Straight Conformation Research Articles

Intrinsic Bending of Microtubule Protofilaments

The complex polymerization dynamics of the microtubule (MT) plus end are closely linked to the hydrolysis of the GTP nucleotide bound to the β-tubulin. The destabilization is thought to be associated with the conformational change of the tubulin dimers from the straight conformation in the MT lattice to a curved conformation. It remains under debate whether this transformation is directly related to the nucleotide state, or a consequence of the longitudinal or lateral contacts in the MT lattice. Here, we present large-scale atomistic simulations of short tubulin protofilaments with both nucleotide states, starting from both extreme conformations. Our simulations indicate that both interdimer and intradimer contacts in both GDP and GTP-bound tubulin dimers and protofilaments in solution bend. There are no observable differences between the mesoscopic properties of the contacts in GTP and GDP-bound tubulin or the intradime and interdimer interfaces.

Direct Visualization of Walking Myosin V Molecules by High-Speed AFM

Facilitated by the processivity of myosin V, numerous single-molecule studies have demonstrated that it moves hand over hand with a ∼36 nm advance for every ATP hydrolysis. However, these results are obtained using optical microscopy and therefore the molecule itself is invisible in the observations. The motor has been visualized using electron microscopy but the obtained images are static. Thus, the detailed structural dynamic of walking myosin V and its functional mechanism are not yet fully described. Here, we directly visualized walking M5-HMM (tail-truncated myosin V) using high-speed AFM with nanometer and sub-second spatiotemporal resolution. The molecular movies provided not only corroborative visual evidence for established and speculated behaviors but also revealed unappreciated ones. First, it was cleared that the forward movement is driven not by bending but by rotation of the leading head. The leading head often showed ‘foot-stomping-like brief detachment and rebinding followed by forward step. In ADP, the coiled-coil tail of the two-headed bound molecule occasionally unwound, which was followed by the leading lever-arm swing, indicating the presence of intramolecular tension responsible for the powerstroke. From these observations, it is concluded that tension for forward movement can be produced without transitioning through a weak-binding ADP-Pi-bound state. Moreover, in low concentrations of ADP, the leading head showed conformational switching between the straight and sharp bend conformations, from which the rate of ADP dissociation from the leading head was estimated to be 0.1/s. This is straightforward evidence that the ATPase cycle is essentially stalled at the leading head. Thus, dynamic visualization of functioning M5-HMM by high-speed AFM lead to a comprehensive understanding of how it operates to function.

Free-Energy Landscape of the Helical Wrapping of a Carbon Nanotube by a Polysaccharide

Carbon nanotubes wrapped by polysaccharide chains like chitosan (CHTS) or its derivatives through noncovalent decoration have been shown to condense effectively and deliver DNA for gene therapy. Despite the importance of these novel nanoscale materials, the detail of the microscopic structure and underlying interaction mechanism is still fragmentary. In the present work, the complex formed by CHTS and a single-walled carbon nanotube (SWNT) has been investigated by means of atomistic molecular dynamics (MD) simulations to explore its propensity toward self-assembly, together with its structural properties in an aqueous environment. The present results reveal that CHTS can wrap spontaneously the tubular surface by regulating backbone torsional stress upon van der Waals attraction by the SWNT, resulting in a steady, right-handed helical conformation. The free-energy landscape characterizing the wrapping process of the CHTS chain from a straight conformation to a tight helical one brings to light two energeti...

Increased paclitaxel cytotoxicity against cancer cell lines using a novel functionalized carbon nanotube

Potential applications of carbon nanotubes have attracted many researchers in the field of drug delivery systems. In this study, multiwalled carbon nanotubes (MWNTs) were first functionalized using hyperbranched poly citric acid (PCA) to improve their hydrophilicity and functionality. Then, paclitaxel (PTX), a potent anticancer agent, was conjugated to the carboxyl functional groups of poly citric acid via a cleavable ester bond to obtain a MWNT-g-PCA-PTX conjugate. Drug content of the conjugate was about 38% (w/w). The particle size of MWNT-g-PCA and MWNT-g-PCA-PTX was approximately 125 and 200 nm, respectively. Atomic force microscopy and transmission electron microscopy images showed a curved shape for MWNT-g-PCA and MWNT-g-PCA-PTX, which was in contrast with the straight or linear conformation expected from carbon nanotubes. It seems that the high hydrophilicity of poly citric acid and high hydrophobicity of MWNTs led to conformational changes from a linear state to a curved state. Paclitaxel can be released from the MWNT-g-PCA-PTX conjugates faster at pH 6.8 and 5.0 than at pH 7.4, which was suitable for the release of the drug in tumor tissues and tumor cells. In vitro cytotoxicity studies were evaluated in the A549 and SKOV3 cell lines. MWNT-g-PCA had an insignificant cytotoxic effect on both cell lines. MWNT-g-PCA-PTX had more of a cytotoxic effect than the free drug over a shorter incubation time (eg, 24 hours versus 48 hours), which suggests improved cell penetration of MWNT-g-PCA-PTX. Therefore, paclitaxel conjugated to MWNT-g-PCA is promising for cancer therapeutics.

