Fluorescence characterization of the interaction of Al3+ and Pd2+ with Suwannee River fulvic acid in the absence and presence of the herbicide 2,4-dichlorophenoxyacetic acid

In an effort to understand the role of environmental metal ions in the interaction of charged pesticides with humic substances, a fluorescence study of the interaction of the widely-used herbicide 2,4-dichlorophenoxyacetic acid (DCPAA) with Al(3+) and Pd(2+) and Suwannee River fulvic acid (SRFA) was undertaken. Initial fluorescence experiments on binary solutions clearly indicated that both Al(3+) and Pd(2+) strongly interact with both SRFA and DCPAA when alone in solution with the metal ion. Titrations of SRFA with Al(3+) at pH values of 4.0, 3.0 and 2.0 revealed decreased degrees of fluorescence emission enhancement (at lambda(emission, max)=424 nm) with decreasing pH, consistent with the expected loss of rigidity in the SRFA-Al(3+) complexes formed as pH is lowered. In contrast, titrations of SRFA with Pd(2+) at all of these pH values resulted in significant fluorescence quenching. Al(3+) additions to solutions of DCPAA at pH values above the pK(a) (2.64) of DCPAA resulted primarily in significant changes in the wavelength of maximum emission (without significant quenching or enhancement of emission intensity), while Pd(2+) additions to DCPAA solutions resulted primarily in very significant fluorescence quenching. The DCPAA fluorescence results strongly support the formation of an Al(3+)-DCPAA complex at pH values above the pK(a) of DCPAA. The fluorescence results obtained for solutions of Pd(2+) and DCPAA are best explained by a collisional quenching mechanism, that is, energy transfer from excited DCPAA molecules to Pd(2+) following the collision of these two species in solution. Excitation-emission matrix plots obtained on ternary solutions (at environmentally-relevant pH 4.0) containing SRFA, DCPAA and metal ions (i.e., either Al(3+) or Pd(2+)) provides evidence (especially for systems containing Al(3+)) for the existence of ternary complexes between fulvic acid species, the herbicide DCPAA and metal ion, suggesting (at least at pH 4.0, where the predominant DCPAA species is negatively-charged) that metal ions may function to "bridge" negatively-charged fulvic acids to negatively-charged pesticides.     

Formation of ternary complexes involving zinc or copper ions, polynucleotides, and polypeptides containing glutamic acid and tyrosine residues

Metal ions such as Zn²⁺ and Cu²⁺ can mediate interactions between copolypeptides (Glu_x, Tyr_y)_n and polynucleotides. CD data show that these ternary complexes are characterized by an unstacking of nucleic acid bases, while the polypeptide adopts an α-helical conformation as observed in the two binary complexes polynucleotide–cation and polypeptide–cation. Fluorescence studies demonstrate that tyrosyl side chains interact with nucleic acid bases in the ternary complexes, leading to a quenching of tyrosine fluorescence.     

Fluorescence study of the interaction of Suwannee River fulvic acid with metal ions and Al3+-metal ion competition

In this study emission and synchronous-scan fluorescence spectroscopy have been used to investigate the interaction of the class A (oxygen seeking 'hard acid') metal Al(3+), with Suwannee River fulvic acid (SRFA), as well as competition between Al(3+) and several other metal ions (Ca(2+), Mg(2+), Cu(2+), Pd(2+), La(3+), Tb(3+) and Fe(3+)) for binding sites on SRFA. Of the four metal ions possessing very similar (and relatively low) ionic indices (Ca(2+), Mg(2+), Cu(2+) and Pd(2+)) only the latter two paramagnetic ions significantly quenched SRFA fluorescence emission intensity. Of the four metal ions possessing very similar (and relatively low) covalent indices (Ca(2+), Mg(2+), La(3+) and Tb(3+)) only the last paramagnetic ion (Tb(3+)) significantly quenched SRFA fluorescence. None of these metals was able to significantly compete with SRFA-bound Al(3+).Fe(3+), which differs substantially from all of the other metals examined in this study in that it possesses a relatively high ionic index (but not as high as Al(3+)) and a relatively low covalent index (but not as low as Al(3+)), was able not only to quench SRFA fluorescence but also to compete (at least to some extent) with SRFA-bound Al(3+). Synchronous-scan fluorescence SRFA spectra taken in the absence and presence of Fe(3+) and/or Al(3+) support the view that these two metal ions can compete for sites on SRFA. The results of these fluorescence experiments further confirm the Al(3+), and metal ions that have electronic properties somewhat similar to Al(3+) (such as Fe(3+)) are somewhat unique in their ability to interact strongly with binding sites on fulvic acids.     

