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In this study, a high-activity Zn–Co double metal cyanide complex was synthesized and used to catalyze the ternary polymerization of carbon dioxide, propylene oxide, and itaconic anhydride to produce a new class of polypropylene carbonates characterized by excellent performance and low ether content. The number average molecular weight of the terpolymer was as high as 2.14 × 105 g·mol−1, and the polydispersity index was as high as 1.58. In the case of PPCIAn3, the 5% weight loss temperature increased by 70°C, the total weight loss temperature increased by 100°C, the tensile strength increased by 6.6 MPa, and the elongation at break decreased to 14.5% with respect to traditional polypropylene carbonate. The apparent efficiency of the catalyst was as high as 45.79 gpolymer/gcatalyst, indicating its high catalytic activity. 1H-NMR spectrometry, gel permeation chromatography, differential scanning calorimetry, thermogravimetric analysis, and in situ Fourier-transform infrared spectroscopy were used to characterize the polymerization process and the structural properties of the obtained terpolymers.

Bladder cancer is the most common malignant tumor of the urinary system. The intention of the present research is to explore the prognostic value and biological function of solute carrier family 12 member 8 (SLC12A8) in bladder cancer. The analysis based on the TCGA and ONCOMINE database revealed that the expression of SLC12A8 in bladder cancer was notably increased compared with the normal group. SLC12A8 expression was notably correlated with the age, pathological stage, T-stage, and lymph node metastasis of bladder cancer patients. Moreover, the patients’ overall survival was notably shorter in the high SLC12A8 group. Compared with the control, SLC12A8 upregulation enhanced the proliferative, invasive, and migratory capacities of bladder cancer cells and promoted the expression of epithelial–mesenchymal transition (EMT) protein markers including β-catenin, vimentin, snail, and slug, while reduced the expression of E-cadherin. In the case of downregulated SLC12A8 expression, the proliferative, invasive, and migratory capacities of bladder cancer cells and the expression of EMT protein markers presented the opposite trend. This study demonstrated that SLC12A8 was highly correlated with oncogenesis and progression of bladder cancer, indicating that SLC12A8 may be a meaningful biomarker for initial diagnosis and early treatment of bladder cancer.

A series of Salen cobalt complexes have been successfully synthesized and they show high catalytic activity for the copolymerization of propylene oxide (PO) and carbon dioxide (CO2), yielding poly(propylene carbonate) (PPC). After optimizing the reaction conditions such as temperature, pressure and reaction time, the catalytic system achieved excellent conversion (99.47%) and showed high catalytic activity with an initial TOF up to 483.18 h−1. The content of carbonate chain of the PPC and head-to-tail connection are 88.92% and 86.79% respectively. PPC starts to decompose at 230°C and completely decomposed at 260°C. The Mn of the PPC is 109.898 kg/mol, and the PDI is 1.65295.

The paper presents a study and comparative analysis of microfluids and nanofluids based on aqueous glycerol solutions, by study of the thermal conductivity, rheology, interfacial tension and solution stability. Cu, Ni and Al nanoparticles were used for preparation of the nanofluids. A newly synthesized monocrystalline pentanickel (II) metal string complex [Ni 5 (μ 5 -pppmda) 4 Cl 2 ] was used to form the microparticles which were compared with dispersed nickel microparticles.

A series of Ti/Li/Al ternary layered double hydroxides (TiLiAl-LDHs) with different Ti:Li:Al molar ratios were prepared by a coprecipitation method for photocatalytic CO2 reduction. It was demonstrated that the contents of anions between the layers of Ti/Li/Al-LDHs greatly determined the photocatalytic activity for CO2 reduction. With Ti:Li:Al molar ratios optimized to be 1:3:2, the largest contents of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\bf{CO}}}_{{\bf{3}}}^{{\bf{2}}}$$\end{document}CO32− anion and hydroxyl group were obtained for the Ti1Li3Al2-LDHs sample, which exhibited the highest photocatalytic activity for CO2 reduction, with CH4 production rate achieving 1.33 mmol h−1 g−1. Moreover, the theoretical calculations show that Ti1Li3Al2-LDHs is a p-type semiconductor with the narrowest band gap among all the obtained TiLiAl-LDHs. After calcined at high temperatures such as 700 °C, and the obtained TiLiAl-700 sample showed much increased photocatalytic activity for CO2 reduction, with CH4 production rate reaching about 1.59 mmol h−1 g−1. This calcination induced photocatalytic enhancement should be related to the cystal structure transformation from hydrotalcite to mixed oxides containing high reactive oxygen species for more efficient CO2 reduction.