Ethyl 2-(3-amino-4-hydroxyphenyl)acetate

The asymmetric unit of the title compound, C10H13NO3, contains two crystallographically independent mol­ecules with different conformations of the eth­oxy­carbonyl groups; the terminal C—C—O—C torsion angles in the two mol­ecules are 83.6 (6) and −171.1 (3)°, resulting in twisted and straight chain conformations, respectively. The crystal structure is stabilized by inter­molecular N—H⋯O, O—H⋯N and C—H⋯O hydrogen bonds. Intra­molecular hydrogen bonds occur between the amino N and phenolic O atoms.

Stathmin and Interfacial Microtubule Inhibitors Recognize a Naturally Curved Conformation of Tubulin Dimers

Tubulin is able to switch between a straight microtubule-like structure and a curved structure in complex with the stathmin-like domain of the RB3 protein (T(2)RB3). GTP hydrolysis following microtubule assembly induces protofilament curvature and disassembly. The conformation of the labile tubulin heterodimers is unknown. One important question is whether free GDP-tubulin dimers are straightened by GTP binding or if GTP-tubulin is also curved and switches into a straight conformation upon assembly. We have obtained insight into the bending flexibility of tubulin by analyzing the interplay of tubulin-stathmin association with the binding of several small molecule inhibitors to the colchicine domain at the tubulin intradimer interface, combining structural and biochemical approaches. The crystal structures of T(2)RB3 complexes with the chiral R and S isomers of ethyl-5-amino-2-methyl-1,2-dihydro-3-phenylpyrido[3,4-b]pyrazin-7-yl-carbamate, show that their binding site overlaps with colchicine ring A and that both complexes have the same curvature as unliganded T(2)RB3. The binding of these ligands is incompatible with a straight tubulin structure in microtubules. Analytical ultracentrifugation and binding measurements show that tubulin-stathmin associations (T(2)RB3, T(2)Stath) and binding of ligands (R, S, TN-16, or the colchicine analogue MTC) are thermodynamically independent from one another, irrespective of tubulin being bound to GTP or GDP. The fact that the interfacial ligands bind equally well to tubulin dimers or stathmin complexes supports a bent conformation of the free tubulin dimers. It is tempting to speculate that stathmin evolved to recognize curved structures in unassembled and disassembling tubulin, thus regulating microtubule assembly.

The Antibacterial Cell Division Inhibitor PC190723 Is an FtsZ Polymer-stabilizing Agent That Induces Filament Assembly and Condensation

Cell division protein FtsZ can form single-stranded filaments with a cooperative behavior by self-switching assembly. Subsequent condensation and bending of FtsZ filaments are important for the formation and constriction of the cytokinetic ring. PC190723 is an effective bactericidal cell division inhibitor that targets FtsZ in the pathogen Staphylococcus aureus and Bacillus subtilis and does not affect Escherichia coli cells, which apparently binds to a zone equivalent to the binding site of the antitumor drug taxol in tubulin (Haydon, D. J., Stokes, N. R., Ure, R., Galbraith, G., Bennett, J. M., Brown, D. R., Baker, P. J., Barynin, V. V., Rice, D. W., Sedelnikova, S. E., Heal, J. R., Sheridan, J. M., Aiwale, S. T., Chauhan, P. K., Srivastava, A., Taneja, A., Collins, I., Errington, J., and Czaplewski, L. G. (2008) Science 312, 1673-1675). We have found that the benzamide derivative PC190723 is an FtsZ polymer-stabilizing agent. PC190723 induced nucleated assembly of Bs-FtsZ into single-stranded coiled protofilaments and polymorphic condensates, including bundles, coils, and toroids, whose formation could be modulated with different solution conditions. Under conditions for reversible assembly of Bs-FtsZ, PC190723 binding reduced the GTPase activity and induced the formation of straight bundles and ribbons, which was also observed with Sa-FtsZ but not with nonsusceptible Ec-FtsZ. The fragment 2,6-difluoro-3-methoxybenzamide also induced Bs-FtsZ bundling. We propose that polymer stabilization by PC190723 suppresses in vivo FtsZ polymer dynamics and bacterial division. The biochemical action of PC190723 on FtsZ parallels that of the microtubule-stabilizing agent taxol on the eukaryotic structural homologue tubulin. Both taxol and PC190723 stabilize polymers against disassembly by preferential binding to each assembled protein. It is yet to be investigated whether both ligands target structurally related assembly switches.