Fluorescence quenching and bonding properties of some hydroxamic acid derivatives by iron(III) and manganese(II)

Spectrophotometric investigations of highly fluorescent metal chelating molecules are of relevance due to their potential application in novel, selective fluorescence‐based sensors. Benzene and naphthalene chromophores are highly fluorescent while hydroxamic acids are widely used as ligands for complexation of transition metals. In order to develop fluorescence probes, several phenyl derivatives of N‐phenylbenzohydroxamic acid and an aminodihydroxamic acid linked with a naphthalene chromophore were synthesized and their selective ionophoric properties towards iron(III) and manganese(II) ions were investigated using fluorescence and absorption spectroscopy. Both methods confirm the formation of 1:1 and 1:2 complexes for iron(III) and a 1:1 complex for manganese(II). The complex that is formed depends on the concentration of the ligand and pH of the medium. The amino dihydroxamic acid exhibits a prominent selectivity towards iron(III) with a two‐step 1:1 and 1:2 quenching mechanism at pH 3 and towards manganese(II) with a 1:1 quenching mechanism at a probe concentration of 1 × 10^?5 mol dm^?3 at pH 9.5 The logarithm of overall formation constants of 1:1 and 1:2 complexes of iron(III) were estimated as 3.30 and 9.05, respectively. Copyright © 2008 John Wiley & Sons, Ltd.     

Study of fluorescence quenching of mercaptosuccinic acid‐capped CdS quantum dots in the presence of some heavy metal ions and its application to Hg(II) ion determination

In this study, CdS quantum dots (QDs) capped with mercaptosuccinic acid (MSA) were prepared in one step. The size, shape, component and spectral properties of MSA‐capped CdS QDs were characterized by transmission electron microscopy (TEM), photoluminescence (PL) and infrared (IR) spectrometry. The results showed that the prepared QDs with an average diameter of 6 nm have favorable fluorescence, which is greatly influenced by the pH of the environment. The interaction of some heavy metal ions including Ag⁺, Hg²⁺, Cu²⁺, Ni²⁺ and Co²⁺ with MSA‐capped CdS QDs was investigated in different buffering pH media. Based on the fluorescence quenching of the QDs in the presence of each of the metal ions, the feasibility of their determinations was examined according to the Stern–Volmer equation. The investigations showed that Hg(II) ions can be determined in the presence of many co‐existing metal ions at a buffering pH of 5. This method was satisfactorily applied to the measurement of Hg(II) ions in some environmental samples. Copyright © 2014 John Wiley & Sons, Ltd.     

Fluorescence quenching due to mercuric ion interaction with aromatic amino acids and proteins

Mercuric ion interacts with indoles, including tryptophan, to produce complexes whose absorption spectra are broader, less structured, and red-shifted as compared with those of the parent compound. Fluorescence and phosphorescence are totally quenched. In a survey of the effect of transition metal ions on tryptophan fluorescence, the strong quenching by Hg²⁺ was unique among the uncolored ions. Mercuric nitrate quenched the fluorescence of practically every protein tested, but the sensitivity to quenching varied with the protein. Ovalbumin was the most sensitive to quenching by Hg²⁺, over 70% of the intrinsic fluorescence being quenched by 2 moles of mercuric ion. Difference absorption spectra show that sulfhydryl groups are attacked by these reagents and Hg²⁺ is, in addition, perturbing the environment near some tryptophans. In contrast to Hg²⁺, Zn²⁺ had negligible effect on protein fluorescence. The emission spectra of proteins which were partly quenched by mercuric ion showed shifts in their maxima to higher or lower wavelengths. This suggests that mercuric ion quenched certain tryptophans more than others, and supports the idea that protein fluorescence is heterogeneous and arises from tryptophans in different microenvironments.     

