Enzymatic formation of pyrrole-containing novel cyclic polyprenoids by bacterial squalene:hopene cyclase

A convergent synthesis provided two pyrrole-containing squalene analogues, in which a C₂₀ isoprene unit is connected to pyrrole, 2-(geranylgeranyl)pyrrole and 2-(farnesyldimethylallyl)pyrrole. When incubated with recombinant squalene:hopene cyclase from Alicyclobacillus acidocaldarius, 2-(farnesyldimethylallyl)pyrrole was enzymatically converted to a 10:1 mixture of a tricyclic and a bicyclic unnatural novel polyprenoids, whereas 2-(geranylgeranyl)pyrrole was not a substrate for the enzyme.     

Enzymatic cyclization of 22,23-dihydro-2,3-oxidosqualene into euph-7-en-3beta-ol and bacchar-12-en-3beta-ol by recombinant beta-amyrin synthase

Recombinant beta-amyrin synthase from Pisum sativum converted 22,23-dihydro-2,3-oxidosqualene, a substrate analogue lacking the terminal double bond of 2,3-oxidosqualene, into a 4:1 mixture of euph-7-en-3beta-ol and bacchar-12-en-3beta-ol. This is the first demonstration of the enzymatic formation of the baccharene skeleton with a six-membered D-ring. In the absence of the terminal double bond, the proton-initiated cyclization first generated the tetracyclic dammarenyl cation, followed by a backbone rearrangement with loss of H-7alpha leading to the formation of euph-7-en-3beta-ol, while D-ring expansion to the baccharenyl cation and subsequent 1,2-hydride shifts with H-12alpha elimination yielded bacchar-12-en-3beta-ol. It is remarkable that the formation of the anti-Markovnikov six-membered D-ring did not depend on the participation of the terminal pi-electrons.     

Enzymatic cyclization of 26- and 27-methylidenesqualene to novel unnatural C31 polyprenoids by squalene:hopene cyclase

Squalene:hopene cyclase (SHC) from Alicyclobacillus acidocaldarius accepted 26-methylidenesqualene (26-MS) and 27-methylidenesqualene (27-MS) as a substrate and converted to novel pentacyclic C₃₁ polyprenoids; a dammarene derivative with a 6.6.6.5+6 ring system and 26-methylidene-hop-22(29)-ene, respectively. The broad substrate specificity of the enzyme provided important information on the structure and function of SHC. Interestingly, 27-MS was also found to be a potent inhibitor of the bacterial SHC (IC₅₀=5 μM), while 26-MS just showed poor enzyme inhibition (IC₅₀=ca. 100 μM).     

PHOTO-EXCITED TRIPLET STATE OF DIMERIC CHLOROPHYLL a IN 3-METHYLPENTANE RIGID MATRIX AT 77 K STUDIED BY FLASH PHOTOLYSIS

Abstract— Flash photolysis studies of dimeric and monomeric chlorophyll a were carried out at 77 K. The triplet-triplet absorption spectrum of the dimeric chlorophyll a in 3-methylpentane at 77 K is interpreted as a sum of the spectra of chlorophyll a in the ground and triplet states. The dimeric chlorophyll a in the triplet state is considered to have the half-excited structure at 77 K without photodisaggregation owing to high viscosity of the solvent.     

Reaction of Cu+ with dimethoxyethane: competition between association and multiple dissociation channels

The reaction of Cu+ with dimethoxyethane (DXE) is studied using kinetic-energy dependent guided ion beam mass spectrometry. The bimolecular reaction forms an associative Cu(+)(DXE) complex that is long-lived and dissociates into several competitive channels: C4H9O2(+)+CuH, Cu(+)(C3H6O)+CH3OH, back to reactants, and other minor channels. The kinetic-energy dependences of the cross sections for the three largest product channels are interpreted with several different models (including rigorous phase space theory) to yield 0 K bond energies after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, Doppler broadening, and dissociation lifetimes. These values are compared with bond energies obtained from collision-induced dissociation (CID) studies of the Cu(+)(DXE) complex and found to be self-consistent. Although all models provide reasonable thermochemistry, phase space theory reproduces the details of the cross sections most accurately. We also examine the dynamics of this reaction using time-of-flight methods and a retarding potential analysis. This provides additional insight into the unimolecular decay of the long-lived Cu(+)(DXE) association complex. Comparison of results from this study with those from the complementary CID study, thus forming the same energized Cu(+)(DXE) complex in two distinct ways, allows an assessment of the models used to interpret CID thresholds.     

