction of ozone with the 1-alkenes (RCH = CH 2) 3-methyl-l-butene (R = i-propyl), 4-methyl-l-pentene (R = i-butyl), 3-methyl-l-pentene ( R= s-butyl), 3,3-dimethyl-l-butene (R = t-butyl) and styrene (R = C 6H 5) at ambient T and p = 1 atm of air. Sufficient cyclohexane was added to scavenge OH in order to minimize reactions of OH with the alkenes and with their carbonyl products. Formation yields (carbonyl formed/ozone reacted) of primary carbonyls were close to the value of 1.0 that is consistent with the mechanism: O 3 + RCH = CH 2 → α(HCHO + RCHOO) + (1 - α) (H 2COO + RCHO), where formaldehyde and RCHO are the primary carbonyls and H 2COO and RCHOO are the biradicals. Measured sums of the primary carbonyl formation yields were 1.006 ± 0.053 (1 S.D.) for formaldehyde + methylpropanal from3-methyl-l-butene(α = 0.494 ± 0.049), 1.025 ± 0.017 for formaldehyde + 2-methylbutanal from 3-methyl-l-pentene (α = 0.384 ± 0.013),1.147 ± 0.050 for formaldehyde + 3-methylbutanal from 4-methyl-l-pentene (α = 0.384 ± 0.020), 0.986 ± 0.014 for formaldehyde + 2,2-dimethylpropanal from 3,3-dimethyl-l-butene (α = 0.320 ± 0.012) and 0.980 ± 0.086 for formaldehyde + benzaldehyde from styrene (α = 0.347 ± 0.059). Carbonyls other than the primary carbonyls were identified; formation pathways are proposed that involve subsequent reactions of the monosubstituted biradicals RCHOO. Similarities and differences between branched-chain 1-alkenes and n-alkyl-substituted 1-alkenes are discussed.

Carbonyl products of the gas phase reaction of ozone with 1,1-disubstituted alkenes

Carbonyl products have been identified and their formation yields measured in experiments involving the gas phase reaction of ozone with 1,1-disubstituted alkenes at ambient T and p=1 atm. of air. Sufficient cyclohexane was added to scavenge the hydroxyl radical in order to minimize OH-alkene and OH-carbonyl reactions. Formation yields (carbonyl formed/ozone reacted) of primary carbonyls were close to the value of 1.0 that is consistent with the mechanism: O3+R1R2C=CH2→ α(HCHO+R1R2COO)+(1−α)(R1COR2+H2COO) where formaldehyde and the ketone R1 COR2 are the primary carbonyls and R1R2COO and H2COO are the corresponding biradicals. Measured values of α were 0.58–0.82 and indicate modest preferential formation of formaldehyde and the disubstituted biradical as compared to the ketone and the biradical H2COO. Carbonyls other than the primary carbonyls were identified. Their formation is discussed in terms of subsequent reactions of the disubstituted biradicals R1R2COO. Similarities and differences between disubstituted and monosubstituted biradicals are outlined.

Carbonyl Products of the Gas Phase Reaction of Ozone with Symmetrical Alkenes

In this study, carbonyl products have been identified and their yields measured in experiments involving the gas phase reaction of ozone with the eight symmetrical alkenes ethylene, cis-3-hexene, cis-4-octene, trans-4-octene, cis-5-decene, trans-5-decene, trans-2, 5-dimethyl-3-hexene, and (cis+trans)-3,4-dimethyl-3-hexene in purified air. Sufficient cyclohexane was added to scavenge the hydroxyl radical (OH) in order to minimize the reaction of OH with the alkenes and with their carbonyl products. Formation yields (carbonyl formed/ozone reacted) of primary carbonyls were close to the value of 1.0 that is consistent with simple reaction mechanism. Carbonyls other than the primary carbonyls R{sub 1}COR{sub 2} were identified as products. Their formation is discussed in terms of subsequent reactions of the R{sub 1}R{sub 2}COO biradicals CH{sub 3}CH{sub 2}CHOO, CH{sub 3}(CH{sub 2}){sub 2}CHOO, CH{sub 3}(CH{sub 2}){sub 3}CHOO, (CH{sub 3}){sub 2}CHCHOO, and C{sub 2}H{sub 5}C(CH{sub 3})OO. Similarities and differences are discussed for cis and trans isomers and for biradical reactions as a function of the nature and number of the substituents. The results are compared to those for the biradicals H{sub 2}COO, CH{sub 3}CHOO, and (CH{sub 3}){sub 2}COO from simpler symmetrical alkenes and contribute to a better understanding of the ozone-alkene reaction under atmospheric conditions. 51 refs., 1 fig., 3 tabs.

