Final Adduct Research Articles

Agrobacterium Phytochrome as an Enzyme for the Production of ZZE Bilins

Photoconversion of phytochrome from the red-absorbing form Pr to the far-red-absorbing form Pfr is initiated by a Z to E isomerization around the ring C-ring D connecting double bond; the chromophore undergoes a ZZZ to ZZE isomerization. In vivo, phytochrome chromophores are covalently bound to the protein, but several examples of noncovalent in vitro adducts have been reported which also undergo Pr to Pfr photoconversion. We show that free biliverdin or phycocyanobilin, highly enriched in the ZZE isomer, can easily be obtained from chromophores bound in a noncovalent manner to Agrobacterium phytochrome Agp1, and used for spectral assays. Photoconversion of free biliverdin in a methanol/HCl solution from ZZE to ZZZ proceeded with a quantum yield of 1.8%, but was negligible in neutral methanol solution, indicating that this process is proton-dependent. The ZZE form of biliverdin and phycocyanobilin were tested for their ability to assemble with Agp1 and cyanobacterial phytochrome Cph1, respectively. In both cases, a Pfr-like adduct was formed but the chromophore was bound in a noncovalent manner to the protein. Agp1 Pfr undergoes dark reversion to Pr; the same feature was found for the noncovalent ZZE adduct. After dark reversion, the chromophore became covalently bound to the protein. In analogy, the PCB chromophore became covalently bound to Cph1 upon irradiation with strong far-red light which initiated ZZE to ZZZ isomerization. Agrobacterium Agp2 belongs to a yet small group of phytochromes which also assemble in the Pr form but convert from Pr to Pfr in darkness. When the Agp2 apoprotein was assembled with the ZZE form of biliverdin, the formation of the final adduct was accelerated compared to the formation of the ZZZ control, indicating that the ZZE chromophore fits directly into the chromophore pocket of Agp2.

The domino reaction between 4,6-dinitrobenzofuroxan and cyclopentadiene. Insights on the nature of the molecular mechanism

The molecular mechanism of the domino reaction between 4,6-dinitrobenzofuroxan, 1, and cyclopentadiene, Cp, to give the adduct 11 is examined through density functional theory (DFT) calculations at B3LYP/6-31G* level. This domino reaction comprises two consecutive formally [4+2] cycloadditions. The first one is a two-center addition initialized by the nucleophilic attack of Cp to the more electrophilic center of 1. The subsequent cyclization can take place along two competitive channels associated to the formation of a second C–C bond yielding the formally [2+4] cycloadduct 9, or a C–O bond yielding the formally [4+2] cycloadduct 10. The second cycloaddition is a stepwise process that is initialized by the nucleophilic attack of a second molecule of Cp to 10. The energetic results are in agreement with the experiments allowing to explain the formation of the final domino adduct 11. Inclusion of solvent effects, via a polarizable continuum model, decreases the activation energies of these polar processes. A DFT analysis of the indexes of reactivity provides a sound explanation on the reactivity of 1 on this domino reaction.

Understanding the nature of the molecular mechanisms associated with the competitive Lewis acid catalyzed [4+2] and [4+3] cycloadditions between arylidenoxazolone systems and cyclopentadiene: a DFT analysis.

The molecular mechanisms of the reactions between aryliden-5(4H)-oxazolone 1, and cyclopentadiene (Cp), in presence of Lewis acid (LA) catalyst to obtain the corresponding [4+2] and [4+3] cycloadducts are examined through density functional theory (DFT) calculations at the B3LYP/6-31G* level. The activation effect of LA catalyst can be reached by two ways, that is, interaction of LA either with carbonyl or carboxyl oxygen atoms of 1 to render [4+2] or [4+3] cycloadducts. The endo and exo [4+2] cycloadducts are formed through a highly asynchronous concerted mechanism associated to a Michael-type addition of Cp to the beta-conjugated position of alpha,beta-unsaturated carbonyl framework of 1. Coordination of LA catalyst to the carboxyl oxygen yields a highly functionalized compound, 3, through a domino reaction. For this process, the first reaction is a stepwise [4+3] cycloaddition which is initiated by a Friedel-Crafts-type addition of the electrophilically activated carbonyl group of 1 to Cp and subsequent cyclization of the corresponding zwitterionic intermediate to yield the corresponding [4+3] cycloadduct. The next rearrangement is the nucleophilic trapping of this cycloadduct by a second molecule of Cp to yield the final adduct 3. A new reaction pathway for the [4+3] cycloadditions emerges from the present study.