Collision-induced dissociation and theoretical studies of Cu+-dimethoxyethane complexes

Collision-induced dissociation of the dimethoxyethane (DXE) complexes with copper ions, Cu⁺(DXE)_n, n = 1 and 2, is studied using kinetic energy dependent guided ion beam mass spectrometry. For Cu⁺(DXE)₂, the only product formed corresponds to endothermic loss of a neutral ligand, while the Cu⁺(DXE) complex dissociates by several competitive channels. The cross-section thresholds for single ligand loss are interpreted to yield 0 and 298 K bond energies for Cu⁺-DXE and (DXE)Cu⁺-DXE after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. We find absolute 0 K bond dissociation energies for these complexes of 2.74 ± 0.08 and 1.87 ± 0.06 eV, respectively. These values are compared with theoretical values obtained using density functional and second order Møller-Plesset perturbation, MP2, theories. We also compare our results with previously studied alkali cation-ether complexes. Although Cu⁺ and all alkali cations have ¹S electronic ground states, the comparison shows different trends for Cu⁺ because of hybridization effects involving the valence d-electrons.     

Process research on arylnaphthalene lignan aza-analogues: a new palladium-catalyzed benzannulation of α,β-bisbenzylidenesuccinic acid derivatives

The discovery of a new Pd-catalyzed benzannulation reaction of bisbenzylidenesuccinimide during process research on arylnaphthalene lignan aza-analogues is described. An extension of the Pd-catalyzed benzannulation to the regiospecific synthesis of various arylnaphthalene lignan aza-analogues utilizing classical Stobbe condensation is included.     

Temperature effect on inverse (Tn → S1) intersystem crossing

The temperature effects on the quantum yields φ_TS of the inverse (T_n → S₁ intersystem crossings of anthracene, 9-methylanthracene and acridine are studied by the stepwise two-photon excitation technique. Φ_TS of anthracene in ethanol increases with increasing temperature, while that of 9-methylanthracene does not depend on temperature, Φ_TS of acridine in alkaline water decreases with increasing temperature.     

Enzymatic formation of indole-containing unnatural cyclic polyprenoids by bacterial squalene:hopene cyclase

[reaction: see text] Two indole-containing substrate analogues, in which a C20 isoprene unit is connected to indole (3-(geranylgeranyl)indole and 3-(farnesyldimethylallyl)indole), were synthesized and tested for enzymatic cyclization by squalene:hopene cyclase from Alicyclobacillus acidocaldarius. Interestingly, 3-(geranylgeranyl)indole was not a substrate for the bacterial squalene cyclase, while 3-(farnesyldimethylallyl)indole was efficiently converted to a 2:1 mixture of unnatural novel products.     

Enzymatic formation of an unnatural hexacyclic C35 polyprenoid by bacterial squalene cyclase

A C35 polyprene in which a farnesyl C15 unit is connected in a head-to-head fashion to a geranylgeranyl C20 unit was enzymatically converted to an unnatural hexacyclic polyprenoid by squalene:hopene cyclase from Alicyclobacillus acidocaldarius. This is the first demonstration of the remarkable ability of the squalene cyclizing enzyme to perform construction of unnatural hexacyclic skeleton. The cyclization of the C35 polyprene was initiated by a proton attack on the terminal double bond of the C15 unit and proceeded without rearrangement of carbon and hydrogen. The substrate should be folded in chair-chair-chair-chair-boat-boat conformation to achieve the stereochemistry of the cyclization product.