Formation of a long-lived P+BA- state in plant pheophytin-exchanged reaction centers of Rhodobacter sphaeroides R26 at low temperature

Femtosecond transient absorption spectroscopy in the range of 500-1040 nm was used to study electron transfer at 5 K in reaction centers of Rhodobacter sphaeroides R26 in which the bacteriopheophytins (BPhe) were replaced by plant pheophytin a (Phe). Primary charge separation took place with a time constant of 1.6 ps, similar to that found in native RCs. Spectral changes around 1020 nm indicated the formation of reduced bacteriochlorophyll (BChl) with the same time constant, and its subsequent decay in 620 ps. This observation identifies the accessory BChl as the primary electron acceptor. No evidence was found for electron transfer to Phe, indicating that electron transfer from BA- occurs directly to the quinone (QA) through superexchange. The results are explained by a model in which the free energy level of P+Phe- lies above that of P+BA-, which itself is below P*. Assuming that the pigment exchange does not affect the energy levels of P* and P+BA-, our results strongly support a two-step model for primary electron transfer in the native bacterial RC, with no, or very little, admixture of superexchange.     

A new pathway for transmembrane electron transfer in photosynthetic reaction centers of Rhodobacter sphaeroides not involving the excited special pair

It is generally accepted that electron transfer in bacterial photosynthesis is driven by the first singlet excited state of a special pair of bacteriochlorophylls (P*). We have examined the first steps of electron transfer in a mutant of the Rhodobacter sphaeroides reaction center in which charge separation from P* is dramatically slowed down. The results provide for the first time clear evidence that excitation of the monomeric bacteriochlorophyll in the active branch of the reaction center (B(A)) drives ultrafast transmembrane electron transfer without the involvement of P*, demonstrating a new and efficient mechanism for solar energy transduction in photosynthesis. The most abundant charge-separated intermediate state probably is P+B(A)-, which is formed within 200 fs from B(A)* and decays with a lifetime of 6.5 ps into P+H(A)-. We also see evidence for the involvement of a B(A)+H(A)- state in the alternative pathway.     

Potassium permanganate susceptibility of sigma E-RNA polymerase-promoter complexes

Formation of the vibronic wavepacket by 90-fs excitation of the primary electron donor P in bacteriochlorophyll(M)-modified reaction centers is shown to induce nuclear motions accompanied by (1) oscillation of the stimulated emission from excited primary electron donor P* and (2) wavepacket motions leading to electron transfer at 293 K from P* to bacteriochlorophyll (B(L)) and then to bacteriopheophytin (H(L)). The latter motions have low frequency (about 15 cm-1) and are related to protein-nuclear motions which are along the reaction coordinate. When the wavepacket approaches the intersection of the reactant (P*B(L)) and product (P+B(L)-) potential energy surfaces (approximately 1.5 ps delay), about 60% of P* is converted to the P+B(L)- state. The P+H(L)- state formation is delayed by approximately 2 ps with respect to that of P+B(L)-. It is suggested that the wavepacket is transferred to and moves also slowly on the P+B(L)- potential energy surface and approaches the intersection of the surfaces of P+B(L)- and P+H(L)- within approximately 2 ps (approximately 8 cm-1), indicating the electron transfer to H(L).     

Charge separation in the reaction center of photosystem II studied as a function of temperature

In photosystem II of green plants the key photosynthetic reaction consists of the transfer of an electron from the primary donor called P680 to a nearby pheophytin molecule. We analyzed the temperature dependence of this reaction by subpicosecond transient absorption spectroscopy over the temperature range 20-240 K using isolated photosystem II reaction centers from spinach. After excitation in the red edge of the Qy absorption band, the decay of the excited state can conveniently be described by two kinetic components that both accelerate with temperature. This temperature behavior differs remarkably from that observed in purple bacterial reaction centers. We attribute the first component, which accelerates from 2.6 ps at 20 K to 0.4 ps at 240 K, to charge separation after direct excitation of P680, and explain its temperature dependence by an intermediate that lies in energy above the singlet-excited P680 and that possibly has charge-transfer character. The second component accelerates from 120 ps at 20 K to 18 ps at 240 K and is attributed to charge separation after direct excitation of the "trap" state near-degenerate with P680 and subsequent slow energy transfer from this trap state to P680. We suggest that the slow energy transfer from the trap state to P680 plays an important role in the kinetics of radical pair formation at room temperature.     

