Giant magneto-optical response of ferromagnetic EuB<Subscript><Emphasis Type="Bold">6</Emphasis></Subscript>

Intercalation of polyatomic molecules into a superconductor can drastically affect the properties of the compound. A mechanism responsible for a large increase in T _c for such systems is proposed. It explains the recent remarkable observation of high T _c superconductivity in the hole-doped C₆₀/CHX₃ (X≡Cl, Br) compounds and the large shift in their T _c upon Cl↦Br substitution. The increase in T _c is due to contribution to the pairing arising from the interaction of electrons with the vibrational manifold of the molecule. The proposed mechanism opens up the possibility to observe a site-selective isotope effect. We also suggest that intercalating CHI₃ would further increase the critical temperature to T _c≃ 140 K. Received 23 January 2002     

Characterization of copolymer hydroxybutyrate/hydroxyvalerate from saponified vernonia,soybean, and "spent" frying oils

Poly(beta-hydroxyalkanoate)s (PHAs) were biosynthesized by Ralstonia eutropha (formerly known as Alcaligenes eutrophus) by using saponified soybean, vernonia, and "spent" frying oils. These PHAs were isolated and characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), gas chromatography/mass spectrometry (GC/MS), proton nuclear magnetic resonance spectrometry (1H NMR), and 2-dimensional homonuclear (1H-1H) correlation spectroscopy (COSY). The analytical results revealed that the PHAs produced from saponified vernonia and soybean oils were copolymers of hydroxybutyrate (HB) and hydroxyvalerate (HV), that is, P(HB/HV)s, whereas the saponified "spent" frying oil produced only poly(beta- hydroxybutyrate) (PHB) homopolymer. MALDI-MS, GC/MS, and NMR independently confirmed the composition of the PHAs. Saponified soybean oil and vernonia oil PHAs contained approximately 4 and 1% HV units, respectively. For comparison, commercial PHB and P(HB/HV), produced by R. eutropha by using glucose and a cosubstrate of glucose and propionic acid, respectively, as carbon sources, were similarly characterized.     

Coordination of adenosylmethionine to a unique iron site of the [4Fe-4S] of pyruvate formate-lyase activating enzyme: a Mössbauer spectroscopic study

Pyruvate formate-lyase activating enzyme (PFL-AE) generates the catalytically essential glycyl radical of PFL. It is a member of the so-called "radical-SAM superfamily" of enzymes that use a [4Fe-4S] cluster and S-adenosylmethionine (AdoMet or SAM) to catalyze diverse radical-mediated reactions. Evidence suggests that this class of enzymes operate by common initial steps involving the generation of an AdoMet-derived adenosyl radical intermediate, of which the mechanism remains unresolved. The three-cysteine CX3CX2C cluster-binding motif common to all members of this superfamily suggests a unique Fe site in the [4Fe-4S] cluster, which presumably interacts with AdoMet to effect the reductive cleavage and radical generation. Here we employ a dual-iron-isotope (56Fe/57Fe) approach to demonstrate the existence of a unique Fe site in the [4Fe-4S] cluster of PFL-AE by M?ssbauer spectroscopy. Coordination of AdoMet to this unique Fe site was made evident by the observation of a substantial increase in the isomer shift (delta) of the M?ssbauer spectrum associated with the unique Fe site: delta = 0.42 mm/s in the absence of AdoMet increases to delta = 0.72 mm/s in the presence of AdoMet. Further, the M?ssbauer data show that the binding of AdoMet to the unique Fe site occurs in the [4Fe-4S]2+ state, prior to the injection of the reducing equivalent required for catalysis. This observation indicates that AdoMet coordination is a necessary prerequisite to adenosyl radical generation.     

Harmonic generation in a two-dimensional nonlinear quasi-crystal

Second-harmonic generation in a two-dimensional nonlinear quasi-crystal is demonstrated for the first time to our knowledge. Temperature and wavelength tuning of the crystal reveal the uniformity of the pattern while angle tuning reveals the dense nature of the crystal's Fourier spectrum. These results compare well with theoretical predictions showing the excellent uniformity of the crystal and suggest that more-complicated nonlinear holograms should be possible.     

Experimental observation of nonlinear pulse compression in nonuniform Bragg gratings

We demonstrate a scheme for optical pulse compression by cross-phase modulation that utilizes a nonuniform Bragg grating to work in reflection. Our scheme is similar to the conventional optical pushbroom, which works in transmission. This reflection geometry has the advantage of allowing the compressed signal to be observed easily, as it is spatially separate from the pump. This is to our knowledge the first nonlinear effect to be observed that requires a nonuniform grating.     

Nonlinear self-switching and multiple gap-soliton formation in a fiber Bragg grating

We report, for the first time to our knowledge, the experimental observation of quasi-cw nonlinear switching and multiple gap-soliton formation within the bandgap of a fiber Bragg grating. As many as five gap solitons with 100-500-ps durations were generated from a 2-ns pulse at a launched peak intensity of approximately 27 GW/cm(2). A corresponding increase in the grating transmission from 3% to 40% of the incident pulse energy was observed.     

Complexity and directional entropy in two dimensions

We study the directional entropy of a dynamical system associated to a Z² configuration in a finite alphabet. We show that under local assumptions on the complexity, either every direction has zero topological entropy or some direction is periodic. In particular, we show that all nonexpansive directions in a Z² system with the same local assumptions have zero directional entropy.     

Redox control of a polymerization catalyst by changing the oxidation state of the metal center

The activity of cerium alkoxide complexes supported by a Schiff base ligand was controlled using redox reagents during the ring-opening polymerization of L-lactide. The rate of L-lactide polymerization was modified by switching in situ between the cerium(III) and cerium(IV) species.     

Redox control of a ring-opening polymerization catalyst

The activity of an yttrium alkoxide complex supported by a ferrocene-based ligand was controlled using redox reagents during the ring-opening polymerization of L-lactide. The oxidized complex was characterized by X-ray crystallography and (1)H NMR, XANES, and M?ssbauer spectroscopy. Switching in situ between the oxidized and reduced yttrium complexes resulted in a change in the rate of polymerization of L-lactide. Synthesized polymers were analyzed by gel permeation chromatography. Polymerization of trimethylene carbonate was also performed with the reduced and oxidized forms of an indium alkoxide complex. The indium system showed the opposite behavior to that of yttrium, revealing a metal-based dependency on the rate of polymerization.