Synthesis,crystal structure and fluorescence properties of terbium complexes with phenoxyacetic acid and 2,4,6‐tris‐(2‐pyridyl)‐s–triazine

Two complexes of Tb³⁺, Gd³⁺/Tb³⁺ and one heteronuclear crystal Gd³⁺/Tb³⁺ with phenoxyacetic acid (HPOA) and 2,4,6‐tris‐(2‐pyridyl)‐s–triazine (TPTZ) have been synthesized. Elemental analysis, rare earth coordination titration, inductively coupled plasma atomic emission spectrometry (ICP‐AES) and thermogravimetric analysis‐differential scanning calorimetry (TG‐DSC) analysis show that the two complexes are Tb₂(POA)₆(TPTZ)₂·6H₂O and TbGd(POA)₆(TPTZ)₂·6H₂O, respectively. The crystal structure of TbGd(POA)₆(TPTZ)₂·2CH₃OH was determined using single‐crystal X‐ray diffraction. The monocrystal belongs to the triclinic system with the P‐1 space group. In particular, each metal ion is coordinately bonded to three nitrogen atoms of one TPTZ and seven oxygen atoms of three phenoxyacetic ions. Furthermore, there exist two coordinate forms between C₆H₅OCH₂COO^– and the metal ions in the crystal. One is a chelating bidentate, the other is chelating and bridge coordinating. Fluorescence determination shows that the two complexes possess strong fluorescence emissions. Furthermore, the fluorescence intensity of the Gd³⁺/Tb³⁺ complex is much stronger than that of the undoped complex, which may result from a decrease in the concentration quench of Tb³⁺ ions, and intramolecular energy transfer from the ligands coordinated with Gd³⁺ ions to Tb³⁺ ions. Copyright © 2015 John Wiley & Sons, Ltd.     

Fluorescence and FT-IR spectroscopic studies of Suwannee River fulvic acid complexation with aluminum, terbium and calcium.

In this study fluorescence emission and IR spectroscopy have been used to investigate the interaction of the class A (oxygen seeking "hard acid") metal Al(3+), with Suwannee River fulvic acid. Addition of Al(3+) ion results in a significant enhancement in fulvic acid fluorescence emission (at lambda(em)=424 nm) and significant red shift of the excitation wavelength (from lambda(ex)=324 nm to lambda(ex)=344 nm) at low pH values (pH approximately 4.0-5.0). At pH 4.0 (0.1 M ionic strength), where the predominant aluminum ion species is the "free" (aquo) ion, the fulvic acid fluorescence reaches 142% of the value in the absence of added metal ion. Analysis of the pH 4.0 and pH 5.0 fluorescence enhancement data with the nonlinear (single site) model of Ryan and Weber indicated binding constants in the range of 4.67.10(4)-2.87.10(6) M(-1) and concentrations of ligand sites in the range of 18.6.10(-6)-24.0.10(-6) M, both consistent with previous studies performed on both aquatic and soil fulvic acids. Companion fluorescence experiments performed on two other class A metal ions, Ca(2+) and Tb(3+), indicated no significant enhancement or quenching with Ca(2+) and only slight quenching with Tb(3+). Comparison of FT-IR spectra collected on fulvic acid alone and fulvic acid in the presence of the three class A metals (Al(3+), Ca(2+) and Tb(3+)) provides strong evidence for the involvement of carboxyl carbonyl functions in the binding of all three metal ions, which is not unexpected. The spectra also reveal, however, a very pronounced difference in the 4000-2000 cm(-1) IR spectral region between the Al(3+) spectrum and the Ca(2+) and Tb(3+) spectra. The -OH stretch spectral region in the Al(3+) spectrum has a major component shifted to higher energy (compared to fulvic acid alone or to fulvic acid in the presence of Ca(2+) or Tb(3+)). Even more striking is the emergence of a pronounced IR band at 2407 cm(-1), which is present only in the Al(3+) spectrum. The results of fluorescence and IR experiments with the model compounds salicylic acid and phthalic acid further confirm that both salicylic acid-like sites and phthalic acid-like sites are likely complexation sites for Al(3+) in fulvic acid and are major contributors to the observed spectroscopic changes associated with Al(3+) ion complexation. From a comparison of both the fluorescence and IR spectral results for all three class A metals, differing most strongly in the value of their ionic index, it seems clear that major sources of the deviation in spectral properties between Al(3+) and Ca(2+)/Tb(3+) is the unusually high value of its charge density and relatively low propensity for involvement in covalent bonding interactions (very high ionic index and relatively low covalent index in the Nieboer and Richardson classification of environmental metals), as well as affinity for certain functional groups.     