Characterization and quantitative analysis of DNA adducts formed from lower chlorinated PCB-derived quinones.

Polychlorinated biphenyls are wide pollutants readily detected in environmental and human specimens. DNA adduction occurs through the corresponding quinones. Polychlorinated biphenyls are first metabolized to arene oxides, which can be further oxidized to dihydroxy metabolites by microsomal cytochrome p450s. The catechol and hydroquinone products are further oxidized by peroxidases to quinones, which are electrophilic and capable of reacting with DNA to form adducts. DNA adduction is initiated by Michael addition preferentially to guanosine followed by stabilization through enolization. Another nucleophilic attack forms a five-membered ring, which aromatizes by dehydration to form the final adduct. This report describes the characterization and quantitative study of DNA adducts formed from lower chlorinated PCB-derived quinones. Quantitative study by HPLC/ESI-MS/MS and (32)P-postlabeling-HPLC gave the adduct levels in the range of 3-1200 adducts per 10(8) nucleotides. These results demonstrate that increasing chlorine substitution is associated with lower yields of DNA adduct. Although (32)P-postlabeling is more sensitive than HPLC/ESI-MS/MS for the quantitative analysis of DNA adducts, modification levels were severely underestimated by the (32)P-postlabeling assay as compared to the HPLC/ESI-MS/MS assay.

Highly selective binding of organometallic ruthenium ethylenediamine complexes to nucleic acids: novel recognition mechanisms.

We have investigated the recognition of nucleic acid derivatives by organometallic ruthenium(II) arene anticancer complexes of the type [(eta(6)-arene)Ru(II)(en)X] where en = ethylenediamine, arene = biphenyl (Bip), tetrahydroanthracene (THA), dihydroanthracene (DHA), p-cymene (Cym) or benzene (Ben), X = Cl(-) or H(2)O using (1)H, (31)P and (15)N ((15)N-en) NMR spectroscopy. For mononucleosides, [(eta(6)-Bip)Ru(en)](2+) binds only to N7 of guanosine, to N7 and N1 of inosine, and to N3 of thymidine. Binding to N3 of cytidine was weak, and almost no binding to adenosine was observed. The reactivity of the various binding sites of nucleobases toward Ru at neutral pH decreased in the order G(N7) > I(N7) > I(N1), T(N3) > C(N3) > A(N7), A(N1). Therefore, pseudo-octahedral diamino Ru(II) arene complexes are much more highly discriminatory between G and A bases than square-planar Pt(II) complexes. Such site-selectivity appears to be controlled by the en NH(2) groups, which H-bond with exocyclic oxygens but are nonbonding and repulsive toward exocyclic amino groups of the nucleobases. For reactions with mononucleotides, the same pattern of site selectivity was observed, but, in addition, significant amounts of the 5'-phosphate-bound species (40-60%) were present at equilibrium for 5'-TMP, 5'-CMP and 5'-AMP. In contrast, no binding to the phosphodiester groups of 3', 5'-cyclic-GMP (cGMP) or cAMP was detected. Reactions with nucleotides proceeded via aquation of [(eta(6)-arene)Ru(en)Cl](+), followed by rapid binding to the 5'-phosphate, and then rearrangement to give N7, N1, or N3-bound products. Small amounts of the dinuclear species, e.g., Ru-O(PO(3))GMPN7-Ru, Ru-O(PO(3))IMPN1-Ru, Ru-O(PO(3))TMPN3-Ru, Ru-N7IMPN1-Ru, and Ru-N7InoN1-Ru were also detected. In competitive binding experiments for [(eta(6)-Bip)Ru(en)Cl](+) with 5'-GMP versus 5'-AMP or 5'-CMP or 5'-TMP, the only final adduct was [(eta(6)-Bip)Ru(en)(N7-GMP)]. Ru-H(2)O species were more reactive than Ru-OH species. The presence of Cl(-) or phosphate in neutral solution significantly decreased the rates of Ru-N7 binding through competitive coordination to Ru. In kinetic studies (pH 7.0, 298 K, 100 mM NaClO(4)), the rates of reaction of cGMP with [(eta(6)-arene)Ru(II)(en)X](n+) (X = Cl(-) or H(2)O) decreased in the order: THA > Bip > DHA >> Cym > Ben, suggesting that N7-binding is promoted by favorable arene-purine hydrophobic interactions in the associative transition state. These findings have revealed that the diamine NH(2) groups, the hydrophobic arene, and the chloride leaving group have important roles in the novel mechanism of recognition of nucleic acids by Ru arene complexes, and will aid the design of more effective anticancer complexes, as well as new site-specific DNA reagents.