We aimed to develop new effective catalysts for the synthesis of propylene carbonate from propylene oxide and carbon dioxide. A kind of Mx+LClx coordination complex was fabricated based on the chelating tridentate ligand 2,6-bis[1-(phenylimino)ethyl] pyridine (L). The obtained products were characterized by elemental analysis, infrared spectroscopy, ultraviolet spectroscopy, thermogravimetric analysis, and single-crystal X-ray diffraction. It was found that the catalytic activity of the complexes with different metal ions, the same ligand differed and co-catalyst, where the order of greatest to least catalytic activity was 2 &gt, 3 &gt, 1. The catalytic system composed of complex 2 and DMAP proved to have the better catalytic performance. The yields for complex 2 systems was 86.7% under the reaction conditions of 100 &deg, C, 2.5 MPa, and 4 h. The TOF was 1026 h&minus, 1 under the reaction conditions of 200 &deg, C, 2.5 MPa, and 1 h. We also explored the influence of time, pressure, temperature, and reaction substrate concentration on the catalytic reactions. A hypothetical catalytic reaction mechanism is proposed based on density functional theory (DFT) calculations and the catalytic reaction results.

Based on the ligand H2dpPzda (1), a novel cobalt complex [Co(H2dpPzda)(NCS)2]&middot, CH3OH(2) has been synthesized and characterized. The Complex 2 exhibited excellent catalytic performance for converting CO2 into cyclic carbonates under mild conditions. For propylene oxide (PO) and CO2 synthesis of propylene carbonate (PC), the catalytic system showed a remarkable TOF as high as 29,200 h&minus, 1. The catalytic system also showed broad substrate scope of epoxide. Additionally, the catalyst could be recycled to maintain the integrity of the structure and remained equal to the level of its catalytic activity even after seven catalytic rounds. Additionally, a possible catalytic mechanism was proposed due to the high catalytic activity which might be owing to the synergism of Lewis acidic metal centers and N group.

A series of rare earth metal complexes RE(TPP)(acac) (RE=NdIII, YbIII, EuIII, ErIII, PrIII and LuIII, TPP=5,10,15,20-tetraphenylporphyrin) were synthesized and characterized via UV-Vis, IR and elemental analyses. Their catalytic activities on the synthesis of cyclic carbonates from carbon dioxide and propylene oxide under different reaction conditions (temperature, pressure and reaction time) were investigated. Catalytic reaction tests demonstrated that the complex Lu(TPP)(acac) could significantly enhance the catalytic reactivity under mild conditions without any co-solvent.

The title compound,catena-poly[[[diaqua(methanol-κO)copper(II)]-μ-N-(4-methylpyrimidin-2-yl-κN1)pyrazin-2-amine-κ2N1:N4] [[aqua(aqua/methanol-κO)(perchlorato-κO)copper(II)]-μ-N-(4-methylpyrimidin-2-yl-κN1)pyrazin-2-amine-κ2N1:N4] tris(perchlorate) methanol monosolvate 1.419-hydrate], {[Cu(C9H9N5)(CH3OH)(H2O)2][Cu(C9H9N5)(ClO4)(CH3OH)0.581(H2O)1.419](ClO4)3·CH3OH·1.419H2O}n, is a one-dimensional straight-chain polymer ofN-(4-methylpyrimidin-2-yl)pyrazin-2-amine (L) with Cu(ClO4)2. The complex consists of two crystallographically independent one-dimensional chains in which the CuIIatoms exhibit two different octahedral coordination geometries. TheLligand coordinates to two CuIIcentres in a tridentate manner, with the pyrazine ring acting as a bridge linking the CuIIcoordination units and building an infinite one-dimensional chain. Extensive hydrogen bonding among perchlorate anions, water molecules andLligands results in three-dimensional networks.

