On the Biosynthesis of Irregular Terpenes: Mechanistic Studies Employing the Thiamine Thiazole as an Isoprenoid Analogue.

The irregular terpene, 2,6,9,13-tetramethyl-6-vinyl-tetradeca-2,8,12-trien-7-one (18), and the terpenoid, 1-phenyl-2,6-dimethyl-2-vinyl-hept-5-en-1-one (19) were synthesized by the Continuous Flow Reformatsky method using geranoyl chloride and benzoyl chloride, respectively. Geranyl bromide was used to generate the required organo-zinc adduct. Citral and geranyl bromide produced 3,6,9,13-tetramethyl-9-vinyl-tetradeca-3,7,12-trien-6-ol (17) by this method. Attempts to isomerize 17 to 2,6,9,13-tetramethyl-6-vinyl-tetradeca-2,8,12-trien-7-ol failed. It is postulated that 17 is more stable due to steric compression around the quaternary carbon. Comparison of 18 and the terpene isolated from yeast enzyme preparations by Bell(\u27135) showed them to be identical. Attempts to synthesize 2-hydroxygeranyl thiamine by the addition of thiamine to citral failed. Cinnamaldehyde also failed to react with thiamine to yield 2-hydroxycinnamyl thiamine. Thiamine and 3-methylbenzothiazolium hemisulfate both react with benzaldehyde to produce 2-benzoyl-3-(2-methyl-4-aminopyrimidin-5-yl) methyl-3a-methylperhydrofuro {2,3-d} thiazole (5) and 3-methylbenzothiazolinyl phenyl ketone (8), respectively. Since 8 is formed via a carbene, a similar proposal is made to account for the behavior of thiamine. Tritiated citronellal, prepared by the selective hydrogenation of the (alpha),(beta)-olefin of tritiated citral, was reacted with thiamine to produce labelled 2-(1-hydroxy-3,7-dimethyl-1-oct-6-enyl) thiamine chloride hydrochloride ((\u273)H-1). When (\u273)H-1 was incubated with geraniol or farnesol 7% of the label was incorporated into 2,6,9,13-tetramethyl-6-vinyl-2,12-tetradecadien-7-one (18a) and 2,6,9,13,17-pentamethyl-9-vinyl-octadeca-2,11,16-trien-8-one (26), respectively. Labelling studies suggest that squalene synthetase is less specific for chain length of the terpene substrate at Site II, than at Site I. Also the thiamine thiazole mimics an isoprenoid moiety of farnesol and the cofactor is not involved in squalene synthesis as proposed by Woodward. The terpenoid, 2-(1-hydroxy-3,7-dimethyloct-6-enyl)-4-methyl-5-(2-hydroxyethyl) thiazole (2) was found to be inactive in yeast enzyme preparations.(\u27). The formation of 2-alkyl derivatives of thiamine via the Hantzsch Thiazole Synthesis was investigated. Using model systems, S-phenacyl thiobenzanilide and S-(p-chlorophenacyl) thiononanilide were synthesized but failed to yield the desired thiazolium salts. Steric hindrance of the substituents was responsible for this failure. Thus, based on the isolation of 18 from the cell-free yeast enzyme preparation, it is suggested that while thiamine is not involved in tail-tail dimerization, the vitamin may play a role in the formation of artemisia ketone and bakuchiol

Crosstalk Correction in Atomic Force Microscopy

Commercial atomic force microscopes usually use a four-segmented photodiode to detect the motion of the cantilever via laser beam deflection. This read-out technique enables to measure bending and torsion of the cantilever separately. A slight angle between the orientation of the photodiode and the plane of the readout beam, however, causes false signals in both readout channels, so-called crosstalk, that may lead to misinterpretation of the acquired data. We demonstrate this fault with images recorded in contact mode on ferroelectric crystals and present an electronic circuit to compensate for it, thereby enabling crosstalk-free imaging

Lateral Signals in Piezoresponse Force Microscopy at Domain Boundaries of Ferroelectric Crystals

In piezoresponse force microscopy a lateral signal at the domain boundaries is occasionally observed. In recent years, a couple of experiments have been reported and varying explanations for the origin of this lateral signal have been proposed. Additionally, elaborated theoretical modeling for this particular issue has been carried out. Here we present experimental data obtained on different crystallographic cuts of $\rm LiNbO_3$, $\rm BaTiO_3$, and $\rm KTiOPO_4$ single crystals. We could thereby rule out some of the explanations proposed so far, introduce another possible mechanism, and quantitatively compare our results to the existing modeling