Spectroscopic investigations of Er(3+) :CdO-Bi(2) O(3) -B(2) O(3) glasses

This article reports on the optical properties of Er³⁺ ions doped CdO–Bi₂O₃–B₂O₃ (CdBiB) glasses. The materials were characterized by optical absorption and emission spectra. By using Judd–Ofelt theory, the intensity parameters Ω_λ (λ = 2, 4, 6) and also oscillatory strengths were calculated from the absorption spectra. The results were used to compute the radiative properties of Er³⁺:CdBiB glasses. The concentration quenching and energy transfer from Yb³⁺–Er³⁺ were explained. The stimulated emission cross‐section, full width at half maximum (FWHM) and FWHM × values are also calculated for all the Er³⁺:CdBiB glasses. Copyright © 2011 John Wiley & Sons, Ltd.     

Facile synthesis of anthracene-appended amino acids as highly selective and sensitive fluorescent Fe3+ ion sensors

We designed new fluorescent chemical sensors for Fe³⁺ ion detection, by conjugating amino acids as receptors into an anthracene fluorophore. The conjugates were synthesized in solid phase by Fmoc-chemistry. Fluorescence sensors containing Asp (1) and Glu (2) both had exclusive selectivity for Fe³⁺ in 100% aqueous solution and in a mixed organic–aqueous solvent system. Other metal ions did not interfere with the detection ability of the sensors for Fe³⁺. The sensors detect Fe³⁺ ions via a chelation-enhanced fluorescent quenching effect. The binding affinity, reversible monitoring, and pH sensitivity of the sensors were investigated. In addition, detection of fluoride ion among halide ions was done by a chemosensing ensemble method with 1–Fe³⁺ and 2–Fe³⁺ complexes.     

Interaction of divalent metal ions with the carboxyl-terminal domain of human voltage-gated proton channel Hv1

The voltage-gated proton channel Hv1 functions as a dimer, in which the intracellular C-terminal domain of the protein is responsible for the dimeric architecture and regulates proton permeability. Although it is well known that divalent metal ions have effect on the proton channel activity, the interaction of divalent metal ions with the channel in detail is not well elucidated. Herein, we investigated the interaction of divalent metal ions with the C-terminal domain of human Hv1 by CD spectra and fluorescence spectroscopy. The divalent metal ions binding induced an obvious conformational change at pH 7 and a pH-sensitive reduction of thermostability in the C-terminal domain. The interactions were further estimated by fluorescence spectroscopy experiments. There are at least two binding sites for divalent metal ions binding to the C-terminal domain of Hv1, either of which is close to His²⁴⁴ or His²⁶⁶ residue. The binding of Zn²⁺ to the two sites both enhanced the fluorescence of the protein at pH 7, whereas the binding of other divalent metal ions to the two sites all resulted fluorescence quenching. The orders of the strength of divalent metal ions binding to the two sites from strong to weak are both Co²⁺, Ca²⁺, Ni²⁺, Mg²⁺, and Mn²⁺. The strength of Ca²⁺, Co²⁺, Mg²⁺, Mn²⁺ and Ni²⁺ binding to the site close to His²⁴⁴ is stronger than that of these divalent metal ions binding to the site close to His²⁶⁶.     

A new principle for activity measurement of ADP or ATP dependent enzymes: fluorescence quenching of epsilon-ADP and epsilon-ATP by divalent metal ions.

