Directed Evolution and Biocatalysis

This review describes the current state of biocatalysis in the chemical industry. Although we recognize the advantages of chemical approaches, we suggest that the use of biological catalysis is about to expand dramatically because of the recent developments in the artificial evolution of genes that code for enzymes. For the first time it is possible to consider the rapid development of an enzyme that is designed for a specific chemical reaction. This technology offers the opportunity to adapt the enzyme to the needs of the process. We describe herein the development of enzyme evolution technology and particularly DNA shuffling. We also consider several classes of enzymes, their current applications, and the limitations that should be addressed. In a review of this length it is impossible to describe all the enzymes with potential for industrial exploitation; there are other classes, which given appropriate activity, selectivity, and robustness, could become useful tools for the industrial chemist. This is an exciting era for biocatalysis and we expect great progress in the future.     

NMR investigations on Stannabicycloundecanes of the type RSn(CH2CH2CH2)3N

The ¹H, ¹³C, and ¹¹⁹Sn NMR data of seven stannabicycloundecanes of the type RSn(CH₂CH₂CH₂)₃N (1, R = Cl; 2 , R = Br; 3 , R = I; 4 , R = OH; 5 , R = SPh; 6 , R = Me; 7 , R = Sn(CH₂CH₂CH₂)₃N) are reported. From ¹H NMR coalescence data at low temperature the free activation enthalpies for the racemisation of the bicyclo[3.3.3]skeleton were estimated to be 37 ± 1 kJ/mol. They are independent of the substituent R. However, it decreases when the tin atom is replaced by silicon for R = Me.     

Copper(I) coordination compounds with two bidentate chelating pyrazole ligands. X-ray crystal structures of DI-μ-chloro-bis[[N,N-bis(1- pyrazolylmethyl)aminoethane]copper(I)] and [bis(N,N-bis(1-pyrazolylmethyl)aminoethane]copper(I) triflate

The pyrazole derivatives of aminoethane N,N-bis(3,5-dimethyl-1-pyrazolyl- methyl)aminoethane (aebd) and N,N-bis(1-pyrazolylmethyl)aminoethane (aebp) form co- ordination compounds with copper(I) of stoichiometry [Cu(L)X], with X = Cl, Br, I and SCN, and [Cu(L)₂X], with X = CF₃SO₃ and BF₄. The ligands chelate in a bidentate manner, with only the pyrazole groups coordinating. The crystal structures of two representative examples have been determined: [Cu(aebp)Cl]₂ is triclinic, space group P , with a = 8.711(2), b = 9.351(1) and c = 9.528(1) Å, = 68.57(1)°, β = 61.47(1)° and γ = 77.82(1)°, and Z = 2. Standard least-squares refinement gave R = 0.029 (R_w = 0.038) for 1804 reflections. [Cu(aebp)₂]CF₃SO₃ is monoclinic, space group P2₁/n, with a = 13.352(5), b = 14.663(3) and c = 15.752(4) Å, β = 117.49(3)°, and Z = 4. Standard least-squares refinement gave R = 0.029 (R_w = 0.032) for 1786 reflections. In both cases the copper environment is slightly-distorted tetrahedral. The chloride compound is dimeric with one ligand molecule (Cu---N distances of 2.011(2) and 2.047(2) Å) and two bridging chlorides per copper (Cu---Cl distances of 2.3874(8) and 2.4094(8) Å). With the non- coordinating triflate anion, a monomeric compound with two ligand molecules per copper was obtained (Cu---N distances of 2.018(4), 2.028(4), 2.049(4) and 2.050(4) Å).     

1s2p resonant inelastic X-ray scattering of iron oxides

1s2p resonant inelastic X-ray scattering (RIXS) spectroscopy has been measured for a series of iron oxides, including octahedral and tetrahedral Fe(II) and Fe(III) systems. Their spectral shapes have been analyzed and explained using crystal-field multiplet simulations. The RIXS planes and the K-edge and L-edge X-ray absorption spectra related to these RIXS planes will be discussed with respect to their analytical opportunities. It is concluded that the full power and possibilities of 1s2p RIXS needs an overall resolution of 0.3 eV. This will yield a technique with more detailed information than K-edge and L-edge X-ray absorption combined, obtained in a single experiment. Another major advantage is that 1s2p RIXS involves only hard X-rays, and experiments under essentially any condition and on any system are feasible.     