Synthesis and electrospray mass spectrometry study of Pd(II) complexes of low-rim amino acid substituted calix[4]arenes

The reactions between Pd(II) and low-rim amino acid substituted calix[4]arenes {La = 5,11,17,23-tetra-4-tert-butyl-25,27-di{[(2′-amino-4′-methylthio)-L-butyryl]aminoethoxy}-26,28-dihydroxycalix[4]arene, Lb = 5,11,17,23-tetra-4- tert-butyl-25,27-di({[(N-tert-butoxycarbonyl)-2′-amino-4′-methylthio]-L-butyryl}aminoethoxy)-26,28-dihydroxycalix[4]arene (diN-BOC-La) and Lc = 5,11,17,23-tetra-4-tert-butyl-25,27-di{[(N-tert-butoxycarbonyl)-2′-aminoacetyl]aminoethoxy}-26,28-dihydroxycalix[4]arene} were studied by electrospray mass spectrometry. When La was refluxed with 2.2 equiv. of trans-[Pd(Py)2Cl2] or 1.1 equiv. of PdCl2, the mononuclear complex PdLaCl2 was formed. When refluxed with 2.2 equiv. of PdCl2, the binuclear complex Pd2LaCl4 was obtained. The same binuclear complex Pd2LaCl4 was also obtained as the final adduct when Lb was refluxed with 3.5 equiv. of PdCl2, during which two BOC groups are removed one-by-one after 6 and 16 h, respectively. However, when Lb was refluxed with trans-[Pd(Py)2Cl2] in methanol, no reaction was observed. In order to understand whether the coordination to Pd(II) of the S atom in methionine is necessary for the removal of BOC groups, the reaction of Lc containing diN-BOC-glycine with PdCl2 was also conducted. The result showed that the BOC groups can also be removed and the Pd(II) complex is not formed until both BOC groups are detached from Lc. These data reveal that the detachment of BOC groups is likely induced by PdCl2, which acts as a Lewis acid during the reaction. This method can be potentially applied to the preparation of analogs of Pd(II) and other metal complexes of calixarenes substituted at the lower rim by BOC-protected amino acids or peptides without prior treatment of the BOC groups.

A density functional theory study of the chemoselectivity and regioselectivity of the domino cycloaddition reactions of nitroalkenes with substituted alkenes

The chemoselectivity and regioselectivity of the domino intermolecular [4 + 2]/[3 + 2] cycloaddition reactions of nitroalkenes with substituted alkenes, vinyl ethers as electron-rich alkenes and vinyl ketones as electron-poor alkenes, have been studied using density functional theory (DFT) methods with the B3LYP functional and the 6-31G* basis set. These domino processes comprise two consecutive cycloaddition reactions: the first one is an intermolecular [4 + 2] cycloaddition of the vinyl ether to the nitroalkene to give a nitronate intermediate, which then affords the final nitroso acetal adduct through an intermolecular [3 + 2] cycloaddition reaction with the vinyl ketone. The two consecutive cycloadditions present total chemoselectivity and ortho regioselectivity. While first [4 + 2] cycloaddition reaction takes place along the attack of the electron-rich alkene to nitroalkene, the [3 + 2] one takes place along the attack of the electron-poor alkene to the corresponding nitronate intermediate. This DFT study is in complete agreement with the experimental results.