Time Resolved Studies of Intermediate Products in the Oxidation of HCFC 141b (CFCl2CH3) and HCFC 142 b (CF2ClCH3)

AbstractThe oxidative degradation of HCFC 141 b (CFCl2CH3) and HCFC 142 b (CF2ClCH3) in the presence of NO at T = 293 K and p = 50 mbar has been studied using laser pulse initiation combined with cw laser long path absorption/LiF for the detection of OH and NO2. The absolute yield and temporal behaviour of these products have been found to be sensitive indicators for the reaction behaviour of the alkoxy radicals CFCl2CH2O and CF2ClCH2O, respectively. The rate coefficients for the reactions of the alkoxy radicals with O2 were determined to be k(CFCl2CH2O+O2→CFCl2CHO+HO2) = (2.0±1.0)×10−15 cm3/s and k(CF2ClCH2O+O2→CF2ClCHO+HO2) = (2.5±1.5)×10−15 cm3/s. Upper limits for the rate coefficients for unimolecular decomposition of the alkoxy radicals by C‐C‐bond fission of k(CFCl2CH2O+M→CFCl2+CH2O+M)×1.0±103 s−1 and k(CF2ClCH2O+M→CF2Cl+CH2O+M)≤1.5 T≤ 103 s−1 were derived. We conclude from these data that CFCl2CHO and CF2ClCHO will be formed as the dominant primary carbonyl products in the oxidation of HCFC 141 b and 142 b throughout the troposphere. Moreover, the rate coefficients for the reactions of the HCFCs with OH radicals at T = 293 K have been measured. The values k(CFCl2CH3+OH→CFCl2CH2+H2O) = (4.6±0.8)×10−15 cm3/s and k(CF2ClCH3+OH→CF2ClCH2+H2O) = (2.6±0.4)×10−15 cm3/s were obtained.

Carbonyl Products of the Gas-Phase Reaction of Ozone with Simple Alkenes

Carbonyl products have been identified and their yields measured in experiments involving the gas-phase reaction of ozone with eight simple alkenes in purified air. Sufficient cyclohexane was added to scavenge the hydroxyl radical (OH) in order to minimize the reaction of OH with the alkenes and with their carbonyl products. Formation yields of primary carbonyls (carbonyl formed/ozone reacted) were 1.060 ± 0.071 (one standard deviation) for formaldehyde from ethylene, 1.150 ± 0.104 for acetaldehyde from 2-butene (ca. 40% cis and 60% trans isomers), 1.011 ± 0.049 for propanal from trans-3-hexene, 1.006 ± 0.049 for acetone from 2,3-dimethyl-2-butene, 0.980 ± 0.036 for formaldehyde + propanal from 1-butene, 0.987 ± 0.020 for acetaldehyde + acetone for 2-methyl-2-butene, 1.300 ± 0.030 for formaldehyde + acetaldehyde from propene, and 1.290 ± 0.103 for formaldehyde + acetone from 2-methylpropene. After correction for the estimated contribution of biradical reactions to formaldehyde, primary carbonyl yields were ca. 1.22 ± 0.03 for acetaldehyde + formaldehyde from propene and 1.08 ± 0.11 for acetone + formaldehyde from 2-methylpropene. These yields are consistent with the value of 1.0 for primary carbonyl yields according to the reaction mechanism : O 3 + R 1 R 2 C = CR 3 R 4 →α(R 1 -COR 2 + R 3 R 4 COO) + (1 - α)(R 3 COR 4 + R 1 R 2 COO) which, for symmetrical alkenes, reduces to O 3 + R 1 R 2 C = CR 1 R 2 → one primary carbonyl (R 1 G(O)R 2 ) + one biradical (R 1 R 2 COO). The results suggest modest preferential formation of the more substituted biradical for 1-butene (CH 3 CH 2 CHOO vs H 2 COO, α = 0.643 ± 0.039), 2-methyl-2-butene [(CH 3 ) 2 COO vs CH 3 CHOO, α = 0.694 ± 0.024], 2-methylpropene [(CH 3 ) 2 COO vs H 2 -COO, estimated α = 0.68 ± 0.12], and perhaps propene (CH 3 CHOO vs H 2 COO, estimated α = 0.57 ± 0.03). Reaction of the biradicals resulted in the formation of carbonyls and of hydroxycarbonyls and/or dicarbonyls, which together accounted for ca. 0.25, 0.31, and 0.57 of the biradical reactions for CH 3 CHOO, CH 3 CH 2 -CHOO, and (CH 3 ) 2 COO, respectively. Carbonyls and hydroxycarbonyls and/or dicarbonyls from propene, 2-methylpropene, 1-butene, and 2-methyl-2-butene were consistent in nature and yields with those that form from the symmetrical alkenes that lead to the same biradicals.

