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At least 19 recordsLinked to original sources

PubMed2003

"Acceptor-donor-acceptor" motifs recognize the Watson-Crick, Hoogsteen and Sugar "donor-acceptor-donor" edges of adenine and adenosine-containing ligands.

Nucleotides are among the most extensively exploited chemical moieties in nature and, as a part of a handful of different protein ligands, nucleotides play key roles in energy transduction, enzymatic catalysis and regulation of protein function. We have previously reported that in many proteins with different folds and functions a distinctive adenine-binding motif is involved in the recognition of the Watson-Crick edge of adenine. Here, we show that many proteins do have clear structural motifs that recognize adenosine (and some other nucleotides and nucleotide analogs) not only through the Watson-Crick edge, but also through the sugar and Hoogsteen edges. Each of the three edges of adenosine has a donor-acceptor-donor (DAD) pattern that is often recognized by proteins via a complementary acceptor-donor-acceptor (ADA) motif, whereby three distinct hydrogen bonds are formed: two conventional N-H...O and N-H...N hydrogen bonds, and one weak C-H...O hydrogen bond. The local conformation of the adenine-binding loop is betabetabeta or betabetaalpha and reflects the mode of nucleotide binding. Additionally, we report 21 proteins from five different folds that simultaneously recognize both the sugar edge and the Watson-Crick edge of adenine. In these proteins a unique beta-loop-beta supersecondary structure grasps an adenine-containing ligand between two identical adenine-binding motifs as part of the betaalphabeta-loop-beta fold.

Adenine↗

PubMed1990

Preferential ligand binding to multi-state acceptor systems: the unexplored paradox of acceptor self-association that is ligand-mediated but detrimental to ligand binding.

Consideration is given to the interactions of ligand with self-associating acceptor systems for which preferential ligand binding is an ambiguous term, in that the acceptor species with greater affinity for ligand possesses relatively fewer binding sites. A paradoxical situation wherein ligand-mediated self-association is seemingly detrimental to ligand binding is shown to be the predicted outcome for a transient range of ligand concentrations. This outcome reflects the existence of a critical point in the dependence of the extent of acceptor self-association upon ligand concentration that coincides with a cross-over point of ligand-binding curves for different, fixed total concentrations of acceptor. By classical differentiation methods the conditions for the existence of these critical points are established not only for two-state acceptor systems but also for three-state acceptor systems in which the ligand-binding form of monomer also undergoes reversible isomerization to an inactive state. Similar procedures are used to comment upon the forms of binding curves for the three-state acceptor systems, the Scatchard representations of which may exhibit as many as three critical points (two maxima and a minimum). This delineation of quantitative expressions for critical points and other distinctive features associated with the conflicting interplay of ligand-binding and self-association behaviour should provide a more definitive means of characterizing systems with one acceptor state the preferred binding form on affinity grounds but with the other the preferred state from the viewpoint of binding-site numbers.

Animals↗

PubMed2000

Electron acceptors of the fluorene series. 10.(1) novel acceptors containing butylsulfanyl, butylsulfinyl, and butylsulfonyl substituents: synthesis, cyclic voltammetry, charge-transfer complexation with anthracene in solution, and X-ray crystal structures of two tetrathiafulvalene complexes