The relationship between curvature, flexibility and persistence length in the tropomyosin coiled-coil

The inherent flexibility of rod-like tropomyosin coiled-coils is a significant factor that constrains tropomyosin's complex positional dynamics on actin filaments. Flexibility of elongated straight molecules typically is assessed by persistence length, a measure of lengthwise thermal bending fluctuations. However, if a molecule's equilibrium conformation is curved, this formulation yields an "apparent" persistence length ( approximately 100nm for tropomyosin), measuring deviations from idealized straight conformations which then overestimate actual dynamic flexibility. To obtain the "dynamic" persistence length, a true measurement of flexural stiffness, the average curvature of the molecule must be taken into account. Different methods used in our studies for measuring the dynamic persistence length directly from Molecular Dynamics (MD) simulations of tropomyosin are described here in detail. The dynamic persistence length found, 460+/-40nm, is approximately 12-times longer than tropomyosin and 5-times the apparent persistence length, showing that tropomyosin is considerably stiffer than previously thought. The longitudinal twisting behavior of tropomyosin during MD shows that the amplitude of end-to-end twisting fluctuation is approximately 30 degrees when tropomyosin adopts its near-average conformation. The measured bending and twisting flexibilities are used to evaluate different models of tropomyosin motion on F-actin.

Structural Change of N-terminus in Smooth Muscle Myosin Regularly Light Chain using Accelerated Molecular Dynamics

A novel accelerated molecular dynamics method (the Orthogonal Space Random Walk algorithm, OSRW) is applied to study the effect of the regulatory light chain (RLC) phosphorylation on the structure of the light chain binding domain of smooth muscle myosin. Smooth muscle myosin is activated by phosphorylation on the S19 (and T18 subsequently) at the N-terminus of the RLC that causes a conformational change from the closed inhibited asymmetric structure (Wendt et al. PNAS 2001) to the open structure by an unknown mechanism. The N-terminus also plays an important role in stabilizing the folded 10S conformation that is soluble at physiological ionic strength. However, X-ray structures of the RLC do not show the 24-residue N-terminus, which holds the phosphoration site. Thus, we are performing MD simulations on the 21 residues of the N-terminus as well as the RLC with part of the heavy chain. The phosphorylated N-terminus shows a bent α-helical conformation, where the S19 interacts with the R16. The unphosphorylated N-terminus has showed a straight α-helical conformation. Those simulations are carried out in explicit water under near-physiological conditions. The OSRW has demonstrated hundreds of times sampling capacity in compared with regular MD, which will promote our understanding on the phophorylation activation mechanism. This work is supported by NIAMSD.

Structural dynamics of the myosin relay helix by time-resolved EPR and FRET

We have used two complementary time-resolved spectroscopic techniques, dipolar electron-electron resonance and fluorescence resonance energy transfer to determine conformational changes in a single structural element of the myosin motor domain, the relay helix, before and after the recovery stroke. Two double-Cys mutants were labeled with optical probes or spin labels, and interprobe distances were determined. Both methods resolved two distinct structural states of myosin, corresponding to straight and bent conformations of the relay helix. The bent state was occupied only upon nucleotide addition, indicating that relay helix, like the entire myosin head, bends in the recovery stroke. However, saturation of myosin with nucleotide, producing a single biochemical state, did not produce a single structural state. Both straight and bent structural states of the relay helix were occupied when either ATP (ADP.BeF(x)) or ADP.P(i) (ADP.AlF(4)) analogs were bound at the active site. A greater population was found in the bent structural state when the posthydrolysis analog ADP.AlF(4) was bound. We conclude that the bending of the relay helix in the recovery stroke does not require ATP hydrolysis but is favored by it. A narrower interprobe distance distribution shows ordering of the relay helix, despite its bending, during the recovery stroke, providing further insight into the dynamics of this energy-transducing structural transition.