Carbonyl products of the ozone‐unsaturated alcohol reaction

Carbonyl products of the gas phase reaction of ozone with four monosubstituted and three 1,2‐disubstituted unsaturated alcohols that are relevant to biogenic emissions have been identified in laboratory experiments involving unsaturated alcohol‐ozone mixtures in the dark. Cyclohexane was added to scavenge the hydroxyl radical, which forms as a product of the ozone‐unsaturated alcohol reaction. Major carbonyl products included formaldehyde and hydroxyacetaldehyde from allyl alcohol, acetaldehyde, and hydroxyacetaldehyde from 2‐buten‐1‐ol, propanal, and hydroxyacetaldehyde from cis‐2‐penten‐1‐ol, formaldehyde from 3‐buten‐2‐ol, 1‐penten‐3‐ol, and 2‐methyl‐3‐buten‐2‐ol, and hydroxyacetaldehyde from 2‐buten‐1, 4 diol. Yields of the primary carbonyls (ozone + unsaturated alcohol → 2 primary carbonyls + 2 biradicals) averaged 0.91±0.29, which compares to a theoretical value of 1.0. Yields of carbonyls that are not hydroxy substituted averaged 0.48±0.13, which compares to a theoretical value of 0.50 if one assumes that the alkyl‐substituted and the hydroxyalkyl‐substituted biradicals are formed in equal yields. The corresponding reaction mechanisms are outlined with focus on similarities between unsaturated alcohols and alkene structural homologues. Possible decomposition pathways for hydroxyalkyl‐substituted biradicals are examined. The atmospheric persistence and the fate of unsaturated alcohols from biogenic emissions and of their carbonyl products are briefly discussed.

Zeolite-supported transition metal catalysts by design

Zeolite supported metal Catalysts are prepared by ion exchange or incipient wetness impregnation, or by deposition of metal complexes, followed by calcination and reduction. Hydrogen reduction of encaged transition metal ions yields metal dusters or isolated atoms and protons of high Brønsted acidity. The elementary steps in the genesis of mono- and bimetallic metal dusters have been unravelled by a combination of dynamic and spectroscopic methods. Small metal clusters can be anchored to zeolite cage walls by transition metal ions or protons; however, adsorption of a CO which can displace the protons from the metal, initiates migration of the primary carbonyl dusters leading to their coalescence. At low temperature, this process is limited by the geometry of the metal core vis-à-vis the cage window. Protons of high Brensted acidity can reoxidize metals at elevated temperature, even selectively “leach” the less noble metal atoms out of bimetal clusters. Large metal particles, present in zeolite voids or at the external surface, can be transformed into small particles, exposing virtually all their metal atoms, by either of two novel in situ techniques which are based on oxidative dispersion with oxygen or chlorine gas.

Solubilization and localization of triolein in phosphatidylcholine bilayers: a 13C NMR study.