Toward an understanding of the selectivity in domino reactions. A DFT study of the reaction between acetylenedicarboxylic acid and 1, 3-Bis(2-furyl)propane

The mechanism of the domino reaction between acetylenedicarboxylic acid and 1,3-bis(2-furyl)propane has been theoretically studied in the framework of density functional theory. This domino process comprises two consecutive cycloaddition reactions: the first one is initialized by the nucleophilic attack of the C5 position of the furan ring to a conjugate position of acetylenedicarboxylic acid to give a zwitterionic intermediate, which by a subsequent ring-closure process affords an oxanorbornadiene intermediate. The second reaction is an intramolecular concerted cycloaddition of this intermediate to give the final dioxapentacyclic adduct. For the second cycloaddition, which corresponds to the step controlling the selectivity, eight alternative reaction pathways are found. Chemoselectivity, facial selectivity, and stereoselectivity of this domino reaction are related with the different approach modes of the tethered furan to the oxanorbornadiene system of the intermediate. The most favorable pathway takes place along an endo/syn approach of the furan ring relative to the bridged oxygen atom of the oxanorbornadiene system, with participation of the substituted double bond. An analysis of energetic contributions to the potential energy barriers for the intramolecular cycloadditions identifies the different factors controlling the reactive channels. Selectivity outcome is reproduced by these calculations.

A DFT study of the domino inter

The molecular mechanism of the domino inter [4 + 2]/intra [3 + 2] cycloaddition reactions of nitroalkenes with enol ethers to give nitroso acetal adducts has been characterized using density functional theory methods with the B3LYP functional and the 6-31G basis set. The presence of Lewis acid catalyst and solvent effects has been taken into account to model the experimental environment. These domino processes comprise two consecutive cycloaddition reactions: the first one is an intermolecular [4 + 2] cycloaddition of the enol ether to the nitroalkene to give a nitronate intermediate, which then affords the final nitroso acetal adduct through an intramolecular [3 + 2] cycloaddition reaction. The intermolecular [4 + 2] cycloaddition can be considered as a nucleophilic attack of the enol ether to the conjugated position of the nitroalkene, with concomitant ring closure and without intervention of an intermediate. For this cycloaddition process, the presence of the Lewis acid favors the delocalization of the negative charge that is being transferred from the enol ether to the nitroalkene and decreases the activation energy of the first cycloaddition. The [4 + 2] cycloaddition presents a total regioselectivity, while the endo/exo stereoselectivity depends on the bulk of the Lewis acid used as catalyst. Thus, for small Lewis acid catalyst, modeled by BH(3), the addition presents an endo selectivity. The [3 + 2] cycloaddition reactions present an total exo selectivity, due to the constraints imposed by the tether. Inclusion of Lewis acid catalyst and solvent effects decrease clearly the barrier for the first [4 + 2] cycloaddition relative to the second [3 + 2] one. Calculations for the activation parameters along this domino reaction allow to validate the results obtained using the potential energy barriers.