2,4,5,7-Tetranitro-9-fluorenone (1b) reacts readily with n-butanethiol in dipolar aprotic solvents with selective substitution of nitro groups by butylsulfanyl groups in positions 2 and 7 (2, 3); the 2,5-isomer 4 was formed only as a minor product (<1%). Condensation of fluorenones 2-4 with malononitrile yielded 9-dicyanomethylene derivatives 5-7, which showed strong intramolecular charge transfer (lambda approximately 510-560 nm) and were found to sensitize the photoconductivity of carbazole-containing polymer films. Oxidation of sulfides 2-4 gave sulfoxide 8 or sulfones 9-11, which then were converted into their corresponding dicyanomethylene derivatives 12-15. All these novel acceptors showed three reversible single-electron reduction waves (cyclic voltammetry) yielding radical anion, dianion, and radical trianion; moreover, acceptors 13-15 showed also a fourth reduction wave, representing reversible tetraanion formation. Substitution of the oxygen of the carbonyl group in the fluorenones by a dicyanomethylene group increased the thermodynamic stability (K(SEM) growth) of the radical anion; K(SEM) ranged from 3 x 10(5) to 3 x 10(9) M(-1). CV measurements characterize compounds 3, 4 (EA = 1. 86-1.89 eV) as poor acceptors, 2, 6-11 (EA = 2.13-2.31 eV) as moderate acceptors, and 5, 12-15 (EA = 2.53-2.66 eV) as strong electron acceptors. Charge-transfer complex (CTC) formation between acceptors 9, 10, 13, 14, and anthracene as a donor was monitored by the appearance of additional low-energy bands in the visible region (CTC bands) of their electron absorption spectra. Increasing the EA of the acceptors from 9-fluorenones to the corresponding 9-dicyanomethylenefluorenes increases the complexation constants K(CTC) by 2.5-3 times, while sulfonyl substituents present substantial steric hindrance for complexation (as compared to the nitro group), decreasing K(CTC) values. Two CTCs for acceptors 14 and 17 with tetrathiafulvalene (TTF) were obtained, and their structures were solved by single-crystal X-ray diffractometry, giving the stoichometries 14:TTF, 2:3, and 17:TTF:PhCl, 1:1:0.5. In the former complex the packing motif is a mixed.DDAD'A. stack; in the latter complex the D and A moieties form unusually close CT pairs, which pack in a herringbone motif.

Journal Article↗

PubMed1991

Four sites in the acceptor helix and one site in the variable pocket of tRNA(Ala) determine the molecule's acceptor identity.

The structural features that determine tRNA(Ala) acceptor identity have been studied with amber-suppressor tRNAs in Escherichia coli cells. Previous work established that a wobble pair composed of guanosine at position 3 and uridine at position 70 (G3-U70) in the acceptor helix of tRNA(Ala) is a determinant of the molecule's acceptor identity. We show that additional determinants are located at three other sites in the acceptor helix and at one site in the variable pocket of tRNA(Ala). These latter determinants are less important than G3.U70 since their individual alterations in mutants of tRNA(Ala) have smaller degrading effects on the functions of the molecules, and subsets of the determinants, when combined with G3.U70, are sufficient to switch the identities of several other tRNAs to that of tRNA(Ala). Other workers are using fragments of the tRNA(Ala) acceptor helix to study the molecule's acceptor identity. Our demonstration that the variable pocket contributes to tRNA(Ala) acceptor identity means that such fragments do not faithfully replicate the structure-function relationship of the cellular process.

Anticodon↗

PubMed1980

Dissection of the active site of rabbit liver tRNA nucleotidyltransferase. Specificity and properties of the tRNA and acceptor subsites determined with model acceptor substrates.

The specificity of rabbit liver tRNA nucleotidyltransferase with respect to its interaction with acceptor residues at the 3' end of tRNA was analyzed using a model acceptor system consisting of dinucleoside monophosphates or nucleosides. Of all the dinucleoside monophosphates tested, only CpC was an active AMP acceptor, indicating that the specificity of the enzyme conforms exactly to the structure present at the 3' terminus of the natural acceptor, tRNA-C-C. Similarly, CMP incorporation into model acceptors closely paralleled the specificity seen with tRNA-C and tRNA-X. Competition studies between the model acceptors and tRNAs with modified 3' termini suggested that the model compounds bind to the enzyme at the site normally recognizing the 3' terminus of tRNA. Comparison of nucleotide incorporation into tRNAs and into the model acceptors revealed a number of differences which allowed us to separate effects on tRNA structure from direct effects on the reaction. These studies enabled us to distinguish several subsites on the enzyme: an ATP-donor site, two sites specifically recognizing the 2 terminal C residues on tRNA, and a site recognizing the nonreacting part of the tRNA. Thus, these results support several features of the multisite model previously proposed (Deutscher, M. P. (1972) J. Biol. Chem. 247, 459-468) to explain tRNA nucleotidyltransferase action.