The divalent metal ions Cu²⁺, Co²⁺, Mn²⁺, and Zn²⁺ form complexes with the fluorescent etheno analogs of the adenine nucleotides. The fluorescence intensity is thereby diminished. The binding strength of the metals to etheno-adenosine triphosphate is higher than to etheno-adenosine di- and monophosphate. The quenching effect of the divalent metal ions can be exploited as a simple routine activity measurement for various kinases and phosphohydrolases.     

Fluorescent studies on the interaction of DNA and ternary lanthanide complexes with cinnamic acid‐phenanthroline and antibacterial activities testing

Twelve lanthanide complexes with cinnamate (cin^–) and 1,10‐phenanthroline (phen) were synthesized and characterized. Their compositions were assumed to be RE(cin)₃phen (RE³⁺ = La³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Tm³⁺, Yb³⁺, Lu³⁺). The interaction mode between the complexes and DNA was investigated by fluorescence quenching experiment. The results indicated the complexes could bind to DNA and the main binding mode is intercalative binding. The fluorescence quenching constants of the complexes increased from La(cin)₃phen to Lu(cin)₃phen. Additionally, the antibacterial activity testing showed that the complexes exhibited excellent antibacterial ability against Escherichia coli, and the changes of antibacterial ability are in agreement with that of the fluorescence quenching constants. Copyright © 2014 John Wiley & Sons, Ltd.     

Spectroscopic Analysis of the Cu<Superscript>2+</Superscript>-Induced Fluorescence Quenching of Fluorescent Proteins AmCyan and mOrange2

Fluorescent proteins show fluorescence quenching by specific metal ions, which can be applied towards metal biosensing applications. In order to develop metal-biosensor, we performed spectroscopic analysis of the fluorescence quenching of fluorescent protein AmCyan and mOrange2 by various metal ions. The fluorescence intensity of AmCyan was reduced to 48.54% by Co²⁺ and 67.77% by Zn²⁺; Cu²⁺ reduced the fluorescence emission of AmCyan to 19.30% of its maximum. The fluorescence intensity of mOrange2 was quenched by only Cu²⁺, to 11.48% of its maximum. When analyzed by Langmuir equation, dissociation constants for AmCyan and mOrange2 were 56.10 and 21.46 µM, respectively. The Cu²⁺ quenching of AmCyan and mOrange2 were reversible upon treatment with the metal chelator EDTA, indicating that the metal ions were located on the protein surface. Their model structures suggest that AmCyan and mOrange2 have novel metal-binding sites.     

A new heavy lanthanide-dependent DNAzyme displaying strong metal cooperativity and unrescuable phosphorothioate effect

In vitro selection of RNA-cleaving DNAzymes was performed using three heavy lanthanide ions (Ln³⁺): Ho³⁺, Er³⁺ and Tm³⁺. The resulting sequences were aligned together and about half of the library contained a new family of DNAzyme. These DNAzymes have a simple loop structure, and they are active only with the seven heavy Ln³⁺. Among the tested non-lanthanide ions, only Y³⁺ induced cleavage and even Pb²⁺ failed to cleave, suggesting a very high specificity. A representative DNAzyme, Tm7, has a sigmoidal metal binding curve with a Hill coefficient of 3, indicating that three metal ions are involved in the catalytic step. Its pH-rate profile has a slope of 1, suggesting a single deprotonation step is involved in the rate-limiting step. Tm7 has a cleavage rate of 1.6 min⁻¹ at pH 7.8 with 10 μM Er³⁺. Phosphorothioate substitution at the cleavage junction completely inhibits the activity, which cannot be rescued by Cd²⁺ alone, or by a mixture of Er³⁺ and Cd²⁺, suggesting that two interacting metal ions are involved in direct bonding to both non-bridging oxygen atoms. A new model involving three lanthanide ions is proposed based on this study. A biosensor is engineered using Tm7 to detect Dy³⁺ down to 14 nM.     