Mutational analysis of a leucine heptad repeat motif in a class I aminoacyl-tRNA synthetase

Aminoacyl-tRNA synthetases activate amino acids with ATP to form aminoacyl adenylates as the essential intermediates for aminoacylation of their cognate tRNAs. The class I Escherichia coli cysteine tRNA synthetase contains an N-terminal nucleotide binding fold that provides the catalytic site of adenylate synthesis. The C-terminal domain of the cysteine enzyme is predominantly alpha-helical and contains a leucine heptad repeat motif. We show here that specific substitutions of leucines in the leucine heptad repeats reduced tRNA aminoacylation. In particular, substitution of Leu316 with phenylalanine reduced the catalytic efficiency of aminoacylation by 1000-fold. This deleterious effect was partially alleviated by a more conservative substitution of leucine with valine. Filter binding assays show that neither the phenylalanine nor the valine substitution at Leu316 had a major effect on the ability of the cysteine enzyme to bind tRNA(Cys). In contrast, pyrophosphate exchange assays show that both substitutions decreased the adenylate synthesis activity of the enzyme. Analysis of these results suggests that the primary defect of the valine substitution is executed at adenylate synthesis while that of the phenylalanine substitution is at both adenylate synthesis and the transition state of tRNA aminoacylation. Thus, although Leu316 is located in the C-terminal domain of the cysteine enzyme, it may modulate the capacity of the N-terminal domain for amino acid activation and tRNA aminoacylation through a domain-domain interaction.     

Heterologous biosynthesis and characterization of the [2Fe-2S]-containing N-terminal domain of Clostridium pasteurianum hydrogenase

The primary structure of Clostridium pasteurianum hydrogenase I appears to be composed of modules suggesting that the various iron-sulfur clusters present in this enzyme might be segregated in structurally distinct domains. On the basis of this observation, a gene fragment encoding the 76 N-terminal residues of this enzyme has been expressed in Escherichia coli. The polypeptide thus produced contains a [2Fe-2S]n+ cluster of which the oxidized level (n = 2) has been monitored by UV-visible absorption, circular dichroism, and resonance Raman spectroscopy. This cluster can be reduced by dithionite or electrochemically to the n = 1 level which has been investigated by EPR and by low-temperature magnetic circular dichroism. The redox potential of the +2 to +1 transition is -400 mV (vs the normal hydrogen electrode). The spectroscopic and redox results indicate a [2Fe-2S]2+/+ chromophore coordinated by four cysteine ligands in a protein fold similar to that found in plant- and mammalian-type ferredoxins. Among the five cysteines present in the N-terminal hydrogenase fragment, four (in positions 34, 46, 49, and 62) are conserved in other sequences and are therefore the most likely ligands of the [2Fe-2S] site. The fifth cysteine, in position 39, can be dismissed on the grounds that the Cys39Ala mutation does not alter any of the properties of the iron-sulfur cluster. The spectroscopic signatures of this chromophore are practically identical with some of those reported for full-size hydrogenase. This confirms that C. pasteurianum hydrogenase I contains a [2Fe-2S] cluster and indicates that the polypeptide fold around the metal site of the N-terminal fragment is very similar, if not identical, to that occurring in the full-size protein. The N-terminal sequence of this hydrogenase is homologous to sequences of a number of proteins or protein domains, including a subunit of NADH-ubiquinone oxidoreductase of respiratory chains. From that, it can be anticipated that the structural domain isolated and described here is a building block of electron transfer complexes involved in various bioenergetic processes.     

Glutamate 286 in cytochrome aa3 from Rhodobacter sphaeroides is involved in proton uptake during the reaction of the fully-reduced enzyme with dioxygen

The reaction with dioxygen of solubilized fully-reduced wild-type and EQ(I-286) (exchange of glutamate 286 of subunit I for glutamine) mutant cytochrome c oxidase from Rhodobacter sphaeroides has been studied using the flow-flash technique in combination with optical absorption spectroscopy. Proton uptake was measured using a pH-indicator dye. In addition, internal electron-transfer reactions were studied in the absence of oxygen. Glutamate 286 is found in a proton pathway proposed to be used for pumped protons from the crystal structure of cytochrome c oxidase from Paracoccus denitrificans [Iwata et al. (1995) Nature 376, 660-669; E278 in P.d. numbering]. It is the residue closest to the oxygen-binding binuclear center that is clearly a part of the pathway. The results show that the wild-type enzyme becomes fully oxidized in a few milliseconds at pH 7.4 and displays a biphasic proton uptake from the medium. In the EQ(I-286) mutant enzyme, electron transfer after formation of the peroxy intermediate is impaired, CuA remains reduced, and no protons are taken up from the medium. Thus, the results suggest that E(I-286) is necessary for proton uptake after formation of the peroxy intermediate and transfer of the fourth electron to the binuclear center. The results also indicate that the proton uptake associated with formation of the ferryl intermediate controls the electron transfer from CuA to heme a.     