Contrast Mechanisms for the Detection of Ferroelectric Domains with Scanning Force Microscopy

We present a full analysis of the contrast mechanisms for the detection of ferroelectric domains on all faces of bulk single crystals using scanning force microscopy exemplified on hexagonally poled lithium niobate. The domain contrast can be attributed to three different mechanisms: i) the thickness change of the sample due to an out-of-plane piezoelectric response (standard piezoresponse force microscopy), ii) the lateral displacement of the sample surface due to an in-plane piezoresponse, and iii) the electrostatic tip-sample interaction at the domain boundaries caused by surface charges on the crystallographic y- and z-faces. A careful analysis of the movement of the cantilever with respect to its orientation relative to the crystallographic axes of the sample allows a clear attribution of the observed domain contrast to the driving forces respectively.Comment: 8 pages, 8 figure

Depth resolution of Piezoresponse force microscopy

Given that a ferroelectric domain is generally a three dimensional entity, the determination of its area as well as its depth is mandatory for full characterization. Piezoresponse force microscopy (PFM) is known for its ability to map the lateral dimensions of ferroelectric domains with high accuracy. However, no depth profile information has been readily available so far. Here, we have used ferroelectric domains of known depth profile to determine the dependence of the PFM response on the depth of the domain, and thus effectively the depth resolution of PFM detection

Precision nanoscale domain engineering of lithium niobate via UV laser induced inhibition of poling

Continuous wave ultraviolet (UV) laser irradiation at lambda=244 nm on the +z face of undoped and MgO doped congruent lithium niobate single crystals has been observed to inhibit ferroelectric domain inversion. The inhibition occurs directly beneath the illuminated regions, in a depth greater than 100 nm during subsequent electric field poling of the crystal. Domain inhibition was confirmed by both differential domain etching and piezoresponse force microscopy. This effect allows the formation of arbitrarily shaped domains in lithium niobate and forms the basis of a high spatial resolution micro-structuring approach when followed by chemical etching

Critical Casimir effect in classical binary liquid mixtures

If a fluctuating medium is confined, the ensuing perturbation of its fluctuation spectrum generates Casimir-like effective forces acting on its confining surfaces. Near a continuous phase transition of such a medium the corresponding order parameter fluctuations occur on all length scales and therefore close to the critical point this effect acquires a universal character, i.e., to a large extent it is independent of the microscopic details of the actual system. Accordingly it can be calculated theoretically by studying suitable representative model systems. We report on the direct measurement of critical Casimir forces by total internal reflection microscopy (TIRM), with femto-Newton resolution. The corresponding potentials are determined for individual colloidal particles floating above a substrate under the action of the critical thermal noise in the solvent medium, constituted by a binary liquid mixture of water and 2,6-lutidine near its lower consolute point. Depending on the relative adsorption preferences of the colloid and substrate surfaces with respect to the two components of the binary liquid mixture, we observe that, upon approaching the critical point of the solvent, attractive or repulsive forces emerge and supersede those prevailing away from it. Based on the knowledge of the critical Casimir forces acting in film geometries within the Ising universality class and with equal or opposing boundary conditions, we provide the corresponding theoretical predictions for the sphere-planar wall geometry of the experiment. The experimental data for the effective potential can be interpreted consistently in terms of these predictions and a remarkable quantitative agreement is observed.Comment: 30 pages, 17 figure

Collective magnetism at multiferroic vortex domain walls

Topological defects have been playgrounds for many emergent phenomena in complex matter such as superfluids, liquid crystals, and early universe. Recently, vortex-like topological defects with six interlocked structural antiphase and ferroelectric domains merging into a vortex core were revealed in multiferroic hexagonal manganites. Numerous vortices are found to form an intriguing self-organized network. Thus, it is imperative to find out the magnetic nature of these vortices. Using cryogenic magnetic force microscopy, we discovered unprecedented alternating net moments at domain walls around vortices that can correlate over the entire vortex network in hexagonal ErMnO3 The collective nature of domain wall magnetism originates from the uncompensated Er3+ moments and the correlated organization of the vortex network. Furthermore, our proposed model indicates a fascinating phenomenon of field-controllable spin chirality. Our results demonstrate a new route to achieving magnetoelectric coupling at domain walls in single-phase multiferroics, which may be harnessed for nanoscale multifunctional devices.Comment: 18 pages, 10 figure