Adenosine Monophosphate↗

PubMed1983

Comparative rates of transfer of N-acetylneuraminic acid to acceptors bearing one or more Gal(beta 1-4)GlcNAc terminus by the Gal(beta 1-4)GlcNAc(NeuAc-Gal) (alpha 2-6)-sialyltransferase from embryonic chicken liver. Utilization of oligosaccharides as acceptors in sialyltransferase assays.

Using a number of branched and unbranched oligosaccharides, glycoproteins and artificial glycoproteins bearing Gal(beta 1-4)GlcNAc-R termini as acceptors (where R represents H, oligosaccharide, oligosaccharide-protein or fatty acid-protein), the comparative rates of transfer of NeuAc by the Gal(beta 1-4)GlcNAc(NeuAc-Gal) (alpha 2-6)-sialyltransferase of embryonic chicken liver were determined. Acceptor substrates were utilized at levels approximating physiological, near the Km value of the best acceptor, desialylated alpha 1 acid glycoprotein. The sialyltransferase has a marked preference for multi-branched acceptors. From the specificity data, it is concluded that the enzyme binds at least two Gal(beta 1-4)GlcNAc termini of an acceptor molecule, and that the relative orientation of the branches is an important factor determining the rate of catalysis by the enzyme. The use of oligosaccharides as acceptors to study sialyltransferase catalyses is emphasized. Results are discussed in the context of the mode of assembly of sialoside termini of known glycoprotein structures in vivo.

Animals↗

PubMed1981

The involvement of a high potential acceptor in the acid-base induced reduction of the acceptor Q in chloroplasts.

Study of the acid-base induced fluorescence transient, the so-called 'reverse electron flow', in chloroplasts revealed the following new properties: (1) Experiments in which the acid-base transition was performed in the absence of the measuring beam showed that the high fluorescence state induced by a pH-jump was attained and decayed even in complete darkness. These results indicated that a non-photochemical electron transfer was affected by the pH transition. A pH-induced reverse electron transfer from some secondary electron acceptor to Q probably occurred during that process. (2) This conclusion was supported by the effect of the Photosystem I electron acceptor methyl viologen. Methyl viologen accelerated the decay phase of the transient showing that this phase was controlled by the rate of electron flow to Photosystem I, but this acceptor did not diminish the size of the transient's initial rise, probably because this rise reflected a pH effect on a non-photochemical step located between Q and P-700. (3) The size of the fluorescence transient was dependent upon the reduction state of both parts of the secondary pool of electron acceptors, A2 and A1. (4) Redox potential measurements using ferricyanide-ferrocyanide mixtures showed that the size of the transient was directly dependent on a midpoint potential of +385 mV at pH 6.9 and with n = 1. This suggested the involvement of a high potential secondary electron acceptor in the acid-base induced reduction of Q.

Chloroplasts↗

PubMed2006

Bifunctional charge transfer operated fluorescent probes with acceptor and donor receptors. 2. Bifunctional cation coordination behavior of biphenyl-type sensor molecules incorporating 2,2':6',2' '-terpyridine acceptors.