The CSIC [ Carbanion mediated Sulfonate ( Sulfonamido) Intramolecular Cyclization] reaction: Scope and limitations

The CSIC (Carbanion-mediated Sulfonate -Sulfonamide- Intramolecular Cyclization) reaction has been extended to new carbonyl containing substrates, showing the scope and limitations of this process. Suitable derivatives of ketones (e.g acetophenone ( 1)), β-keto esters (e.g ethyl acetoacetate ( 4)), γ-keto esters (e.g ethyl 2-oxocyclohexaneacetate ( 5) and ethyl levulinate ( 6)) proved reluctant to undergo this protocol. Cyclopropyl methyl ketone ( 2) gave the heterocycle ( 3), only in the “sulfonamide” synthetic sequence of the CSIC reaction. Cyclic azaketones (e.g tropinone ( 7)) failed also, but 4-piperidones ( 9, 10) afforded the novel 3,8-disubstituted 4-amino-8-aza-1-oxa-2-thiaspiro[4.5]dec-3-ene 2,2-dioxide ( 12, 15a-c) and 8-substituted 4-amino-1,8-diaza-2-thiaspiro[4.5]dec-3-ene 2,2-dioxide ( 18a, 18b, 21a, 21b) ring systems; the former compounds are the first examples of such ring systems substituted at the 3-position, whereas the latter represent the first ever representatives of spiro fused systems containing the 4-amino-2,3-dihydroisothiazole 1,1-dioxide moiety. Base promoted (NaH or DBU) cyclization of precursors 11b, 14a-c, 17b, 17c and 20 give the final adducts in good overall yield. Finally, we were unsuccessful with some conveniently functionalized anthranilonitrile derivatives ( 8a-d), in an attempt to extend the CSIC reaction to β-aminonitriles. As a result of these studies the substrate dependent reactivity in the CSIC reaction has been analyzed in depth and some restrictions and limitations have been observed and discussed.

Domino reaction between 2-acylfurans and diethyl azodicarboxylate: a combined experimental, theoretical, X-ray and dynamic NMR study

Treatment of 2-acetylfuran 6a with diethyl azodicarboxylate gave the cycloadduct 7, 6,7-diethoxycarbonyl-6,7-diaza-8-oxabicyclo[3.2.1]oct-3-en-2-one, whereas 5-methyl-2-formylfuran 6b reacted giving a simple hydrazide product derived from radical reaction on the formyl group. The structure of the bicyclic compound 7, established by NMR measurements, was confirmed by an X-ray crystallographic analysis. Variable temperature 1H NMR and 13C NMR studies of 7 indicate that this compound undergoes two distinct dynamic conformational changes with ΔG ‡ 10.2 and 13.1 kcal mol–1 respectively. The reaction mechanism associated with the domino reaction between furfural 1 and dimethyl azodicarboxylate 9 to give the cycloadduct 10 has been theoretically characterized using ab initio methods at the RHF/6-31G* level. The reaction pathway can be described as a three-step process. The first step corresponds with a [4 + 2] cycloaddition between 1 and 9, while the two subsequent steps are associated with a structural isomerization of the initial formyl cycloadduct to a more stable final adduct.

Crystallographic Evidence of [1,5]Prototropic Shifts Occurring upon Transannular Diels–Alder Reaction of a 14-Membered Macrocycle leading to a 5α-Steroid Skeleton

Reversible prototropic shifts from a stable 14-membered ring macrocycle (4Z,10E,12Z)-[1R*,14S*]-1,8,8-tris(methoxycarbonyl)-5-methyl-3,17- dioxobicyclo[12.3.0]heptadeca-4,10,12-triene, C24H30O8, occurred when the compound was heated to form rac-(5Z,11Z,13Z)-1,8,8-tris(methoxycarbonyl)-5-methyl-3,17-dioxobicyclo[12.3.0]heptadeca-5,11,13-triene, C24H30O8. The resulting compound crystallizes in a highly disordered fashion. After a transannular Diels-Alder reaction, the resulting adduct rac-3,3-bis­(methoxycarbonyl)-18- oxo-5α,9β-androst-6-en-11,17-dione, C24H30O8, was then easily converted to a 5α-steroid core, rac-3,3-bis(methoxycarbonyl)-18-oxo-5α-androst-6-en-11,17-dione, C24H30O8. The final adduct is an highly advanced intermediate for the total synthesis of naturally occurring aldosterone.

