Nicholas H. Rees

Biotransformation of the sesquiterpene (+)-valencene by cytochrome P450cam and P450BM-3

The sesquiterpenoids are a large class of naturally occurring compounds with biological functions and desirable properties. Oxidation of the sesquiterpene (+)-valencene by wild type and mutants of P450cam from Pseudomonas putida, and of P450BM-3 from Bacillus megaterium, have been investigated as a potential route to (+)-nootkatone, a fine fragrance. Wild type P450cam did not oxidise (+)-valencene but the mutants showed activities up to 9.8 nmol (nmol P450)(-1) min(-1), with (+)-trans-nootkatol and (+)-nootkatone constituting >85% of the products. Wild type P450BM-3 and mutants had higher activities (up to 43 min(-1)) than P450cam but were much less selective. Of the many products, cis- and trans-(+)-nootkatol, (+)-nootkatone, cis-(+)-valencene-1,10-epoxide, trans-(+)-nootkaton-9-ol, and (+)-nootkatone-13S,14-epoxide were isolated from whole-cell reactions and characterised. The selectivity patterns suggest that (+)-valencene has one binding orientation in P450cam but multiple orientations in P450BM-3.

Macrochelation, cyclometallation and G-quartet formation: N3- and C8-bound PdII complexes of adenine and guanine

The reactions of Pd(II) ions with a series of chelate-tethered derivatives of adenine and guanine have been studied and reveal a difference in the reactivity of the purine bases. Reactions of [PdCl2(MeCN)2] and A-alkyl-enH x Cl (alkyl = propyl or ethyl, A adenine, en = ethylenediamine) yield the monocationic species [PdCl(A-N3-Et-en)]+ (1) and [PdCl(A-N3-Pr-en)]+ (2). Both involve co-ordination at the minor groove site N3 of the nucleobase as confirmed by single-crystal X-ray analysis. Reactions with the analogous G-alkyl-enH x Cl derivatives (G=guanine, alkyl = ethyl or propyl) were more complex with a mixture of species being observed. For G-Et-en HCI a product was isolated which was identified as [PdCl(G-C8-Et-en)]+ (3). This compound contains a biomolecular metal-carbon bond involving C8 of the purine base. Crystallography of a product obtained from reaction of G-Pr-enH x Cl and [Pd(MeCN)4][NO3]2 reveals an octacationic tetrameric complex (4), in which each ligand acts to bridge two metal ions through a combination of a tridentate binding mode involving the diamine and N3 and monodentate coordination at N7.

Anion induced and inhibited circumrotation of a [2]catenane.

The first example of a catenane capable of performing circumrotation via an anion switching methodology is described; of particular interest is a conformational locking mechanism which results from chloride coordination in the catenane binding cavity.

NMR assignment of absolute configuration of a P-chiral diphosphine and mechanics of its stereoselective formation

Abstract Two-dimensional rotating-frame nuclear Overhauser enhancement (ROESY) NMR spectra are used to determine the absolute configuration of (+)-diphenyl-phosphine-2,3-dimethyl-7-phenyl-7-phosphabicyclo[2.2.1]hept-2-ene. This diphosphine ligand is obtained from the palladium complex-promoted Diels-Alder reaction between diphenylvinylphosphine and 1-phenyl-3,3-dimethylphosphole in the presence of ( R )-dimethyl(1-(α-naphthyl)ethylamine as the chiral auxiliary. The origin of the stereoselectivity in this asymmetric reaction is also revealed by solution NMR studies.

Sr3Co2O4.33H0.84: An Extended Transition Metal Oxide-Hydride

Reaction of the n = 2 Ruddlesden-Popper oxide Sr(3)Co(2)O(5.80) with CaH(2) yields an extended oxide-hydride phase: Sr(3)Co(2)O(4.33)H(0.84). Neutron powder diffraction data reveal the material adopts a body-centered orthorhombic structure (Immm: a = 3.7551(5) Å, b = 3.7048(4) Å, c = 21.480(3) Å) in which the hydride ions are accommodated within disordered CoO(1.16)H(0.46) layers. Low temperature neutron powder diffraction data show no evidence for long-range magnetic order, suggesting the chemical disorder in the anion lattice of the material leads to magnetic frustration.

Solid-State Synthesis and Characterization of σ-Alkane Complexes, [Rh(L2)(η2,η2-C7H12)][BArF4] (L2 = Bidentate Chelating Phosphine)

The use of solid/gas and single-crystal to single-crystal synthetic routes is reported for the synthesis and characterization of a number of σ-alkane complexes: [Rh(R2P(CH2)nPR2)(η(2),η(2)-C7H12)][BAr(F)4]; R = Cy, n = 2; R = (i)Pr, n = 2,3; Ar = 3,5-C6H3(CF3)2. These norbornane adducts are formed by simple hydrogenation of the corresponding norbornadiene precursor in the solid state. For R = Cy (n = 2), the resulting complex is remarkably stable (months at 298 K), allowing for full characterization using single-crystal X-ray diffraction. The solid-state structure shows no disorder, and the structural metrics can be accurately determined, while the (1)H chemical shifts of the Rh···H-C motif can be determined using solid-state NMR spectroscopy. DFT calculations show that the bonding between the metal fragment and the alkane can be best characterized as a three-center, two-electron interaction, of which σCH → Rh donation is the major component. The other alkane complexes exhibit solid-state (31)P NMR data consistent with their formation, but they are now much less persistent at 298 K and ultimately give the corresponding zwitterions in which [BAr(F)4](-) coordinates and NBA is lost. The solid-state structures, as determined by X-ray crystallography, for all these [BAr(F)4](-) adducts are reported. DFT calculations suggest that the molecular zwitterions within these structures are all significantly more stable than their corresponding σ-alkane cations, suggesting that the solid-state motif has a strong influence on their observed relative stabilities.