Constitutive activation of the m5 muscarinic receptor by a series of mutations at the extracellular end of transmembrane 6

The m5 muscarinic acetylcholine receptor was constitutively activated by a wide range of amino acid substitutions at a residue (serine 465) that is positioned at the junction of the sixth transmembrane domain and the extracellular loop. Of 13 substitutions tested, 11 produced significant increases in constitutive activity. Replacement of serine 465 with large (phenylalanine and valine) or basic residues (arginine and lysine) increased the constitutive activity of the receptor to between 55 and 110% of the maximum response of the wild-type receptor to the agonist carbachol. Other substitutions (e.g., cysteine and leucine) increased the constitutive activity to an intermediate level (30%), while small and acidic residues (glycine, aspartate, and glutamate) caused small or insignificant increases. The increase in the constitutive activity of each mutant receptor correlated with an increase in the potency of carbachol in both binding and functional assays, with the most constitutively activated receptors showing a 40-fold decrease in the EC50 of carbachol. The negative antagonist atropine bound to and reversed the constitutive activity of all mutant receptors with equal potency. These data were fitted to a two-state model of receptor function. The data are consistent with the primary effect of substitutions to serine 465 being to selectively destabilize the inactive state of the receptor, thus favoring formation of the active state in the absence of agonists. Our data strongly support this two-state model of receptor function and identify a critical role of this domain in the activation of muscarinic receptors.     

Influence of iron-removal procedures on sequential electron transfer in photosynthetic bacterial reaction centers studied by transient EPR spectroscopy

Electron spin polarized electron paramagentic resonance (ESP EPR) spectra were obtained with deuterated iron-removed photosynthetic bacterial reaction centers (RCs) to specifically investigate the effect of the rate of primary charge separation, metal-site occupancy, and H-subunit content on the observed P865+QA- charge-separated state. Fe-removed and Zn-substituted RCs from Rb. sphaeroides R-26 were prepared by refined procedures, and specific electron transfer rates (kQ) from the intermediate acceptor H- to the primary acceptor QA of (200 ps)-1 vs (3-6 ns)-1 were observed. Correlation of the transient EPR and optical results shows that the observed slow kQ rate in Fe-removed RCs is H-subunit-independent, and, in some cases, independent of Fe-site occupancy as Zn2+ substitution does not ensure retention of the native kQ. In addition, shifts in the optical spectrum of P865 and differences in the high-field region of the Q-band ESP spectrum for Fe-removed RCs with slow kQ indicate possible structural changes near P865. The experimental X-band and Q-band spin-polarized EPR spectra for deuterated Fe-removed RCs where kQ is at least 15-fold slower at room temperature than the (200 ps)-1 rate observed for native Fe-containing RCs have different relative amplitudes and small g-value shifts compared to the spectra of Zn-RCs which have a kQ unchanged from native RCs. These differences reflect the trends in polarization predicted from the sequential electron transfer polarization (SETP) model [Morris et al. (1995) J. Phys. Chem. 99, 3854-3866; Tang et al. (1996) Chem. Phys. Lett. 253, 293-298]. Thus, SETP modeling of these highly resolved ESP spectra obtained with well-characterized proteins will provide definitive information about any light-induced structural changes of P865, H, and QA that occur upon formation of the P865+QA- charge-separated state.     

Control of the phosphorylation of the astrocyte marker glial fibrillary acidic protein (GFAP) in the immature rat hippocampus by glutamate and calcium ions: possible key factor in astrocytic plasticity