By measuring the vibrational wavenumbers of their stretching modes in Raman and surface-enhanced Raman scattering (SERS) spectra, we investigated the strength of the Cr-Cr bonds in metal-string complexes Cr 5 (tpda) 4 X 2 and Cr 7 (teptra) 4 (NCS) 2 (tpda = tripyridyldiamido; teptra = tetrapyridyltriamido; X = Cl ― , NCS ― ). The bands in SERS and Raman differ insignificantly in spectral positions, indicating no major structural variation between the solid and solution forms. For SERS measurements, these complexes were bound to silver or gold nanoparticles in aqueous solution to eliminate the constraint of a crystal lattice and to maintain the complexes in thermal equilibrium; this method is convenient to identify the stable structure. We identified both penta- and heptachromium complexes in both symmetric (s) and unsymmetric (u) forms. For pentachromium complexes, our data agree with the results obtained from structural determination of the crystalline form, but for the heptachromium complex, this experimental evidence is the first for the existence of the u-form structure. From our analysis of the vibrational normal modes, we assign the band at 280 cm ―1 to the Cr―Cr symmetric stretching mode of the s-form pentachromium complex. According to comparisons of SERS spectra obtained at either high temperatures or under oxidizing conditions, we assign 570 cm ―1 to the stretching mode of the Cr―Cr quadruple bond in the u-form for the pentachromium complex and 554/571 cm ―1 analogously for the heptachromium complex. The bands for metal-related modes in SERS spectra might be enhanced because of interaction with the metal nanoparticles. The metal-string complexes with a linear arrangement of metal ions have an increased absorption coefficient in the visible spectra and, consequently, an increased resonance Raman intensity for the metal-metal stretching modes, yielding information about the strength of chromium―chromium multiple bonding.

We recorded infrared, Raman, and surface-enhanced Raman scattering (SERS) spectra of metal−string complexes Ni5(tpda)4X2 and Co5(tpda)4X2 (tpda = tripyridyldiamido, X = Cl−, NCS−) and free ligand tripyridyldiamine (H2tpda) to determine their vibrational wavenumbers and the strength of the metal−metal bonds. For SERS measurements, these complexes were adsorbed on silver or gold nanoparticles in aqueous solution to eliminate the constraint of a crystal lattice and to maintain the complexes in thermal equilibrium. The spectra of SERS and Raman modes show insignificant deviation in spectral features and band positions. We observe a single breathing band of pyridyl in Co5(tpda)4X2, indicating the existence of only the symmetric form, whereas split pyridyl lines are observed for Ni5(tpda)4X2 and assigned to arise because of a varied environment of coordination: square planar for the inner nickels and square pyramidal for the outer nickels in the complexes. From our analysis of the vibrational normal modes, we a...

An unsymmetrical pyrazine-modulated oligo-α-pyridylamine ligand as N2-(pyrazin-2-yl)-N6-(6-(pyridin-2-ylamino)pyridin-2-yl)pyridine-2,6-diamine (H3tpz) and its copper(II) and cobalt(II) complexes have been synthesized and structurally characterized. The free ligand H3tpz (1) exhibits a syn-anti-syn-anti-syn-anti conformation, and shows a double chain supramolecular structure through hydrogen bonds and π-π interactions. H3tpz is a quadridentate ligand in both copper and cobalt mononuclear complexes and coordinates to metal atoms in an all-anti mode. The copper(II) ion in [Cu(H3tpz)(ClO4)]-ClO4 (2) is five-coordinated in tetragonal pyramid geometry with one of the perchlorate anions being weakly coordinated at the apex position. The cobalt(II) ion in [Co(H3tpz)(CH3OH)2](ClO4)2 (3) is six-co-ordinated in octahedron geometry. Extensive hydrogen bonds are formed, and construct the complexes 2 and 3 into 1D supramolecular structure.