Image analysis of / -tubulin rings in two-dimensional crystalline arrays of periodic mesoporous nanostructures

Two-dimensional crystalline arrays of periodic mesoporous nanostructures, consisting of alpha/beta-tubulin ring-shaped oligomers in the double-layered architecture in an apolar arrangement are successfully prepared with PEM-G buffer containing Arginine nonamers and observed by electron microscopy. The ring-shaped alpha/beta-tubulin assemblies are conformations related to a protofilament, although protofilaments are usually highly constrained in a straight conformation in microtubules. The two-dimensional crystalline arrays appear to have the structure of a square lattice with the layer group P4, the unit cell dimensions of a = b = 36.9 nm and gamma = 90 degrees . The isolated torus-like shaped projections of a pseudo-repeat structural unit are analysed with the method for single particle reconstruction. The average values of outer- and inner-diameters for the torus-like shapes are 49.0 +/- 1.0, and 25.4 +/- 1.1 nm, respectively. The thickness of the peripheral masses consisting of two concentric sub-rings is 11.8 +/- 1.0 nm, where alpha/beta-tubulin heterodimers appear to reside alternately. Taking the average sizes of the rings and the crystallographic unit cell dimensions into consideration, the discussion is focused on the most probable arrangement of the alpha/beta-tubulin rings in the two-dimensional crystalline arrays.

Micelle-bound structures and dynamics of the hinge deleted analog of melittin and its diastereomer: Implications in cell selective lysis by d-amino acid containing antimicrobial peptides

Melittin, the major component of the honey bee venom, is a 26-residue hemolytic and membrane active peptide. Structures of melittin determined either in lipid environments by NMR or by use of X-ray demonstrated two helical regions at the N- and C-termini connected by a hinge or a bend at the middle. Here, we show that deletion of the hinge residues along with two C-terminal terminal Gln residues (Q25 and Q26), yielding a peptide analog of 19-residue or Mel-H, did not affect antibacterial activity but resulted in a somewhat reduced hemolytic activity. A diastereomer of Mel-H or Mel- dH containing d-amino acids [ dV5, dV8, dL11 and dK16] showed further reduction in hemolytic activity without lowering antibacterial activity. We have carried out NMR structures, dynamics (H–D exchange and proton relaxation), membrane localization by spin labeled lipids, pulse-field-gradient (PFG) NMR and isothermal titration calorimetry (ITC) in dodecylphosphocholine (DPC) micelles, as a mimic to eukaryotic membrane, to gain insights into cell selectivity of these melittin analogs. PFG-NMR showed Mel-H and Mel- dH both were similarly partitioned into DPC micelles. ITC demonstrated that Mel-H and Mel- dH interact with DPC with similar affinity. The micelle-bound structure of Mel-H delineated a straight helical conformation, whereas Mel- dH showed multiple β-turns at the N-terminus and a short helix at the C-terminus. The backbone amide-proton exchange with solvent D 2O demonstrated a large difference in dynamics between Mel-H and Mel- dH, whereby almost all backbone protons of Mel- dH showed a much faster rate of exchange as compared to Mel-H. Proton T 1 relaxation had suggested a mobile backbone of Mel- dH peptide in DPC micelles. Resonance perturbation by paramagnetic lipids indicated that Mel-H inserted deeper into DPC micelles, whereas Mel- dH is largely located at the surface of the micelle. Taken together, results presented in this study demonstrated that the poor hemolytic activity of the d-amino acid containing analogs of antimicrobial peptides may be correlated with their flexible dynamics at the membrane surface.

Single Myosin Lever Arm Orientation in a Muscle Fiber Detected with Photoactivatable GFP