The present review describes recent research on the regulation by glutamate and Ca2+ of the phosphorylation state of the intermediate filament protein of the astrocytic cytoskeleton, glial fibrillary acidic protein (GFAP), in immature hippocampal slices. The results of this research are discussed against a background of modern knowledge of the functional importance of astrocytes in the brain and of the structure and dynamic properties of intermediate filament proteins. Astrocytes are now recognized as partners with neurons in many aspects of brain function with important roles in neural plasticity. Site-specific phosphorylation of intermediate filament proteins, including GFAP, has been shown to regulate the dynamic equilibrium between the polymerized and depolymerized state of the filaments and to play a fundamental role in mitosis. Glutamate was found to increase the phosphorylation state of GFAP in hippocampal slices from rats in the post-natal age range of 12-16 days in a reaction that was dependent on external Ca2+. The lack of external Ca2+ in the absence of glutamate also increased GFAP phosphorylation to the same extent. These effects of glutamate and Ca2+ were absent in adult hippocampal slices, where the phosphorylation of GFAP was completely Ca(2+)-dependent. Studies using specific agonists of glutamate receptors showed that the glutamate response was mediated by a G protein-linked group II metabotropic glutamate receptor (mGluR). Since group II mGluRs do not act by liberating Ca2+ from internal stores, it is proposed that activation of the receptor by glutamate inhibits Ca2+ entry into the astrocytes and consequently down-regulates a Ca(2+)-dependent dephosphorylation cascade regulating the phosphorylation state of GFAP. The functional significance of these results may be related to the narrow developmental window when the glutamate response is present. In the rat brain this window corresponds to the period of massive synaptogenesis during which astrocytes are known to proliferate. Possibly, glutamate liberated from developing synapses during this period may signal an increase in the phosphorylation state of GFAP and a consequent increase in the number of mitotic astrocytes.     

Energy transduction on the nanosecond time scale: early structural events in a xanthopsin photocycle

Photoactive yellow protein (PYP) is a member of the xanthopsin family of eubacterial blue-light photoreceptors. On absorption of light, PYP enters a photocycle that ultimately transduces the energy contained in a light signal into an altered biological response. Nanosecond time-resolved x-ray crystallography was used to determine the structure of the short-lived, red-shifted, intermediate state denoted [pR], which develops within 1 nanosecond after photoelectronic excitation of the chromophore of PYP by absorption of light. The resulting structural model demonstrates that the [pR] state possesses the cis conformation of the 4-hydroxyl cinnamic thioester chromophore, and that the process of trans to cis isomerization is accompanied by the specific formation of new hydrogen bonds that replace those broken upon excitation of the chromophore. Regions of flexibility that compose the chromophore-binding pocket serve to lower the activation energy barrier between the dark state, denoted pG, and [pR], and help initiate entrance into the photocycle. Direct structural evidence is provided for the initial processes of transduction of light energy, which ultimately translate into a physiological signal.     

Triplet energy transfer in bacterial photosynthetic reaction centres

[3-vinyl]-132-OH-bacteriochlorophyll a has been selectively exchanged against native bacteriochlorophyll a in the monomer binding sites at the A- and B-branch of the photosynthetic reaction centre from Rhodobacter sphaeroides. Transient absorption difference measurements were performed on these samples over a temperature range from 4.2 to 300 K with 20 ns time resolution. Specifically the decay of the primary donor triplet state, 3P870, as well as the rise and decay rates of the carotenoid triplet state, 3Car (spheroidene), were measured. The observed rates revealed a thermally activated carotenoid triplet formation corresponding to the decay of the primary donor triplet state. The activation energies for the triplet energy transfer process were 100(+/-10) cm-1 for reaction centers from wild-type Rhodobacter sphaeroides 2.4.1, with and without exchange of the monomeric bacteriochlorophyll on the electron transfer-active branch, BA. For reaction centers from Rhodobacter sphaeroides R26.1 with both monomers exchanged against [3-vinyl]-132-OH-bacteriochlorophyll a, and subsequent spheroidene reconstitution the activation energy was 460(+/-20) cm-1. These activation energies correspond to the energy difference between the triplet states of the accessory BChl monomer, BB, and the primary donor when native BChl a or [3-vinyl]-132-OH-BChl a is present in the BB binding site. In all samples the 3Car formation rates were bi-phasic over a large temperature range. A fast temperature-independent rate was observed on the wavelength of the carotenoid triplet-triplet absorption which dominated at very low temperatures. Additionally, a slower temperature-independent 3Car formation rate was observed at low temperatures which could be explained with the assumption of heterogeneity in the energy barrier (3BB) and/or the primary donor triplet state (3P870). A tunneling mechanism as proposed earlier by Kolaczkowski (PhD thesis, Brown University, 1989) is not only unnecessary but also incompatible with the available experimental data.     