Cosonicated mixtures of egg phosphatidylcholine and small amounts (less than 5% wt/wt) of triolein have been studied by 13C NMR spectroscopy. The 50.3-MHz 13C NMR spectrum of vesicles preparations containing 90% isotopically substituted [1-13C]triolein showed two carbonyl resonances at chemical shift values that indicate hydrogen bonding of H2O molecules with the carbonyl groups. The extent of hydration, estimated from the chemical shift values that indicate hydrogen bonding of H2O molecules with the chemical shift values (173.07 ppm and 172.39 is approximately 50%. The data suggest that the triolein is located in the bilayer with the three carbonyl groups at the aqueous interface. The acyl chains are extended toward the bilayer interior, with a conformation of the glyceryl region such that the primary (alpha) carbonyls are closer to the aqueous medium than is the secondary (beta) carbonyl. Thus, triolein is present in the bilayer in an orientation appropriate for enzymatic hydrolysis, with the second substrate (H2O) in close proximity to the hydrolytic site, and with a conformation that could explain, in part, enzymatic specificity for hydrolysis at the alpha position. Spectra of vesicles containing greater than or equal to 3% triolein showed two additional carbonyl peaks characteristic of pure (neat) triolein. This allowed a determination of the maximum solubility (approximately 2.8%) of surface-oriented triolein in the bilayer phase. Beyond this limit all excess triolein partitions into a separate oil phase.

Specificity and regulation of methionine transport in filamentous fungi

l-Methionine uptake by mycelium of Penicillium chrysogenum (under physiological conditions) is mediated by two distinctly different, independently regulated, stereospecific membrane transport systems (permeases). One of the systems is relatively specific for l-methionine. The other is a general (nonspecific) amino acid permease. The specific l-methionine permease develops upon sulfur-starvation. l-methionine transport by sulfur-starved mycelium is at least ten times more rapid than by sulfur-sufficient mycelium at low external methionine concentrations and is strongly inhibited only by analogs and derivatives of l-methionine (e.g., l-ethionine, ethionine and methionine sulfoxides, sulfones, sulfoximines, methyl and ethyl esters, and homocysteine). Nonsulfur amino acids show little or no inhibition. Sulfur-starvation has little effect on the rates of transport of nonsulfur amino acids. The specific system is (a) pH-dependent (optimum at pH 6 with 50% of maximum velocity at pH 3 and 9), (b) temperature-dependent ( Q 10 of 2 between 5 ° and 35 °), (c) energy-dependent (inhibited by 2,4-dinitrophenol, azide, and p-chloromercuribenzoate), (d) relatively insensitive to variations in the ionic strength of the incubation medium, (e) independent of metal ions for activity, and (f) independent of concurrent protein synthesis. Transport is concentration-dependent showing saturation kinetics. The V max in sulfur-sufficient and sulfur-deficient mycelia is 1–3 μmoles/gm-minute. Sulfur starvation results in a decrease in the K m for l-methionine from 10 −3 to 10 −5 m. Refeeding sulfur-starved mycelia with various sulfur compounds results in a rapid increase in the K m back to the sulfur-sufficient level. These results suggest that the specific l-methionine permease is regulated primarily by feedback inhibition-deinhibition rather than by repression-derepression. Inhibition studies suggest that the minimum structural requirements for strong interaction of a compound with the specific methionine permease include (a) a sulfur atom containing no ionizable groups, (b) two methylene groups between the sulfur atom and the α-carbon, (c) an unsubstituted l-α-amino group, (d) an unsubstituted α-hydrogen, and (e) a primary carbonyl group. The nonspecific transport system is deinhibited, or derepressed (or both), upon nitrogen starvation. l-methionine transport by nitrogen-deficient mycelia is 20–100 times more rapid than by nitrogen-sufficient mycelia at low external methionine concentrations, and is strongly inhibited by almost all neutral and basic l-α-amino acids. Nitrogen starvation results in an increase in the rates of transport of all other amino acids tested ( l-tryptophan, l-phenylalanine, l-leucine, l-serine, l-glutamic acid, and l-lysine).