SDS-capped 1-pyrenecarboxaldehyde nanoprobe for selective detection of Cu2+ ion from water samples: Spectroscopic approach

Sodium dodecyl sulfate (SDS)-capped 1-pyrenecarboxaldehyde nanoparticles (PyalNPs) were prepared using a reprecipitation method in an aqueous medium and exhibited red-shifted aggregation-induced enhanced emission (AIEE). The dynamic light scattering (DLS) examination showed narrower particle size distribution with an average particle size of 41 nm, whereas −34.5 mV zeta potential value indicate the negative surface charge and good stability of nanoparticles (NPs) in an aqueous medium. The AIEE was seen at λ_max = 473 nm in a fluorescence spectrum of a PyalNP suspension. In the presence of Cu²⁺ ions, the fluorescence of PyalNPs quenches very significantly, even in the presence of other metal ions like Ba²⁺, Ca²⁺, Cd²⁺, Co²⁺, Al³⁺, Fe²⁺, Hg²⁺, Ni²⁺ and Mg²⁺. The changes in the fluorescence lifetime of PyalNPs in the presence of Cu²⁺ ions suggested that the type of quenching was dynamic. The fluorescence quenching data for the NPs suspension fitted well into a typical Stern–Volmer relationship in the concentration range 1.0–25 μg/ml of Cu²⁺ ions. The estimated value of the correlation coefficient R² = 0.9877 was close to 1 and showed the linear relationship between quenching data and Cu²⁺ ion concentration. The limit of detection (LOD) was found to be 0.94 ng/ml and is far below the tolerable intake limit value of 1.3 μg/ml accepted by the World Health Organization for Cu²⁺ ions in drinking water. The fluorescence quenching approach for a SDS-capped Pyal nanosuspension for copper ion quantification is of high specificity and coexisting ions were found to interfere very negligibly. The developed method was successfully applied for the estimation of copper ions in river water samples.     

Effect of Bi3+ addition on the upconversion luminescence properties of NaYbF4:Er

In this study, Bi³⁺ incorporation in NaYbF₄:Er lattice and its influence on upconversion luminescence properties have been investigated in detail using techniques such as temperature‐dependent luminescence, Fourier transform infrared spectroscopy and X‐ray diffraction (XRD). The study was carried out to develop phosphors with improved upconversion luminescence. From photoluminescence and lifetime measurements it is inferred that luminescence intensity from NaYbF₄:Er increases with Bi³⁺ addition. The sample containing 50 at.% Bi³⁺ ions exhibited optimum upconversion luminescence. Increased distance between Yb³⁺–Yb³⁺ and Er³⁺–Er³⁺ due to Bi³⁺ incorporation into the lattice and associated decrease in the extent of dipolar interaction/self‐quenching are responsible for increase in lifetime values and luminescence intensities from Er³⁺ ions. Incorporation of Bi³⁺ into NaYbF₄:Er lattice reduced self‐quenching among Yb³⁺–Yb³⁺ions and this facilitated energy transfer from Yb³⁺ to Er³⁺. This situation also explains decrease in the extent of temperature‐assisted quenching of emission from thermally coupled ²H_11/2 and ⁴S_3/2 levels of Er³⁺. Based on Rietveld refinement of XRD patterns it was confirmed that a maximum of 10 at.% of Bi³⁺added was incorporated into the NaYbF₄:Er lattice and the remaining complex co‐exists as a BiOF phase. These results are of significant interest in the area of development of phosphors based on Yb³⁺–Er³⁺ upconversion luminescence.     

Increase in the stoichiometry of the functioning active sites of horse liver aldehyde dehydrogenase in the presence of magnesium ions