A pyrazine-modulated oligo-α-pyridylamine ligand H3pzpz and its mononuclear copper(II) and cobalt(II) complexes have been synthesized and structurally characterized. The protonated ligand cation 1 exhibits an anti-anti-anti-anti-syn-anti conformation. H3pzpz can be a quadridentate ligand and coordinates to metal atoms with all-anti mode in both copper and cobalt mononuclear complexes. The copper(II) ion in 2 is five-coordinated in tetragonal pyramid geometry with one of the perchlorate anions being weakly coordinated in the apex position. The cobalt(II) ion in 3 and 4 is six-coordinated in octahedron geometry. Extensive hydrogen bonds are formed, and construct the complexes 1 and 2 into 2D networks and 3 and 4 into 3D networks. X-band EPR spectra of 2 showed well-resolved hyperfine structures resulting from the two paramagnetic isotopes 63Cu and 65Cu, yielding g = 2.16 and A0 = 67 × 10−4 cm−1. Complexes 3 and 4 showed spin–orbital coupling and zero-field splitting that is reflected in the magnetic properties.

The synthesis, crystal structures, electrochemical, and magnetic properties of a linear pentanuclear Ni5 compound derived from an oligo-α-pyridylamino ligand, [Ni5(μ-dmpdda)4(NCS)2] [dmpdda-H2 = N,N′-di(4-methylpyridin-2-yl)pyridine-2,6-diamine], are reported. Ni5(μ-dmpdda)4(NCS)2 involve a Ni5 linear chain unit with all of the Ni–Ni–Ni angles being nearly 180°, terminated by two axial ligands. The pentanuclear linear metal chain is helically wrapped by four syn–syn–syn–syn type dmpdda2− ligands. There are two types of Ni–Ni distances in this complex. Terminal Ni–Ni distances bonded with the axial ligand are longer (2.377 A); the inner Ni–Ni distances are short at 2.2968 A. Terminal Ni(II) ions bonded with the axial ligands are square-pyramidal (NiN4NCS) with long Ni–N bonds (2.092 A), consistent with a high-spin Ni(II) configuration. The inner three Ni(II) ions have short Ni–N (1.901–1.925 A) bond distances, consistent with a square planar (NiN4), diamagnetic arrangement of a low-spin Ni(II) configuratio...

Using pyrazine-modulated oligo-α-pyridylamido ligands N2-(pyrazin-2-yl)-N6-[6-(pyrazin-2-ylamino)pyridin-2-yl]pyridine-2,6-diamine (H3pzpz) and N2-(pyrazin-2-yl)-N6-[6-(pyridin-2-ylamino)pyridin-2-yl]pyridine-2,6-diamine (H3tpz), linear heptametal(II) extended metal atom chains (EMACs) [M7(μ7-L)4X2] [L = pzpz3–, M = NiII, X = Cl– (1), NCS– (2); M = CrII, X = Cl– (3), NCS– (4); L = tpz3–, M = CrII, X = Cl– (5), NCS– (6)] were synthesized and structurally characterized. Electrochemical studies showed that heptanickel(II) complexes can undergo one reversible oxidation at +0.46 V for 1 and +0.52 V for 2. Chromium(II) species 3 exhibited two reversible, one-electron oxidation peaks at +0.61 and +0.88 V, and 5 exhibited three reversible, one-electron oxidation peaks at +0.40, +0.68 and +1.07 V. The redox peaks shifted positively when axial ligands changed from chloride to thiocyanate anions, at +0.67 and +0.92 V for 4 and +0.44, +0.73 and +1.11 V for 6. The introduction of electron-withdrawing pyrazine rings to the spacer ligand retarded oxidation of the heptametal EMACs and stabilized the complexes. In nickel(II) species 1 and 2, both terminal nickel atoms exist in spin state S = 1 whereas all the inner nickel atoms exist in spin state S = 0. Temperature-dependent magnetic research revealed an antiferromagnetic interaction between the two terminal atoms through a superexchange pathway along metal cores with a parameter of about–4 cm–1. Chromium(II) species 3–6 showed a localized structure consisting of three quadruple Cr–Cr bonds and a single terminal CrII atom. Magnetic study revealed a quintet ground state resulting from the isolated, high-spin CrII atom.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