Enantioselective radical-chain hydrosilylation of alkenes using homochiral thiols as polarity-reversal catalysts

The thiol-catalysed radical-chain additions of triphenylsilane and of tris(trimethylsilyl)silane to a number of cyclic prochiral terminal alkenes have been carried out at 60 °C in the presence of di-tert-butyl hyponitrite as initiator. The function of the thiol catalyst is to promote the overall abstraction of hydrogen from the silane by the nucleophilic carbon-centred radical intermediate, formed by addition of the silyl radical to the alkene, and the stereogenic centre in the final adduct is set by hydrogen-atom transfer from the thiol to this β-silylalkyl radical. When the thiol is homochiral the transfer of hydrogen becomes enantioselective and an optically active adduct results. A number of homochiral thiols were investigated and the highest enantiomeric excesses (up to 95%) were achieved using the tetra-O-acetyl derivatives of 1-thio-β-D-glucopyranose and 1-thio-β-D-mannopyranose. The absolute configuration of an enantiopure triphenylsilane adduct (upgraded by recrystallisation) was determined by X-ray crystallography and it was shown that this adduct could be oxidatively desilylated to the corresponding alcohol and acetate with no loss of enantiomeric purity.

Stereoselective Radical Addition to .ALPHA.-Methylenebutyrolactones by Indirect Electroreduction Catalyzed by a Nickel(II) Complex.

The addition of butyl radicals to β- and/or γ-disubstituted α-methylenebutyrolactones by a nickel(II) complex-catalyzed indirect electroreduction using equimolar amounts of butyl iodides and α-methylenebutyrolactones provides cis-(C-α) : (C-β)-trisubstituted lactones as the major product. The addition was successful because the present method to permits long lifetimes for the initial butyl radicals and decreases lifetimes of the final adduct radicals by selective reduction of these radicals.

New catalytic roles for serine esterases: a 19F-NMR study of the interaction of 3,3,3-trifluoro-2,2-dihydroxy-l-phenyl-l-propanone with chicken liver carboxylesterase

The reactions of 3,3,3--trifluoro-2,2-dihydroxy-l-phenyl-l-propanone (TDPP[ with chicken liver carboxylesterase have shown that this ketone hydrate is not only a potent inhibitor of the enzyme, but also a substrate for a number of enzyme-catalyzed reactions. The kinetics of inhibition are consistent with a mechanism in which the bound hydrate is initially dehydrated in a rate-limiting step catalyzed by the enzyme. Nucleophilic attack by the active-site serine on parent ketone then produces a hemiketal adduct. However, the slow reactivation (by dialysis[ of TDPP-inhibited enzyme indicates that the interaction with this inhibitor is more complex. At equilibrium, a dissociation constant of 2.4 pM was obtained for this interaction. 19F-NMR studies of the enzyme-TDPP complex show that after pre-equilibration, the major adduct is not the hemiketal adduct. It is proposed that this final adduct is a cross-linked adduct formed between TDPP, the active-site serine and the active-site histidine. 19F-NMR studies reveal that chicken liver carboxylesterase catalyses the cleavage of TDPP to yield either fluoride ion or trifluoroacetate, and also the benzilic acid rearrangement of TDPP to α-trifluoromethylmandelai.e. These products have also been identified in model studies of the reaction between TDPP and imidazole.

KINETICS OF NUCLEOPHILIC ATTACK ON COORDINATED ORGANIC MOIETIES. PART 31 ADDITION OF IMIDAZOLES TO [Fe(CO)3(1-5-η-DIENYL)]+CATIONS

Abstract Synthetic and spectroscopic studies show that imidazole, 2-methylimidazole, and 4-methylimidazole (BH) react with the dienyl cations [Fe(CO)3(1-5-η-dienyl)]+ (Ia-Ic; dienyl = C6H7, 2-MeOC6H6, or C7H9) in CH3CN or acetone solvent via the two-step sequence shown below. With N-methylimidazole, which contains no ring N-H group, reaction stops at the monocationic adduct (III). In all cases the rate law, kobs = k1 [amine] is obeyed. This may be rationalised in terms of rate-determining addition (k1) at the dienyl ring to yield (III), followed where applicable by rapid deprotonation (k2) to yield the final neutral adduct (IV). For attack by imidazole, the rate trend is C6H7 > 2-MeOC6H6 > C7H9 (relative rates: 47:4:1), revealing a previous k1 value for cation (Ib) to be erroneous. This rate trend, together with the exo-configuration established for adducts (III), support direct addition (k1) of imidazole from above the dienyl ring. The reactivity order 2-methylimidazole > 4-methylimidazole > N-methylimid...