Protein-induced bonding perturbation of the rhodopsin chromophore detected by double-quantum solid-state NMR

We have obtained carbon-carbon bond length data for the functional retinylidene chromophore of rhodopsin, with a spatial resolution of 3 pm. The very high resolution was obtained by performing double-quantum solid-state NMR on a set of noncrystalline isotopically labelled bovine rhodopsin samples. We detected localized perturbations of the carbon-carbon bond lengths of the retinylidene chromophore. The observations are consistent with a model in which the positive charge of the protonated Schiff base penetrates into the polyene chain and partially concentrates around the C13 position. This coincides with the proximity of a water molecule located between the glutamate-181 and serine-186 residues of the second extracellular loop, which is folded back into the transmembrane region. These measurements support the hypothesis that the polar residues of the second extracellular loop and the associated water molecule assist the rapid selective photoisomerization of the retinylidene chromophore by stabilizing a partial positive charge in the center of the polyene chain.     

Lower Rim-Upper Rim Hydrogen-Bonded Adducts of Calix[4]arenes

An upper-rim-substituted calix[4]arene tetracarboxylic acid forms hydrogen-bonded duplexes with lower-rim-substituted tetra(4-pyridyl)- and tetra(3-pyridyl)calix[4]arenes in chloroform. The formation of these adducts was studied by extraction experiments. The association constants determined via (1)H NMR dilution experiments in CDCl(3) are 7.6 x 10(3) and 1.3 x 10(3) M(-1) for the 4-pyridyl and the 3-pyridyl derivative, respectively. IR studies in the solid state and in solution indicate that the interaction is based on hydrogen bonding and that the degree of proton transfer is negligible. VPO measurements support the formation of 1:1 adducts.     

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].     

UVB exposure impairs immune responses after hepatitis B vaccination in two different mouse strains

Ultraviolet light exposure can impair immune responses that are not restricted to the exposed skin but is also found at other sites, i.e. systemic immunosuppression. Therefore, we investigated the UV-induced modulating effects on vaccination against hepatitis B in a mouse model. Two different mouse strains, BALB/c and C57B1/ 6, were vaccinated intramuscularly against hepatitis B. Mice were exposed to different doses of ultraviolet B (UVB) for five consecutive days on shaved back skin before the vaccination. Vaccination against hepatitis B induced cellular (delayed-type hypersensitivity [DTH] and lymphocyte stimulation test) as well as humoral immune responses in both mouse strains. The DTH responses in C57BB1/6 mice were statistically significantly higher compared with BALB/c mice. UVB exposure induced a dose-dependent suppression of cellular immunity in both strains of mice. C57B1/6 mice seemed to be more susceptible to this suppression. Anti-hepatitis B surface antibodies (total-Ig) were only marginally suppressed after UVB exposure. IgG2a and interferon-gamma levels, both indicators for Th1 immune response, were suppressed in both mouse strains after UVB exposure. In summary, UVB exposure induced a dose-dependent suppression of both cellular and humoral immune responses after hepatitis B vaccination, although the suppressive effects on humoral immunity were limited to IgG2a production. Susceptibility to UVB-induced immunomodulation depended on the strain of mice and their predilection for developing different T cell responses.     

The potential energy surface for the [C2H2O]+˙ system: The ketene radical cation [CH2CO]+˙ and its isomers

Ab initio molecular orbital calculations with large, polarization basis sets and incorporating valence electron correlation have been employed to examine the [C₂H₂O]^+˙ potential energy surface. Four [C₂H₂O]^+˙ isomers have been identified as potentially stable, observable ions. These are the experimentally well-known ketene radical cation, [CH₂?C?O]^+˙ (a), and the presently unknown ethynol radical cation, [CH₂?C? OH]^+˙ (b), the oxirene radical cation (c) and an ion resembling a complex of CO with [CH₂]^+˙, (d). The calculated energies of b, c and d relative to a are 189, 257 and 259 kJ mol^?1, respectively. Dissociation of ions a and d is found to occur without reverse activation energy.