Myosin 2 is the molecular motor in muscle. It binds actin and executes a power stroke by rotating its lever arm through an angle of approximately 70 degrees to translate actin against resistive force. Myosin 2 has evolved to function optimally under crowded conditions where rates and equilibria of macromolecular reactions undergo major shifts relative to those measured in dilute solution. Hence, an important research objective is to detect in situ the lever arm orientation. Single-molecule measurements are preferred because they clarify ambiguities that are unavoidable with ensemble measurements; however, detecting single molecules in the condensed tissue medium where the myosin concentration exceeds 100 muM is challenging. A myosin light chain (MLC) tagged with photoactivatable green fluorescent protein (PAGFP) was constructed. The recombinant MLC physically and functionally replaced native MLC on the myosin lever arm in a permeabilized skeletal muscle fiber. Probe illumination volume was minimized using total internal reflection fluorescence microscopy, and PAGFP was sparsely photoactivated such that polarized fluorescence identified a single probe orientation. Several physiological states of the muscle fiber were characterized, revealing two distinct orientation populations in all states called straight and bent conformations. Conformation occupancy probability varies among fiber states with rigor and isometric contraction at extremes where straight and bent conformations predominate, respectively. Comparison to previous work on single rigor cross-bridges at the A-band periphery where the myosin concentration is low suggests molecular crowding in the A-band promotes occupancy of the straight myosin conformation [Burghardt, T. P., et al. (2007) Biophys. J. 93, 2226]. The latter may have a role in contraction because it provides additional free energy favoring completion of the cross-bridge power stroke.

Cortical cell types and intermediate filament arrangements correlate with fiber curvature in Japanese human hair

Naturally straight and curved human scalp hairs were examined using fluorescence and electron microscopy techniques to determine morphological and ultrastructural features contributing to single fiber curvature. The study excluded cuticle and medulla, which lack known bilateral structural asymmetry and therefore potential to form curved fibers. The cortex contained four classifiable cell types, two of which were always present in much greater abundance than the remaining two types. In straight hair, these cell types were arranged annularly and evenly within the cortex, implying that the averaging of differing structural features would maintain a straight fiber conformation. In curved fibers, the cell types were bilaterally distributed approximately perpendicular to fiber curvature direction with one dominant cell type predominantly located closest to the convex fiber side and the other, closest to the concave side. Electron tomography confirmed that the dominant cell type closest to the convex fiber side contained discrete macrofibrils composed of helically arranged intermediate filaments, while the dominant cell type closest to the concave side contained larger fused macrofibrils composed of intermediate filament arrangements varying from helical to hexagonal arrays approximately parallel to the longitudinal fiber axis. These findings concur with the current hypothesis of hair curvature formation and behavior.

Taxol Allosterically Alters the Dynamics of the Tubulin Dimer and Increases the Flexibility of Microtubules

Taxol is a commonly used antitumor agent that hyperstabilizes microtubules and prevents cell division. The interaction of Taxol with tubulin and the microtubule has been studied through a wide array of experimental techniques; however, the exact molecular mechanism by which Taxol stabilizes microtubules has remained elusive. In this study, through the use of large-scale molecular simulations, we show that Taxol affects the interactions between the M and H1-S2 loops of adjacent tubulin dimers leading to more stable interprotofilament interactions. More importantly, we demonstrate that Taxol binding leads to a significant increase in the dynamics and flexibility of the portion of β-tubulin that surrounds the bound nucleotide and makes contact with the α-monomer of the next dimer in the protofilament. We conclude that this increase in flexibility allows the microtubule to counteract the conformational changes induced by nucleotide hydrolysis and keeps the protofilaments in a straight conformation, resulting in a stable microtubule.

Intrinsic Bending and Structural Rearrangement of Tubulin Dimer: Molecular Dynamics Simulations and Coarse-Grained Analysis

Microtubules are long polymers of αβ-tubulin heterodimers. They undergo a process known as dynamic instability, in which the ends of a microtubule switch stochastically between phases of slow growth and rapid shrinkage. The molecular mechanisms inducing the depolymerization of microtubules were attributed to the hydrolysis of the guanosine triphosphate (GTP) nucleotide bound to the β-tubulin. The hydrolysis of GTP is thought to cause microtubule instability by promoting outward curving of the protofilaments constituting the microtubule lattice. The bending of protofilaments is associated with the structural transformation of a tubulin dimer from straight to curved conformations. However, the nature of intrinsic bending of the dimer remains elusive. This study uses molecular dynamics (MD) simulations and coarse-grained analysis to reveal the intrinsic bending, as well as the local structural rearrangements, of the unassembled tubulin dimer as the dimer relaxes from its lattice-constrained, straight conformation of a zinc-induced tubulin sheet. The effect of the nucleotide state on dimer-bending is investigated by the introduction of γ-phosphate into the β-tubulin to form GTP-bound tubulin. In agreement with recent experimental studies that proposed nucleotide-independent curved conformations, both guanosine diphosphate (GDP)-bound and GTP-bound tubulin dimers were found to have curved conformations, but with a tendency toward smaller bending in the GTP-tubulin than in the GDP-tubulin. The perturbation induced through the introduction of γ-phosphate is posited to play a role in straightening the intradimer bending. The local structural rearrangements of GDP-tubulin because of the bending mode of motion of the dimer reveal that one of the three functional domains, the intermediate domain, exhibits significantly lower bending deformation compared with the others, signifying a dynamic connection to the functionally defined domains.