Based on donor (D)-acceptor (A) biphenyl (b) type molecules, a family of fluorescent reporters with integrated acceptor receptors and noncoordinating and coordinating donor substituents of varying strength has been designed for ratiometric emission sensing and multimodal signaling of metal ions and protons. In part 2 of this series on such charge transfer (CT) operated mono- and bifunctional fluorescent devices, the cation coordination behavior of the sensor molecules bpb-R equipped with a proton- and cation-responsive 2,2':6',2' '-terpyridine (bp) acceptor and either amino-type donor receptors (R = DMA, A15C5 = monoaza-15-crown-5) or nonbinding substituents (R = CF(3), H, OMe) is investigated employing the representative metal ions Na(I), Ca(II), Zn(II), Hg(II), and Cu(II) and steady-state and time-resolved fluorometry. The bpb-R molecules, the spectroscopic behavior and protonation behavior of which have been detailed in part 1 of this series, present rare examples for CT-operated bifunctional fluorescent probes that can undergo consecutive and/or simultaneous analyte recognition. The analyte-mediated change of the probes' intramolecular CT processes yields complexation site- and analyte-specific outputs, i.e., absorption and fluorescence modulations in energy, intensity, and lifetime. As revealed by the photophysical studies of the cation complexes of these fluoroionophores and the comparison to other neutral and charged D-A biphenyls, the spectroscopic properties of the acceptor chelates of bpb-R and A- and D-coordinated bpb-R are governed by CT control of an excited-state barrier toward formation of a forbidden charge transfer state, by the switching between analytically favorable anti-energy and common energy gap law type behavior, and by the electronic nature of the ligated metal ion. This accounts for the astonishingly high fluorescence quantum yields of the acceptor chelates of bpb-R equipped with weak or medium-sized donors and the red emission of D- and A-coordinated bpb-R observed for nonquenching metal ions.

Biphenyl Compounds↗

PubMed1995

High efficiency of glycerol 2-phosphate and sn-glycerol 3-phosphate as nucleotidyl acceptors in snake venom phosphodiesterase esterifications. Formation of primary and secondary AMP-O-glyceryl and AMP-O-glycerophosphoryl esters and evidence for an acceptor-binding enzyme site.

Snake venom phosphodiesterase (SVP) catalyzes the alcoholysis of ATP by primary R-CH2OH alcohols with uncharged R residues, yielding AMP-O-CH2R esterification products. The alcohols compete with water for an SVP-bound adenylyl intermediate. In this study, it has been shown that SVP also catalyzes the reactions of glycerol 2-phosphate and sn-glycerol 3-phosphate with ATP to yield AMP-O-glycerophosphoryl esters. The products were identified by HPLC, the dependency of the reactions on glycerol phosphates, ultraviolet spectroscopy, and conversion to AMP by phosphodiesterase, or to AMP-O-glyceryl esters by alkaline phosphatase. The results demonstrated that R-CH2OH alcohols with negatively charged R residues, as well as secondary alcohols, act as adenylyl acceptors in SVP reactions, thus extending the usefulness of SVP as a tool to produce 5'-nucleotide derivatives. The efficiencies (EA) of glycerol phosphates as adenylyl acceptors were very high at low, millimolar concentrations, but decreased abruptly when the acceptor concentration was increased and, for glycerol 2-phosphate, when Pi or NaCl was present. In contrast, glycerol EA was independent of its own concentration, Pi, and NaCl. The responses of glycerol phosphates indicate that they act as adenylyl acceptors via a mechanism different from uncharged R-CH2OH alcohols. The occurrence of an acceptor-binding enzyme site, specific for negatively charged R residues, and its potential relevance to the in vivo role of 5'-nucleotide phosphodiesterases as 5'-nucleotidyl transferases are discussed.

Adenosine Monophosphate↗

PubMedDate not supplied

[Effect of the complexes tocopherol-protein-acceptors and tocopherylquinone-protein-acceptors on the activity cytosol phospholipase A2 in rat liver].

A new method of tocopherol and tocopherylquinone binding with rat liver cytosol proteins-acceptors. Complexes tocopherol-proteins-acceptors and tocopheryl-quinone-proteins acceptors have been studied for their effect on phospholipase A2 activity. It is established that the both complexes are the inhibitors of phospholipase A2, under these conditions tocopherylquinone complex with proteins-acceptors more intensively decreases Km of the enzyme than the complex tocopherol-proteins-acceptors.

Animals↗

PubMed1990

Specificity of acceptor binding to Leuconostoc mesenteroides B-512F dextransucrase: binding and acceptor-product structure of alpha-methyl-D-glucopyranoside analogs modified at C-2, C-3, and C-4 by inversion of the hydroxyl and by replacement of the hydroxyl with hydrogen.

