Novel Technique for All-Optical Clock Extraction Using Fiber Bragg Gratings

We demonstrate a simple passive technique, using phase shifted fibre Bragg gratings, for extraction of optical clock signals from RZ optical data signals . The generation of an optical clock signal at 12.5 GHz from a 12.5 Gbs RZ optical data stream is experimentally demonstrated with simulation of optical clock extraction at 10 GHz fro m 40 Gbs OTDM data streams .     

A comparison of double-focusing sector field ICP-MS, ICP-OES and octopole collision cell ICP-MS for the high-accuracy determination of calcium in human serum

Human serum is routinely measured for total calcium content in clinical studies. A definitive high-accuracy and low-uncertainty method is required for reference measurements to underpin medical diagnoses. This study presents a novel octopole collision cell ICP-MS, high-accuracy, methodology and comparison of that technique with double-focusing sector field ICP-MS and an ICP-OES method. Double-matched isotope dilution mass spectrometry (IDMS) was employed for ICP-MS techniques and an exact matching bracketing technique using scandium as an internal standard was used for ICP-OES analysis. Medium resolution mode was utilised for double-focusing sector field ICP-MS analysis to resolve the dominant interferences on the ⁴⁴Ca/⁴²Ca isotope pair. Hydrogen reaction gas was employed to chemically resolve a number of polyatomic interferences predominantly through charge transfer reactions in the octopole collision cell. Comparison data presented for NIST CRM 909b human serum analysis from all three techniques demonstrates highest accuracy (99.6%) and lowest uncertainty (1.1%) for octopole collision cell ICP-MS. Data from ICP-OES using a non-IDMS technique produces comparably accurate data and low-uncertainties. The much higher total expanded uncertainties for double-focusing sector field ICP-MS compared with octopole collision cell data are explained by lower precision on the measurement of the ⁴⁴Ca/⁴²Ca isotope ratio. Data for octopole collision cell ICP-MS submitted for an international blind trial comparison (CCQM K-14) demonstrated excellent agreement with the mean of all participants with a low expanded uncertainty.     

Germacrene A is a product of the aristolochene synthase-mediated conversion of farnesylpyrophosphate to aristolochene

The biosynthesis of several sesquiterpenes has been proposed to proceed via germacrene A. However, to date, the production of germacrene A has not been proven directly for any of the sesquiterpene synthases for which it was postulated as an intermediate. We demonstrate here for the first time that significant amounts of germacrene A (7.5% of the total amount of products) are indeed released from wild-type aristolochene synthase (AS) from Penicillium roqueforti. Germacrene A was identified through direct GC-MS comparison to an authentic sample and through production of beta-elemene in a thermal Cope rearrangement. AS also produced a small amount of valencene through deprotonation of C6 rather than C8 in the final step of the reaction. On the basis of the X-ray structure of AS, Tyr 92 was postulated to be the active-site acid responsible for protonation of germacrene A (Caruthers, J. M.; Kang, I.; Rynkiewicz, M. J.; Cane, D. E.; Christianson, D. W. J. Biol. Chem. 2000, 275, 25533-25539). The CD spectra of a mutant protein, ASY92F, in which Tyr 92 was replaced by Phe, and of AS were very similar. ASY92F was approximately 0.1% as active as nonmutated recombinant AS. The steady-state kinetic parameters were measured as 0.138 min(-1) and 0.189 mM for k(cat) and K(M), respectively. Similar to a mutant protein of 5-epi-aristolochene (Rising, K. A.; Starks, C. M.; Noel, J. P.; Chappell, J. J. Am. Chem. Soc. 2000, 122, 1861-1866), the mutant released significant amounts of germacrene A (approximately 29%). ASY92F also produced various amounts of a further five hydrocarbons of molecular weight 204, valencene, beta-(E)-farnesene, alpha- and beta-selinene, and selina-4,11-diene.     