We recorded infrared, Raman, and surface-enhanced Raman scattering (SERS) spectra of metal−string complexes Cr3(dpa)4X2 (dpa = di(2-pyridyl)amido, X = Cl, NCS) and [Cr3(dpa)4Cl2](PF6) and dipyridylamine (Hdpa) to determine their vibrational frequencies and to study their structures. For the SERS measurements these complexes were adsorbed on silver nanoparticles in aqueous solution to eliminate the constraints of a crystal lattice. From the results of analysis of the vibrational normal modes we assign the infrared band at 346 cm−1 to the Cr3 asymmetric stretching vibration of the symmetric form and the Raman line at 570 cm−1 to the Cr−Cr stretching mode for the unsymmetric form of Cr3(dpa)4Cl2. Complex Cr3(dpa)4Cl2 exhibits both symmetric (s-) and unsymmetric (u-) forms in solution but Cr3(dpa)4(NCS)2 only the s-form. The structures for both complexes in their ground states have the s-form. The oxidized complex [Cr3(dpa)4Cl2](PF6) has only a u-form for which the Cr−Cr stretching mode is assigned to the ban...

The new pyrimidyl- and naphthyridyl-modulated pentapyridyltetramine ligand (H3N9-2pm) and its linear nona- and octanickel chain complexes were successfully synthesized. The nonanickel complex [Ni9(μ9-N9-2pm)4Cl2](PF6)2 (1) contains two fully delocalized, mixed-valence [Ni2(napy)4]3+ units. Magnetic measurements performed on the defective octanuclear complex [Ni8(μ8-N9-2pm)4Cl2](PF6)2 (2) demonstrate that the metal chain provides a pathway of magnetic superexchange coupling over the extended metal atom chain (EMAC). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Infrared and Raman spectra for metal–string complexes M 3 (dpa) 4 X 2 (M = Ni, Co, dpa = di(2-pyridyl)amido, and X = Cl, NCS) are studied. We assign the Ni 3 asymmetric stretching vibration to infrared lines at 304 and 311 cm −1 for Ni 3 (dpa) 2 Cl 2 and Ni 3 (dpa) 2 (NCS) 2 , respectively. A Raman shift at 242 cm −1 is assigned to the Ni 3 symmetric stretching mode. For Co 3 complexes a line for the Co 3 asymmetric stretching mode appears at 313 and 331 cm −1 for Co 3 (dpa) 2 Cl 2 and Co 3 (dpa) 2 (NCS) 2 , respectively.

Four unsymmetrical pyrazine- and/or 4-picoline-modulated N,N′-bis(α-pyridyl)-2,6-diaminopyridine ligands (PMN5) were synthesized and their 1D copper(II) complexes studied. When pyrazine is a terminal ring, the ligands showed in-plane and out-of-plane coordination modes and led to a one-dimensional zigzag chain polymer. The orthogonal overlap of orbitals through a bridging pyrazine ring resulted in ferromagnetic interaction. Middle-ring pyrazine-modulated ligands resulted in straight one-dimensional compounds and had antiferromagnetic coupling. The EPR spectra of the compounds showed significant covalency in the Cu–N bonds.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

One-pot reaction of Meldrum’s acid (1) and heterocyclic ketene aminal (HKA, 2) in acetone affords an unusual amidine HKA derivative 4. The reaction catalysts were optimized and it was found that l-proline is the best catalyst for the one-pot reaction. The mechanism is discussed. The structure of compound 4 was determined by single crystal X-ray analysis.

Recent studies have indicated that telomerase activity promotes cancer invasion and metastasis, but the underlying mechanism remains unclear. Several studies have shown that expression of exogenous human telomerase reverse transcriptase (hTERT) can promote motility and invasiveness among telomerase-negative tumor cells, and inhibition of endogenous telomerase activity can reduce invasiveness in tumor cells. However, whether overexpression of hTERT can further enhance the motility and invasiveness of telomerase‑positive tumor cells has yet to be determined. In the present study, we showed that stable overexpression of hTERT can increase telomerase activity and telomere length, which significantly promotes the invasive and metastatic potential of telomerase‑positive HepG2 cells but does not affect cell proliferation. Further analysis suggested that enhanced invasiveness and metastasis may act through corresponding upregulation of mRNA and protein expression of matrix metalloproteinase 9 (MMP9) and Ras homolog gene family member C (RhoC). Our study indicated that exogenous expression of hTERT may promote invasiveness and metastasis through upregulation of MMP9 and RhoC.