The V of horse liver aldehyde dehydrogenase is enhanced twofold in the presence of 0.5 mm Mg²⁺ ions when assayed in the dehydrogenase reaction. The mechanism of this activation appears to be related to the fact the enzyme changes from functioning with half-of-the-sites reactivity to functioning with all-of-the-sites reactivity. That is, the presteady-state burst magnitude increases from 2 mol NADH formed per mole of tetrameric enzyme to 4 mol formed per mole (K. Takahashi and H. Weiner, J. Biol. Chem., 1980, 255, 8206–8209). Whether this twofold enhancement correlates, in fact, to a change from half-of-the-sites to all-of-the-sites reactivity of the enzyme by Mg²⁺ ions was investigated by determining the Stoichiometry of coenzyme binding by fluorescence quenching and enhancement methods in the absence and presence of the metal ions. The biphasic Scatchard plots for NAD binding to the enzyme were similar in the absence and presence of Mg²⁺ ions, while that of NADH binding was monophasic (-Mg²⁺) and biphasic (+Mg²⁺). In the presence of p-methoxyacetophenone, a competitive inhibitor for substrate, the stoichiometric titration of coenzyme binding to the ternary complexes (enzyme-NAD(H)-inhibitor) revealed that only 2 mol of NAD or NADH bind in the absence of Mg²⁺ ions but 4 bind per mole of tetrameric enzyme in the presence of added metal. The fluorescence intensity of NAD's fluorescent derivative, 1,N⁶-ethenoadenine dinucleotide, bound to the enzyme was also doubled by the addition of Mg²⁺ ions.The combined binding data show that the stoichiometry of coenzyme binding to aldehyde dehydrogenase in the ternary complex increases from 2 to 4 mol binding per mole of tetrameric enzyme with the addition of Mg²⁺ ions. This increase in stoichiometry corresponds to the observed changes of burst magnitude obtained from the presteady-state and V in the steady-state kinetics assays. From both results of the kinetics and stoichiometry, we show that horse liver aldehyde dehydrogenase exhibits half-of-the-sites reactivity when in the tetrameric state in the absence of Mg²⁺ ions, and all-of-the-sites reactivity in the dimeric state in the presence of the metal.     

An inducible hydrolase from Mortierella isabellina (basidiomycetes). The deacylation of (+)-usnic acid

An inducible enzyme catalysing the hydrolysis of (+)-usnic acid to (+)-2-desacetylusnic acid and acetic acid has been purified 150-fold from the mycelium of Mortierella isabellina grown in the presence of (+)-usnic acid. Purification was achieved by treatment with protamine sulfate, (NH₄)₂SO₄ fractionation, negative adsorption on alumina Cγ gel and hydroxylapatite followed by chromatography on DEAE-cellulose and Sephadex G-200. The elution pattern from a Sephadex G-200 column indicated a MW of ca 7.6 × 10⁴ for the enzyme. The apparent K_m value for (+)-usnic acid at the pH optimum (pH 7) was 4.0 × 10^?5 M. The enzyme was specific for (+)-usnic acid and inactive towards (?)-usnic acid, (+)-isousnic acid or certain phloracetophenone derivatives. Its activity was enhanced in the presence of divalent metal ions such as Co²⁺, Ni²⁺, Mn²⁺, Mg²⁺ and Zn²⁺.     

A highly selective fluorescence and absorption sensor for rapid recognition and detection of Cu2+ ions in aqueous solution and film

A fluorescence and absorption chemosensor (SAAT) based on 5-(hydroxymethyl)-salicylaldehyde (SA) and o-aminothiophenol (AT) was designed and synthesized. SAAT in DMSO–HEPES (20.0 mM, v/v, 1:99, pH = 7.0) solution shows a highly selective and sensitive absorption and an ‘on–off’ fluorescence response to Cu²⁺ ions in aqueous solutions over all other competitive metal ions including Na⁺, Ag⁺, Ba²⁺, Ca²⁺, Cd²⁺, Mg²⁺, Zn²⁺, Cr³⁺, Al³⁺, Hg²⁺, K⁺, Mn²⁺, Ni²⁺, Sr²⁺, Tb³⁺ and Co²⁺. SAAT exhibits ratiometric absorption sensing ability for Cu²⁺ ions. Importantly, SAAT also can sense Cu²⁺ ions using fluorescence quenching, the fluorescence intensity of SAAT showed a good linear relationship with Cu²⁺ concentration, and the detection limit of Cu²⁺ was 0.34 μM. The results of Job's plot, Benesi–Hildebrand plot, mass spectra, and density functional theory calculations confirmed that the selective absorption and fluorescence response were attributed to the formation of a 1:1 complex between SAAT and Cu²⁺. SAAT in test film could identify Cu²⁺ in water samples using the intuitive fluorescence colour change under a UV lamp. SAAT has great application value as a selective and sensitive chemosensor to discriminate and detect Cu²⁺ ions.