Vitamin B12 as an allosteric cofactor; dual fluorescence, hysteresis, oscillations and the selection of corrin over porphyrin

Studies of the emission spectra of four Co(III) cobinamides (diaquo-, aquohydroxo-, dihydroxo- and dicyano-) show (1) that the excited states corresponding to the alphabeta and epsilon absorption bands behave like the S(1) and S(2) levels in the non-alternant hydrocarbon azulene (with emission from S(2)> S(1) in violation of Kasha's rule) and (2) that the excited states include a TICT (twisted intramolecular charge transfer) mechanism, as in the simpler cyanines, but where the TICT state gives rise to dual fluorescence instead of cis-trans isomerisation. Combined with the previously reported dual fluorescence from the S(1) level in synthetic metal corrinoids and in the naturally-occurring metal-free corrin, this provides evidence that the existence of an additional (metastable) ground state with a significantly different vibronic splitting and nuclear configuration is an intrinsic property of the basic corrin ligand (irrespective of the nature of the side-chains and the metal ion or even the absence of a metal) which distinguishes it from porphyrin. The occurrence of hysteresis (and its associated oscillations) in redox reactions of the cobinamides involving both the Co(III/II) and Co(II/I) couples indicates that the corrin ligand also has an intrinsic ability to exist in different conformations or "allosteric" forms with differing redox potential, which further distinguishes it from the porphyrin ligand. Possible links between the existence of an additional metastable ground state and of allosteric changes and the likely reasons for the selection of corrin over a porphyrin for the vitamin B(12)-dependent enzymes are discussed.     

Dihydroaromatic compounds in the Diels-Alder reaction—IV : Synthesis of bicyclic ketones

The in situ isomerisation and Diels-Alder reaction with 2-acetoxyacrylonitrile, of 2,5-dihydroanisole, leads to the adducts (1) which may be used as starting point for the synthesis of a number of bicyclo [2,2,2] octenones, bicyclo [3,2,1] octenones and bicyclo [3,2,2] nonenones. Under similar conditions, 2,5-dihydrotoluene reacts via an ene-reaction to produce, after hydrolysis, the acetylcyclohexadiene (27). The base-catalysed equilibration and subsequent Diels-Alder reactions of 2,5-dihydrotoluene have been investigated.     

A photochemically driven molecular-level abacus

A molecular-level abacus-like system driven by light inputs has been designed in the form of a [2]rotaxane, comprising the pi-electron-donating macrocyclic polyether bis-p-phenylene-34-crown-10 (BPP34C10) and a dumbbell-shaped component that contains 1) a Ru(II) polypyridine complex as one of its stoppers in the form of a photoactive unit, 2) a p-terphenyl-type ring system as a rigid spacer, 3) a 4,4'-bipyridinium unit and a 3,3'-dimethyl-4,4'-bipyridinium unit as pi-electron-accepting stations, and 4) a tetraarylmethane group as the second stopper. The synthesis of the [2]rotaxane was accomplished in four successive stages. First of all, the dumbbell-shaped component of the [2]rotaxane was constructed by using conventional synthetic methodology to make 1) the so-called "west-side" comprised of the Ru(II) polypyridine complex linked by a bismethylene spacer to the p-terphenyl-type ring system terminated by a benzylic bromomethyl function and 2) the so-called "east-side" comprised of the tetraarylmethane group, attached by a polyether linkage to the bipyridinium unit, itself joined in turn by a trismethylene spacer to an incipient 3,3'-dimethyl-4,4'-bipyridinium unit. Next, 3) the "west-side" and "east-side" were fused together by means of an alkylation to give the dumbbell-shaped compound, which was 4) finally subjected to a thermodynamically driven slippage reaction, with BPP34C10 as the ring, to afford the [2]rotaxane. The structure of this interlocked molecular compound was characterized by mass spectrometry and NMR spectroscopy, which also established, along with cyclic voltammetry, the co-conformational behavior of the molecular shuttle. The stable translational isomer is the one in which the BPP34C10 component encircles the 4,4'-bipyridinium unit, in keeping with the fact that this station is a better pi-electron acceptor than the other station. This observation raises the question- can the BPP34C10 macrocycle be made to shuttle between the two stations by a sequence of photoinduced electron transfer processes? In order to find an answer to this question, the electrochemical, photophysical, and photochemical (under continuous and pulsed excitation) properties of the [2]rotaxane, its dumbbell-shaped component, and some model compounds containing electro- and photoactive units have been investigated. In an attempt to obtain the photoinduced abacus-like movement of the BPP34C10 macrocycle between the two stations, two strategies have been employed-one was based fully on processes that involved only the rotaxane components (intramolecular mechanism), while the other one required the help of external reactants (sacrificial mechanism). Both mechanisms imply a sequence of four steps (destabilization of the stable translational isomer, macrocyclic ring displacement, electronic reset, and nuclear reset) that have to compete with energy-wasteful steps. The results have demonstrated that photochemically driven switching can be performed successfully by the sacrificial mechanism, whereas, in the case of the intramolecular mechanism, it would appear that the electronic reset of the system is faster than the ring displacement.     