Molecular associations between indolizines and tetracyanoethylene

AbstractSpectroscopic, thermodynamic and kinetic properties of molecular interactions between indolizines and tetracyanoethylene are reported. The interactions involved in the complexes are weak as evidenced by the low formation constants. The kinetic behaviour indicates the initial formation of a pI‐complex that is rapidly transformed into a σ‐complex and thereafter into the final adduct(s). Apart from the results obtained by other techniques, infrared data clearly evidence vibrational contributes characteristic of the complexes, the tricyanovinyl derivatives and the tetracyanoethylene radical anion.

Spectroscopic characterization of a newly isolated cytochrome P450 from Rhodococcus rhodochrous

Cytochrome P450 (P450) from Rhodococcus rhodochrous have been characterized through circular dichroism and nuclear magnetic resonance (NMR) spectroscopy, both in the substrate-free and substrate-bound forms. The data are compared with those of P450cam and indicate a close similarity of the structure of the active site in the two proteins. The substrate-free species contains low-spin iron(III), while the 2-ethoxyphenol bound species contains high-spin iron(III). The substrate is in slow exchange on the NMR time scale. The binding of CN- has been investigated and the final adduct characterized through NMR spectra. Nuclear relaxation times of the isotropically shifted signals turn out to be shorter than in other heme proteins, both in the high- and in the low-spin species. This is the result of longer electron relaxation times in P450s than in peroxidases and metmyoglobin. This property, as well as the electron paramagnetic resonance (EPR) spectrum of the substrate-free form, are discussed in terms of the presence of the cysteine as the fifth ligand of the iron ion instead of a histidine as it occurs in peroxidases and myoglobin.

Trimethylsilylcyanid als Umpolungsreagenz, XIX[1] Dreikomponentenreaktionen zur Synthese von ungesättigten γ‐Ketoestern

Trimethylsilyl Cyanide – A Reagent for Umpolung, XIX[1]. – Unsaturated γ‐Keto Esters by Three‐Component ReactionsAddition products 1a–c of α,β‐unsaturated aldehydes and trimethylsilyl cyanide are deprotonated and added to methyl acrylate (2). On alkylation with the allylic bromo dienes 3a, b the intermediate ester enolates yield the final adducts 4aa–4cb (83–95%). Deprotection transforms these adducts into 5aa‐5cb (72–98%), which contain the functionalities of an α,β‐unsaturated ketone, a carboxylic ester group, and a diene in the appropriate arrangement for intramolecular Diels‐Alder reactions. Yields between 64 and 79% of 5 are achieved if the reaction sequence 1 + 2 + 3 → 4 → 5 is performed without isolation of 4.

Binding of kunitz and eglin c inhibitors to serine proteinases: thermodynamic, kinetic and molecular aspects

Protein-protein recognition is not a simple process, but consists of a space-time series of events involving among other aspects: molecular flexibility; induced fit; entropy, enthalpy and free energy changes; solvent; as well as hydrophobic, van der Waals and electrostatic interactions. Since the reaction of serine proteinases with macromolecular inhibitors appears to be a good model for studying protein-protein interactions, thermodynamics and kinetics for BPTI and eglin c binding to human γ-thrombin, human leukocyte elastase, bovine β-trypsin, bovine α-chymotrypsin and/or subtilisin Carlsberg have been investigated and analyzed in the light of the available molecular models. Analysis of the data indicates that proteinase: inhibitor interactions follow multi-step processes involving various intermediates, until the system leads to the final adduct.