Synthesis and Chloride Affinity of Sterically Demanding Ditopic Lithium Bis(pyrazol‐1‐yl)borates

AbstractThe synthesis and full characterization of the sterically demanding ditopic lithium bis(pyrazol‐1‐yl)borates Li2[p‐C6H4(B(Ph)pzR2)2] is reported (pzR = 3‐phenylpyrazol‐1‐yl (3Ph), 3‐t‐butylpyrazol‐1‐yl (3tBu)). Compound 3Ph crystallizes from THF as THF‐adduct 3Ph(THF)4 which features a straight conformation with a long Li···Li distance of 12.68(1) Å. Compound 3tBu was found to function as efficient and selective scavenger of chloride ions. In the presence of LiCl it forms anionic complexes [3tBuCl]− with a central Li‐Cl‐Li core (Li···Li = 3.75(1) Å).

Synthesis and ex vivo profiling of chemically modified cytomegalovirus CMVpp65 epitopes

The effect of substituting unnatural hydrophobic amino acids into the critical MHC binding residues of an HLA-A*0201-restricted cytomegalovirus CMVpp65 epitope, NLVPMVATV, has been investigated. A new set of peptides containing the amino acids tert-butyl glycine (Tgl), cyclohexyl glycine (Chg), neo-pentyl glycine (Npg), cyclohexyl alanine (Cha) and cyclo leucine (Cyl), at either position 2, to mimic Leu, or position 9, to mimic Val, have been synthesised. Immunological profiling using class I MHC stabilisation assays to assess MHC binding affinity, and enzyme-linked immunospot (ELISPOT) assays to assess the ability of the modified peptides to re-stimulate a specific cytotoxic T-lymphocyte (CTL) response, compared to the native epitope, have been performed. It was found that the majority of the unnatural substitutions resulted in a decrease in either HLA-A*0201 binding affinity or cytotoxic T-cell activity. However, the HLA-A*0201 binding affinity was unrelated to the ability to re-stimulate a T-cell response. Minimisation and molecular dynamics studies proved helpful in dissecting the ELISPOT responses. Two principal peptide binding modes were found by minimisation, designated kinked and straight. Peptides that bound in a kinked conformation were poor at re-stimulating a T-cell response. Of the peptides that bound in a straight conformation, molecular dynamics (MD) simulations revealed that those capable of re-stimulating the strongest responses had the greatest degree of flexibility (as determined by RMSD values across the MD simulation) around the P6 residue, one of the residues important for T-cell receptor recognition.

The Nucleotide Switch of Tubulin and Microtubule Assembly: A Polymerization-Driven Structural Change

GTP-binding proteins from the tubulin family, including alphabeta-tubulin, gamma-tubulin, bacterial tubulin, and FtsZ, are key components of the cytoskeleton and play central roles in chromosome segregation and cell division. The nucleotide switch of alphabeta-tubulin is triggered by GTP hydrolysis and regulates microtubule assembly dynamics. The structural mechanism of the switch and how it modulates assembly are beginning to be understood. A conserved structural change between the active and inactive states, different from other GTPases, may be extracted from recent tubulin and FtsZ structures. From these and the biochemical properties of tubulin, the new concept emerges that, contrary to what was thought, unassembled tubulin-GTP is in the inactive, curved conformation as in tubulin-GDP rings, and it is driven into the straight microtubule conformation by the assembly contacts; binding of the GTP gamma-phosphate only lowers the free energy difference between the curved and straight forms.

Influence of dehydrated nanotubed titanic acid on polymer light-emitting diodes with phosphorescent dye

In this letter, we demonstrate that hole injection and transport in polymer light-emitting diodes with phosphorescent dye Ir(ppy)3 can be significantly enhanced by doping p-type conductive dehydrated nanotubed titanic acid into poly(vinylcarbazole) (PVK) films at 2wt.%. At the same time, both energy transfer and exciton recombination efficiency are improved because of the open and straight conformation of the PVK molecule in the nanocomposite. The performance of these devices was greatly improved, showing higher luminance, enhanced efficiency, and a lower turn-on voltage.