The specificity of acceptor binding to the active site of dextransucrase was studied by using alpha-methyl-D-glucopyranoside analogs modified at C-2, C-3, and C-4 positions by (a) inversion of the hydroxyl group and (b) replacement of the hydroxyl group with hydrogen. 2-Deoxy-alpha-methyl-D-glucopyranoside was synthesized from 2-deoxyglucose; 3- and 4-deoxy-alpha-methyl-D-glucopyranosides were synthesized from alpha-methyl-D-glucopyranoside; and alpha-methyl-D-allopyranoside was synthesized from D-glucose. The analogs were incubated with [14C]sucrose and dextransucrase, and the products were separated by thin-layer chromatography and quantitated by liquid scintillation spectrometry. Structures of the acceptor products were determined by methylation analyses and optical rotation. The relative effectiveness of the acceptor analogs in decreasing order were 2-deoxy, 2-inverted, 3-deoxy, 3-inverted, 4-inverted, and 4-deoxy. The enzyme transfers D-glucopyranose to the C-6 hydroxyl of analogs modified at C-2 and C-3, to the C-4 hydroxyl of 4-inverted, and to the C-3 hydroxyl of 4-deoxy analogs of alpha-methyl-D-glucopyranoside. The data indicate that the hydroxyl group at C-2 is not as important for acceptor binding as the hydroxyl groups at C-3 and C-4. The hydroxyl group at C-4 is particularly important as it determines the binding orientation of the alpha-methyl-D-glucopyranoside ring.

Glucosyltransferases↗

PubMed1994

Multiphasic modelling of ligand/acceptor interactions. The hydrophobicity-dependent binding of relatively small amphiphilic substances to acceptor proteins and the nature and facedness of acceptor sites.

The modelling of multiphasic ligand/acceptor equilibrium binding systems proceeds at three logically distinct levels: (1) A suitable response quantity, e.g. the amount of acceptor-bound ligand nEL, is expressed as a function of the ligand concentrations [Li] (L = A,B,...) in the compartment i that contains the acceptor sites. One thus obtains a response function nEL = f1([Li]). In general, the equilibrium constants KL contained in such mathematical models are physically ill-defined. (2) Each local concentration [Li] is further expressed as a function of [Laq], the corresponding concentration in the aqueous phase, leading to nEL = f2([Laq]). In this way, the constants KL are transformed into effective constants K'L which (i) can be assessed experimentally and (ii) depend on ligand hydrophobicity in a way that is characteristic of the binding site. Formulation of the functions f1 and F2 only requires knowledge of the reactions in which the acceptor sites participate directly. (3) For each ligand, the experimentally accessible total ligand concentration Lt is expressed as a function of [Laq], leading to concentration balance equations Lt = Lt([Laq]). The latter transformation takes account of any reactions, distinct from ligand/acceptor interaction, in which the ligands are involved, e.g. binding to additional protein sites. As a result of steps 2 and 3, each binding system is described by a set of simultaneous equations dependent on the auxiliary variable [Laq]: (i) the response function f2([Laq]) and (ii) a concentration balance for each ligand Lt = Lt([Laq]). The formulae are rendered more conscise and their discussion and application to data fitting are simplified by introducing, for each ligand L, a function FL characterising the distribution of unbound monomeric ligand over the various partition compartments. When the acceptor acts on unbound ligand, the formulae are further expressed in terms of a new auxiliary variable i.e. the total concentration of unbound monomeric ligand microL. In contrast to data analysis as a function of local concentrations, analysis in terms of total ligand concentrations avoids losing sight of alternate hypotheses about the nature of the binding sites. The present formulation has also permitted clarification of several consequences of the multiphasic nature of the binding systems that, as yet, have been poorly recognised.(ABSTRACT TRUNCATED AT 400 WORDS)

Binding Sites↗

PubMed1983

C3b acceptors on macrophages: inhibition of Fc gamma-receptor-mediated phagocytosis by acceptor-bound C3b.

