Catalytic roles of active-site amino acid residues of coenzyme B12-dependent diol dehydratase: protonation state of histidine and pull effect of glutamate

The hydrogen abstraction and the OH migration processes catalyzed by diol dehydratase are discussed by means of a quantum mechanical/molecular mechanical method. To evaluate the push effect of His143 and the pull effect of Glu170, we considered three kinds of whole-enzyme model, the protonated and two unprotonated His143 models. A calculated activation energy for the hydrogen abstraction by the adenosyl radical is 15.6 (13.6) kcal/mol in the protonated (unprotonated) His143 model. QM/MM calculational results show that the mechanism of the OH migration is significantly changed by the protonation of His143. In the protonated His143 model, the OH group migration triggered by the full proton donation from the imidazolium to the migrating OH group occurs by a stepwise OH abstraction/re-addition process in which the water production reduces the barrier for the C-O bond cleavage. On the other hand, the OH migration in the unprotonated His143 model proceeds in a concerted manner, as we previously proposed using a simple model including only K+ ion and substrate. The latter mechanism seems to be kinetically more favorable from the calculated energy profiles and is consistent with experimental results. The activation barrier of the OH group migration step is only 1.6 kcal/mol reduced by the hydrogen-bonding interaction between the O2 of the substrate and unprotonated His143. Thus, it is predicted that His143 is not protonated, and therefore the main active-site amino acid residue that lowers the energy of the transition state for the OH group migration is determined to be Glu170.     

Siloxyallenes revisited. A useful functional intermediate for the synthesis of (Z)-β-branched Morita-Baylis-Hillman type adducts and (Z)-chalcones

Siloxyallenes proved to be a useful functional intermediate in the preparation of (Z)-β-branched Morita-Baylis-Hillman type adducts by the reaction of aldehydes with silylacetylenes or siloxypropynes. Various (Z)-chalcones were stereoselectively synthesized from siloxypropynes via siloxyallenes.     

Isolation of an inclusion complex of naphthol and its benzoate as an intermediate in the solvent-free benzoylation reaction of naphthol

Solvent-free benzoylation of naphthol was found to proceed via an inclusion complex intermediate of the naphthol and its benzoate by IR spectral monitoring.     

PICOSECOND LASER PHOTOLYSIS OF SQUID RHODOPSIN AT ROOM AND LOW TEMPERATURES

Abstract. Squid rhodopsin extracted with 2% digitonin (pH 10.5 or 7.0) was excited with a 347 nm light pulse from a mode-locked ruby laser at room temperature. Within 19 ps after the excitation, absorbance at 430 nm due to hypsorhodopsin increased and subsequently decreased with a decay time of 45 ± 10 ps. Absorbance at 550 nm due to bathorhodopsin increased with a rise time of 50 ± 10 ps. These results are the first observations of hypsorhodopsin at room temperature and clearly show that hypsorhodopsin is a precursor of bathorhodopsin which has been considered to be the earliest photoproduct in the photobleaching process of rhodopsin.
Hypsorhodopsin appeared with a rise time of 70 ± 10 ps at 421 nm at liquid nitrogen temperature without any bathorhodopsin being observed during the formation of hypsorhodopsin. An experiment using an N₂ laser showed that squid bathorhodopsin converted to lumirhodopsin with a decay time of about 300 ns at room temperature.     

The effects of random fields on the critical and bicritical behavior of Mn x Zn1−x F2

We have carried out a comprehensive study of the static and dynamic spin-spin correlations of Mn _x Zn_1–x F₂ in a magnetic field. Samples withx=0.75 andx=0.5 have been studied. This system exhibits behavior closely related, if not identical, to that of the Random Field Ising Model (RFIM). An additional feature of Mn _x Zn_1–x F₂ is that it exhibits an easily accessible bicritical point; thus one can study the changeover from the RFIM to the uniformXY model with a transverse random field. Quite generally, the instantaneous spin-spin correlations in a field are described by a combination of Lorentzian, Lorentzian-squared and delta function terms the latter corresponds to the long range order (LRO) component. In the Ising phase one finds history dependent behavior as discussed previously. In theXY phase, except very near the spin-flop boundary, one finds ergodic behavior withXY LRO and Lorentzian squared Ising fluctuations. Rather complicated instability effects are found all along the spin-flop boundary. Further, when one establishes LRO in theXY phase and lowers the field through the spin-flop value, one obtains a LRO Ising state in thex=0.75 sample whereas one obtains the field-cooled domain state in thex=0.50 sample. This dramatic difference in behavior is not understood. Our results on the RFIM aspects of the problem are consistent with our previous studies. The transition is dominated by the metastability effects with an underlying equilibrium transition which is either first order or weakly second order (0). The underlying transition manifests itself directly in measurements of the dynamic response nearT _N (H). From the data above the metastability boundary we deduce for the static correlation length exponentv=1.4±0.3 in good agreement with theory. We find for the RFIM crossover exponent _RF=1.5±0.2 where the errors represent the spread in values obtained from different techniques. Finally, we have determined in detail the field-temperature phase diagram of thex=0.5 sample including the critical behavior along the spin-flop line; the latter transition appears to be second order for an extended region.     

Ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pd(en)2] [PdCl2(en)2] (ClO4)4

The ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pd(en)₂] [PdCl₂(en)₂] (ClO₄)₄ (en = ethylenediamine) has been investigated by the use of a femtosecond absorption spectrum, calculated from a pump—probe reflection spectrum at room temperature by the Kramers—Kronig relations. The time dependence of the transient photoinduced absorption around 1.7 eV and the bleaching from 1.9 to 2.5 eV were calculated for three decay components. They are free excitons with a lifetime of about 800 fs, self-trapped excitons with a lifetime of about 3 ps, and polaron pairs which relax within a 100 ps time period.     

First observation of clusters for solvated tropylium ions

We have first observed clusters for solvated tropylium ions (Tr+(ROH)n) which were isolated from ROH-CH3CN (1:1 by vol.; R = Me, Et, and Prn) solutions by using a specially designed mass spectrometer and found the clear-cut essential features concerning the solvation structure around Tr+.     

Syntheses of potential metabolites of a potent kappa-opioid receptor agonist, TRK-820

Chemical syntheses of three kinds of potential metabolites of TRK-820, a potent kappa-opioid receptor agonist, were described. One of the potential metabolites 2, 17-N-dealkylated TRK-820, was synthesized starting from noroxycodone through 8 steps in 21% total yield. Glucuronidation of intermediate 10 and compound 1, the free base of TRK-820, was carried out stereoselectively to give 3-O-beta-D-glucuronides 15 and 16 in good yields, respectively. Syntheses of potential conjugated metabolites 3 and 4 were accomplished through 10 steps and 2 steps in 11% and 43% total yields, respectively. Among the potential metabolites of TRK-820, compounds 2 and 4 were identified as metabolites in human hepatocytes. The results of pharmacological studies of compounds 2, 3, and 4 are described.