The mechanism of decomposition of N-methyl-N-nitrosourea (MNU) in water and a study of its reactions with 2′-deoxyguanosine, 2′-deoxyguanosine 5′-monophosphate and d(GTGCAC)☆

Abstract The carcinogenicity of N-methyl-N-nitrosourea (MNU) arises from its ability to methylate DNA. This occurs in an aqueous environment and therefore an appreciation of the mode of decomposition of MNU in water is essential to understanding the mechanism of DNA methylation and its base sequence dependence. The kinetics of MNU hydrolyses are shown to be first order in MNU with a steep rise in rate above pH 8. Using NMR for in situ monitoring of reaction intermediates and products from hydrolyses of [13CO]MNU, [15NH2]MNU and [13CH3]MNU, it is proved that base-induced hydrolysis of MNU is initiated by deprotonation at the carbamoyl group. The critical reactive species are shown to be the methyldiazonium ion (Me-N2+) and cyanate (NCO−). Investigations of reactions of [13CH3]MNU with 2′-deoxyguanosine (dGuo) and 2′-deoxyguanosine 5′-monophosphate (dGuo-5P) showed that: a) the site of methylation of dGuo is highly pH-dependent (relatively more N-1 and O6-methylation compared to N-7 occurs at higher pH); b) the principal site of methylation of dGuo-5P by MNU is at phosphate; c) incorporation of deuterium into methyl groups occurs in D2O at higher pH. Methylation of the oligonucleotide d(GT[15N]GCAC) by MNU in D2O showed partial deuteriation of the N7-methyl groups of the guanines, whilst methylation by MNU in water indicated no significant preference for either guanine with respect to N7-methylation.

Dearomatisation of o-Xylene by P450BM3 (CYP102A1)

The oxidation of o-xylene by P450(BM3) from Bacillus megaterium yields, in addition to the products formed by microsomal P450s, two metabolites containing an NIH-shifted methyl group, one of which lacks the aromatic character of the substrate. The failure of the epoxide precursor of these two products to rearrange to the more stable 2,7-dimethyloxepin suggests that ring opening is P450-mediated. With m-xylene, the principal metabolite is 2,4-dimethylphenol. The partition between aromatic and benzylic hydroxylation is primarily governed by the steric prescriptions of the active site rather than by C-H bond reactivity. It is also substrate-dependent, o- and m-xylene appearing to bind to the enzyme in different orientations. The product distributions given by variants containing the F87A mutation, which creates additional space in the active site, resemble those reported for microsomal systems.

Structural Analysis of CYP101C1 from Novosphingobium aromaticivorans DSM12444.

CYP101C1 from Novosphingobium aromaticivorans DSM12444 is a homologue of CYP101D1 and CYP101D2 enzymes from the same bacterium and CYP101A1 from Pseudomonas putida. CYP101C1 does not bind camphor but is capable of binding and hydroxylating ionone derivatives including α‐ and β‐ionone and β‐damascone. The activity of CYP101C1 was highest with β‐damascone (kcat=86 s−1) but α‐ionone oxidation was the most regioselective (98 % at C3). The crystal structures of hexane‐2,5‐diol‐ and β‐ionone‐bound CYP101C1 have been solved; both have open conformations and the hexanediol‐bound form has a clear access channel from the heme to the bulk solvent. The entrance of this channel is blocked when β‐ionone binds to the enzyme. The heme moiety of CYP101C1 is in a significantly different environment compared to the other structurally characterised CYP101 enzymes. The likely ferredoxin binding site on the proximal face of CYP101C1 has a different topology but a similar overall positive charge compared to CYP101D1 and CYP101D2, all of which accept electrons from the ArR/Arx class I electron transfer system.

New group 10 complexes of the bulky iminophosphine ligands [Ph2PCH2C(Ph)N(2,6-R2C6H3)], where R = Me, iPr

New neutral and cationic complexes [NiBr2(L1)] (1), [NiBr2(OPHPh2)(L1)] (2), [NiMe2(L1)] (3), [NiBr(PMe3)(L1)](Br) (4), [Ni(CH3CN)(PMe3)(L1)](BF4)2 (5), [PdBr2(L2)] (6), [PdI2(L1)] (7), [PtMeCl(L1)] (8), [PtMe2(L1)] (9), [Pt(CH3CN)2(L1)](BF4)2 (10), [Pt(L1)2](X)2 [X = Cl (11a), Br (11b)], [PtX(L1)2](X) [X = Cl (12a), Br (12b)], where L1: [Ph2PCH2C(Ph)N(2,6-Me2C6H3)2] and L2: [Ph2PCH2C(Ph)N(2,6-iPr2C6H3)2], have been prepared and characterised. The molecular structures of 1, 2, 6, 7 and 9 have been determined. The complexes [PdBr2(L2)] (6), [PdBr2(L1)] and [PdMeCl(L1)] have been found to catalyse the Heck coupling of 4-bromoacetophenone with n-butyl acrylate under aerobic conditions.