Aminocyclodextrins to facilitate the deprotonation of 4-tert-butyl-alpha-nitrotoluene

6A-Amino-6A-deoxy-beta-cyclodextrin enhances the rate of the deprotonation of 4-tert-butyl-alpha-nitrotoluene. The rate constants for reaction of the cyclodextrin-bound species, kinc = 4 x 10(-3), 9 x 10(-3) and 19 x 10(-3) s(-1), at pH 6.0, 6.5 and 7.0, respectively, in 0.1 mol dm(-3) aqueous phosphate buffer containing 1% methanol at 298 K. These rate constants correspond to a rate acceleration (kinc/kun) of ca. 10 times at each pH. Under the same conditions, 6A-dimethylamino-6A-deoxy-beta-cyclodextrin and 6A-(2-aminoethylamino)-6A-deoxy-beta-cyclodextrin are more effective; at pH 6.0, 6.5 and 7.0, for the former, kinc = 3 x 10(-2), 7 x 10(-2) and 12 x 10(-2) s(-1), whilst for the latter, kinc = 4 x 10(-2), 5 x 10(-2) and 9 x 10(-2) s(-1), respectively. Each cyclodextrin also decreases the pKa of the nitrotoluene, from 6.8 in free solution, to 6.2 when bound. The accelerated deprotonation by 6A-amino-6A-deoxy-beta-cyclodextrin is reflected in the enhanced rates of hydrogen-deuterium exchange of the nitrotoluene in deuterium oxide, and in the conjugate addition of the nitrotoluene to methyl vinyl ketone in aqueous solution.     

Ferrocene-containing carbohydrate dendrimers

Aliphatic amines, incorporating one or three (branched) acylated beta-D-glucopyranosyl residues, were coupled with the acid chloride of ferrocenecarboxylic acid and with the diacid chloride of 1,1'-ferrocenedicarboxylic acid to afford four dendrimer-type, carbohydrate-coated ferrocene derivatives in good yields (54-92%). Deprotection of the peracylated beta-D-glucopyranosyl residues was achieved quantitatively by using Zemplén conditions, affording four water-soluble ferrocene derivatives. When only one of the two cyclopentadienyl rings of the ferrocene unit is substituted, strong complexes are formed with beta-cyclodextrin in H2O, as demonstrated by liquid secondary ion mass spectrometry (LSIMS), 1H NMR spectroscopy, electrochemical measurements, and circular dichroism spectroscopy. Molecular dynamics calculations showed that the unsubstituted cyclopentadienyl ring is inserted through the cavity of the toroidal host in these complexes. The electrochemical behavior of the protected and deprotected ferrocene-containing dendrimers was investigated in acetonitrile and water, respectively. The diffusion coefficient decreases with increasing molecular weight of the compound. The potential for oxidation of the ferrocene core, the rate constant of heterogeneous electron transfer, and the rate constant for the energy-transfer reaction with the luminescent excited state of the [Ru(bpy)3]2+ complex (bpy = 2,2'-bipyridine) are strongly affected by the number (one or two) of substituents and by the number (one or three) of carbohydrate branches present in the substituents. These effects are assigned to shielding of the ferrocene core by the dendritic branches. Electrochemical evidence for the existence of different conformers for one of the dendrimers in aqueous solution was obtained.