Role of branched-chain aminotransferase isoenzymes and gabapentin in neurotransmitter metabolism

Because it is well known that excess branched-chain amino acids (BCAAs) have a profound influence on neurological function, studies were conducted to determine the impact of BCAAs on neuronal and astrocytic metabolism and on trafficking between neurons and astrocytes. The first step in the metabolism of BCAAs is transamination with alpha-ketoglutarate to form the branched-chain alpha-keto acids (BCKAs). The brain is unique in that it expresses two separate branched-chain aminotransferase (BCAT) isoenzymes. One is the common peripheral form [mitochondrial (BCATm)], and the other [cytosolic (BCATc)] is unique to cerebral tissue, placenta, and ovaries. Therefore, attempts were made to define the isoenzymes' spatial distribution and whether they might play separate metabolic roles. Studies were conducted on primary rat brain cell cultures enriched in either astroglia or neurons. The data show that over time BCATm becomes the predominant isoenzyme in astrocyte cultures and that BCATc is prominent in early neuronal cultures. The data also show that gabapentin, a structural analogue of leucine with anticonvulsant properties, is a competitive inhibitor of BCATc but that it does not inhibit BCATm. Metabolic studies indicated that BCAAs promote the efflux of glutamine from astrocytes and that gabapentin can replace leucine as an exchange substrate. Studying astrocyte-enriched cultures in the presence of [U-14C]glutamate we found that BCKAs, but not BCAAs, stimulate glutamate transamination to alpha-ketoglutarate and thus irreversible decarboxylation of glutamate to pyruvate and lactate, thereby promoting glutamate oxidative breakdown. Oxidation of glutamate appeared to be largely dependent on the presence of an alpha-keto acid acceptor for transamination in astrocyte cultures and independent of astrocytic glutamate dehydrogenase activity. The data are discussed in terms of a putative BCAA/BCKA shuttle, where BCATs and BCAAs provide the amino group for glutamate synthesis from alpha-ketoglutarate via BCATm in astrocytes and thereby promote glutamine transfer to neurons, whereas BCATc reaminates the amino acids in neurons for another cycle.     

Rate of glutamate synthesis from leucine in rat brain measured in vivo by 15N NMR

The rate of glutamate synthesis from leucine by the branched-chain aminotransferase was measured in rat brain in vivo at steady state. The rats were fed exclusively by intravenous infusion of a nutrient solution containing [15N]leucine. The rate of glutamate synthesis from leucine, determined from the rate of increase of brain [15N]glutamate measured by 15N NMR and the 15N enrichments of brain and blood leucine analyzed by gas chromatography-mass spectrometry, was 0.7-1.8 micromol/g/h at a steady-state brain leucine concentration of 0.25 micromol/g. A comparison of the observed fractional 15N enrichments of brain leucine (0.42 +/- 0.03) and glutamate (0.21 +/- 0.015) showed that leucine provides approximately 50% of glutamate nitrogen under our experimental condition. From the observed rate (0.7-1.8 micromol/g) and the known Km of the branched-chain aminotransferase for leucine (1.2 mM), the rate of glutamate synthesis from leucine at physiological brain leucine concentration (0.11 micromol/g) was estimated to be 0.35-0.9 micromol/g/h, with leucine providing approximately 25% of glutamate nitrogen. The results strongly suggest that plasma leucine from dietary source, transported into the brain, is an important external source of nitrogen for replenishment of brain glutamate in vivo. Implications of the results for treatment of maple-syrup urine disease patients with leucine-restricted diet are discussed.     

Compact thermally-denatured state of a staphylococcal nuclease mutant from resonance energy transfer measurements

Thermal denaturation of a staphylococcal nuclease mutant K78C, where lysine 78 is replaced by cysteine, was studied by circular dichroism (CD) and resonance energy transfer. CD spectra suggest that residual structures remain in the denatured state. Steady-state energy transfer from intrinsic tyrosines to a single and intrinsic tryptophan was measured at different temperatures. In the thermally-denatured state of K78C, there is still a substantial degree of energy transfer from tyrosine(s) to tryptophan, indicating residual structures in the denatured state. The cysteine residue in mutant K78C was labeled with a cysteine specific probe IAEDANS. Fluorescence decays of the tryptophan were measured to estimate distance distributions between Trp 140 and IAEDANS at position 78. Measurements were done as a function of temperature from 4 degrees C (native) to 65 degrees C (denatured) both with and without Ca2+ and inhibitor pdTp. Below 30 degrees C, the apparent distance distribution of both the ligand-free nuclease and the enzyme with bound pdTp can be adequately described by a Gaussian model. Above 40 degrees C, where the ligand-free nuclease but not the ternary complex begins to denature, two different populations are required to fit the data both with and without pdTp. One population has a compact structure and the other has an expanded structure. As temperature rises, the population of the expanded structure increases. At the highest temperature, the non-native compact structure is still the major form (60 to 70%). The overall thermally-denatured states of staphylococcal nuclease mutant K78C in the absence and presence of ligands are thus compact and heterogeneous.     