The title compound, (C4H10NO)2[CoCl4], is an ionic compound consisting of two protonated tetrahydro-1,4-oxazine (morpholine) cations and a [CoCl4]2− dianion. The CoII ion is in a tetrahedral coordination geometry. The cations exhibit chair-shaped conformations. A three-dimensional supramolecular architecture is formed through N—H...Cl and C—H...Cl hydrogen bonds between the dianions and the cations.

Through a new pyrazine-modulated penta-pyridyl-tetraamine ligand, H(4)N(9)-mpz, linear nonachromium(II) complexes with four quadruple metal-metal bonds were successfully obtained, and their structure, magnetic and electrochemistry properties were studied.

In the title compound, [CuCl(C6H4N2)(C6H15N3)]ClO4, the central copper ion is coordinated by three N atoms from 1,4,7-triaza­cyclo­nonane, one N atom from the cyano­pyridine and one Cl atom, forming a square-pyramidal coordination geometry (τ = 0.31). The crystal packing is stabilized by inter­molecular N—H⋯O and N—H⋯Cl hydro­gen bonds.

There are two independent molecules in the asymmetric unit of the title compound, C12H9N5, in which the C—N(amine)—C angles differ slightly [129.63 (11) and 132.02 (11)°]. In each independent molecule, an intramolecular C—H...N hydrogen bond stabilizes the molecular structure, forming an S(6) ring motif. The independent molecules are linked via an N—H...N hydrogen bond. Further N—H...N and C—H...N hydrogen bonds connect the molecules into chains along c axis. Pairs of C—H...π interactions between the chains lead to sheets parallel to the b axis. These are linked by π–π interactions between the naphthyridine and pyrazine rings [centroid–centroid separations of 3.553 (8) Å] into a three-dimensional supramolecular network.

Reaction of [RuCl2(Me2SO)4] with sodium azide in water affords the title complex di-μ-1,3-azido-bis[chlorotris(dimethyl sulfoxide-κS)ruthenium(II)], [Ru2Cl2(N3)2(C2H6OS)6]. X-Ray crystal structure determination has been performed on the title compound. The result reveals that two [Ru(Me2SO)3]2+ units are linked by two bridging azide ligands to form a centrosymmetric complex. The geometry about each Ru atom is essentially octahedral, with three S atoms of Me2SO arranged in a facial configuration, two coordinated N atoms of bridging N3− ligands and a chloride anion.

Pentacobalt EMACs [Co(5)(mu(5)-dpzpda)(4)X(2)] (X = Cl(-) (), NCS(-) ()) with fine-tuning of the supporting ligand based on the tripyridyldiamine ligand, N,N'-di(pyrazin-2-yl)pyridine-2,6-diamine (H(2)dpzpda), and their reduced form (Ph(4)P)[Co(5)(mu(5)-dpzpda)(4)X(2)] (X = Cl(-) (), NCS(-) ()) were first synthesized and structurally characterized. The structures of showed direct Co-Co bonds with Co-Co distances in the range 2.2385(7)-2.2888(15) A, and valence electrons delocalized through the whole metal chain with distances of longer than 9.06 A. The distances of the inner Co-Co bonds and the Co-axial ligands became longer after reduction, whereas no significant change was observed in the distances of the outer Co-Co bonds and Co-N (supporting ligand) bonds, which was consistent with the MO analysis. Electrochemical studies on both and showed one reversible oxidation and one reversible reduction at E(1/2) = +0.82 and -0.05 V for , and at E(1/2) = +0.89 and +0.02 V for , respectively. The redox reactions of the thiocyanate complex happened at higher potentials than the chloride complex . A magnetism study of revealed anomalous magnetic behaviour similar to that of heptacobalt EMACs, and a deviation from the Curie-Weiss law was observed. The chi(M)T value at 300 K is 0.84 and 1.16 emu K mol(-1) for and , respectively, suggesting spin-equilibrium or a spin-admixture between doublet and quartet states arising from the Boltzmann distribution over different energy levels. Similar results were obtained for and , showing intermediate chi(M)T values between a diamagnetic and a triplet state of 0.15-0.96 emu K mol(-1) in the temperature range 5-300 K. The structural and magnetic results were interpreted through an EHMO study.