The binding of nascent human C3b (i.e. the fragment of C3 just after trypsin cleavage) to mouse peritoneal macrophages was demonstrated by immune adherence. Acceptor-bound C3b could be detected longer than 24 h on the cell membrane. The rosette formation and phagocytosis of SRBC coated with anti-SRBC rat IgG was inhibited by preincubation of the cells with C3 and trypsin (15 min, 37 degrees C). However, the phagocytosis of opsonized yeast particles was not influenced by acceptor-bound C3b, proving that C3b-C3b acceptor interaction did not alter the function of C3b-receptors. Acceptor-bound C3b on the macrophages failed to mediate phagocytosis of human 0,Rh+ red cells having C3b-receptors.

Animals↗

PubMed2004

Evidence for pi-pi electron donor-acceptor interactions between pi-donor aromatic compounds and pi-acceptor sites in soil organic matter through pH effects on sorption.

Elucidation of molecular-level interactions controlling the sorption of organic compounds in soils is of major theoretical and practical interest. Sorption of pi-electron donor compounds, pentamethylbenzene (PMB), naphthalene (NAPH), and phenanthrene (PHEN), in a number of soils was found to increase with decreasing pH in the range of approximately pH 2.5-7. This behavior could not be attributed to pH-dependent alteration of the hydrophobic character of humic substances, pi-H-bonding, interaction with mineral surfaces, interaction with black carbons, solute coplanarity, or pH effects on solute activity coefficient. No significant effect of pH was observed for non-pi-donor hydrophobic compounds, whether planar or not: trans-1,2-dichlorocyclohexane (DCCH), hexachloro-1,3-butadiene (HCBD), 1,2,4-trichlorobenzene (TCB), 2,2',5,5'-tetrachlorobiphenyl, and 3,3',4,4'-tetrachlorobiphenyl. The opposite pH effect was observed for 2-nonanol and 2-nonanone, which are non-pi-donors, but capable of H-bonding. Also, no pH-dependent sorption was observed between the pi-donor PHEN and alumina, a model inorganic surface. We propose that the pi-donor solutes interact with pi-acceptor sites in soil organic matter (SOM), including aromatic rings with multiple carboxyl groups, aromatic amines, or heteroaromatic amines. The pi-acceptor ability of such aromatic moieties would increase with protonation. pi-pi Interactions between PMB, NAPH, and PHEN, and model SOM acceptors, 1,3,5-benzenetricarboxylic acid (BTA), 1,4,5,8-naphthalenetetracarboxylic acid (NTA), and pyridine (PY) in methanol and methanol-water, were verified by the appearance of pH-dependent upfield 1H NMR chemical shifts induced by ring current effects. UV/vis spectra showed pH-dependent charge-transfer bands for various donors with NTA. No NMR shifts or charge-transfer bands were found for nondonor compounds paired with the model acceptors.

Adsorption↗

PubMed2005

Fluorinated fullerenes: sources of donor-acceptor dyads with [18]trannulene acceptors for energy- and electron-transfer reactions.

Fine-tuned control over the donor strength in a series of trannulenes-based donor-acceptor ensembles is used to alter the deactivation path of the photoexcited-state chromophore and to modulate the rates of intramolecular electron transfer. For the first time, a detailed analysis of emission spectra, time-dependent spectroscopic measurements, and electrochemistry prove spectroscopically and kinetically that trannulenes can serve, in a manner similar to C(60) and C(60) monoadducts, as both electron and also as energy acceptor in donor-acceptor ensembles, producing widely different electron-transfer regimes. This investigation also shows that the integration of trannulenes, as a versatile electron-acceptor building block, consistently produces charge recombination in the inverted Marcus region.

Journal Article↗

PubMed2006

Quantum-chemical investigation of second-order nonlinear optical chromophores: comparison of strong nitrile-based acceptor end groups and role of auxiliary donors and acceptors.

We report a detailed quantum-chemical investigation of donor-acceptor substituted dipolar nonlinear optical chromophores incorporating the 4-(dimethylamino)phenyl donor end group and a variety of strong heterocyclic acceptor end groups, including tricyanofurans and tricyanopyrroles. In particular, we study the variation of the molecular second-order polarizability (beta) with the acceptor end group and when inserting auxiliary donors (thiophene) and acceptors (thiazole) into the pi bridge. Both finite-field calculations (in the context of local contributions) and sum-over-states calculations were carried out in order to probe the relationship between beta and the chemical structure of the various chromophores. The trends obtained with these two methods are fully consistent. The large beta values (up to 700 x 10(-30) esu) as well as the observed tunability of the optical absorption maximum (lambda(max)) make the chromophores investigated here interesting candidates for use in electro-optic applications at telecommunications wavelengths.