Structure at 0.85 A resolution of an early protein photocycle intermediate

Protein photosensors from all kingdoms of life use bound organic molecules, known as chromophores, to detect light. A specific double bond within each chromophore is isomerized by light, triggering slower changes in the protein as a whole. The initial movements of the chromophore, which can occur in femtoseconds, are tightly constrained by the surrounding protein, making it difficult to see how isomerization can occur, be recognized, and be appropriately converted into a protein-wide structural change and biological signal. Here we report how this dilemma is resolved in the photoactive yellow protein (PYP). We trapped a key early intermediate in the light cycle of PYP at temperatures below -100 degrees C, and determined its structure at better than 1 A resolution. The 4-hydroxycinnamoyl chromophore isomerizes by flipping its thioester linkage with the protein, thus avoiding collisions resulting from large-scale movement of its aromatic ring during the initial light reaction. A protein-to-chromophore hydrogen bond that is present in both the preceding dark state and the subsequent signalling state of the photosensor breaks, forcing one of the hydrogen-bonding partners into a hydrophobic pocket. The isomerized bond is distorted into a conformation resembling that in the transition state. The resultant stored energy is used to drive the PYP light cycle. These results suggest a model for phototransduction, with implications for bacteriorhodopsin, photoactive proteins, PAS domains, and signalling proteins.     

Toward understanding tryptophan fluorescence in proteins

A general approach to dissecting the complex photophysics of tryptophan is presented and used to elucidate the effects of amino acid functional groups on tryptophan fluorescence. We have definitively identified the amino acid side chains that quench tryptophan fluorescence and delineated the respective quenching mechanisms in a simple model system. Steady-state and time-resolved fluorescence techniques, photochemical H-D exchange experiments, and transient absorption techniques were used to measure individual contributions to the total nonradiative rate for deactivation of the excited state, including intersystem crossing, solvent quenching, and excited-state proton and electron transfer rates. Eight amino acid side chains representing six functional groups quench 3-methylindole fluorescence with a 100-fold range in quenching rate constant. Lysine and tyrosine side chains quench by excited-state proton transfer; glutamine, asparagine, glutamic and aspartic acid, cysteine, and histidine side chains quench by excited-state electron transfer. These studies provide a framework for deriving detailed structural and dynamical information from tryptophan fluorescence intensity and lifetime data in peptides and proteins.     

Quantum-Chemical Study of the Charge Transfer Mechanism in the MgTiF<Subscript>6</Subscript>–12MgCl<Subscript>2</Subscript> System

A quantum-chemical analysis of frontier molecular orbitals is used to propose a mechanism for the electrochemical electron transfer in the model system MgTiF₆ + 12MgCl₂. The geometric structure of the transient state is shown not to be intermediate between the initial and final states of the system. The contraction of the Ti–F bonds during totally symmetrical vibrations is found to be the most probable version providing electron transfer to the system.     

Synthesis,characteristic and intramolecular energy transfer mechanism of reactive terbium complex in white light emitting diode

A reactive Tb(III) complex with 2-aminobenzoic acid(2-ABAH) and acrylonitrile(AN) as ligands was synthesized.The structure of the complex was characterized by elemental analysis and Fourier transform infrared spectrometry(FT-IR).The results indicated that the ligands were coordinated with Tb(III) ion.Thermal gravity-derivative thermogravimetric(TG-DTG) analysis indicated that the complex kept stable up to 198 oC.Luminescence properties were investigated by UV-vis absorption spectra and fluorescence spectra.The results suggested that being excited at 361 nm,the complex exhibited characteristic emission of Tb(III) ion,revealing that the complex could be excited by 365 nm ultraviolet chip.The HOMO and LUMO,ΔE(HOMO-LUMO),molecular frontier orbital,and the singlet state and triplet energy state levels of the ligands were calculated at the B3LYP/6-31+G(d) level.The results indicated that intramolecular energy transfer mechanism followed Dexter exchange energy transfer theory.Both the calculation for excited state of ligand and energy transfer mechanism could provide the theoretical basis for the design of high luminescent materials of rare earth complexes with organic ligands.