Journal Article↗

PubMed1972

Structure and function of Escherichia coli formylmethionine transfer RNA: loss of methionine acceptor activity by modification of a specific guanosine residue in the acceptor stem of formylmethionine transfer RNA from Escherichia coli.

The structural requirements of E. coli formylmethionine tRNA for aminoacylation have been examined by chemical modification of the tRNA, followed by separation of the modified molecules into active and inactive components. Photooxidation of tRNA(fMet) at 50 degrees in the presence of methylene blue results in modification of two guanosine (G) residues in the acceptor stem, at positions no. 2 and no. 71 from the 5'-phosphate terminus. Both of these modifications are present in inactive molecules, but only the G residue at position no. 2 is modified in the acceptor stem of active molecules. Loss of methionine acceptance occurs with first-order kinetics, indicating that inactivation by modification of G residue no. 71 is independent of any other modifications taking place under these conditions. The presence of a modified G residue at position no. 2 in the acceptor stem of active photooxidized molecules shows that disruption of normal base-pairing in this region is not sufficient to inactivate tRNA(fMet). These data indicate that the inactivating modification at position no. 71 is lethal due to a specific alteration in the nucleotide base, rather than simply as a result of breaking a hydrogen-bonded base pair in the acceptor stem.

Acylation↗

PubMed2003

Trivalent boron as an acceptor in donor-pi-acceptor-type compounds for single- and two-photon excited fluorescence.

The synthesis, structure, and fluorescence properties of a series of new donor-pi-acceptor (D-pi-A) type compounds, with a trivalent boron, protected by two mesityl groups, as acceptor, and with various typical donors and different pi-conjugated bridges, are reported. All these stable organoboron compounds show intense single-photon excited fluorescence (SPEF) and two-photon excited fluorescence (TPEF) in a wide spectral range from blue to green, with the spectral peak position of the SPEF being basically the same as that of the TPEF. The remarkably strong Cbond;B(mesityl)(2) bonding, and the well-conjugated pi-system, shown in X-ray crystal structures of two compounds, indicate some charge transfer features of the ground state. Meanwhile, spectral data indicate that the charge transfer from donor to acceptor is greatly enhanced in the excited states. Based on typical structural data and comprehensive spectral data, the following structure-property relationships can be drawn: 1) the moderate arylamino donor can more effectively enhance the SPEF and TPEF intensities than can the strong alkylamino donor; 2) stilbene is a better pi-bridge than styrylthiophene for its capability of enhancing and blue-shifting the SPEF and TPEF of the corresponding D-pi-A compounds; and 3) when compared to its boron-free precursors and other analogues, -B(mesityl)(2) invariably and consistently acts as an effective SPEF and TPEF fluorophore in all this series of organoboron compounds, which may result from its strong pi-electron-withdrawing and charge transfer-inducing nature in the ground-state and, more dominantly, in the excited-state. Combining all the above positive structure factors, trans-4'-N,N-diphenylamino-4-dimesitylborylstilbene (compound 3) stands out as the optimized green SPEF and TPEF emitter. This compound exhibits an SPEF quantum yield Phi of 0.91 at 522 nm in THF, a TPEF cross-section sigma' that is an order of magnitude larger than that of its boron-free precursor upon excitation by 800 nm femto-second laser pulses, and a two-photon absorption section sigma of 3.0 x 10(-48) cm(4) s. In the blue light region, trans-4'-N-carbazolyl-4-dimesitylboryl-stilbene (compound 4) shows significant SPEF and TPEF properties, with Phi=0.79 at 464 nm in THF and a large sigma' value, which is five times that of fluorescein upon excitation by 740 nm femto-second laser pulses.

Journal Article↗