Different binding thermodynamics of Ni2+, Cu2+, and Zn2+ to bacitracin A1 determined by capillary electrophoresis

Thermodynamics of the binding of Ni(2+), Cu(2+) and Zn(2+) to bacitracin A(1) was studied by capillary electrophoresis measuring the peptide effective mobility at different pH in the presence of increasing concentration of the three ligands. The affinity follows the order Ni(2+) > Cu(2+) > Zn(2+), with association constant values of (2.3 +/- 0.1)x10(4), (4.9 +/- 0.2)x10(3), and (1.5 +/- 0.1)x10(3) M(-1), respectively. The only model able to rationalize mobility data implies that metal ion binds to the P(0) peptide form. Moreover, mobility values indicated a change of bacitracin A(1) acidic properties on Ni(2+) and Cu(2+) binding, with a shift of the pK(a) of N-terminal Ile-1 from 7.6 to about 5 and of the pK(a) of the delta-amino group of D-Orn-7 from 9.7 to about 7. Even though on Zn(2+) binding a shift of the N-terminal Ile-1 pK(a) was observed, restrictions in the pH range suitable for investigation, due to precipitation phenomena, did not allow establish if the shift of D-Orn-7 lateral chain pK(a) also occurred. Nonetheless, if present, the shift should be limited to the 7.8-9.7 range. Mobility data indicated that the Stokes radius of the complexes is ca. 3 A lower than that of the free peptide. The present results indicate that metal-ion binding to bacitracin A(1) is more complex than previously assumed.     

Capillary electrophoretic study of the binding of zinc(II) ion to bacitracin A1 in water-2,2,2-trifluoroethanol

Binding of Zn(2+) to bacitracin A(1) was studied by capillary electrophoresis in water/2,2,2-trifluoroethanol (70/30 v/v) at different apparent pH values in order to estimate the association constant of metal, the acidic dissociation constants and the Stokes radii of both free and bounded peptide in apolar environment. The Stokes radii of the free peptide species were compared with those in aqueous solution, as obtained in a recent study performed by our group, indicating that apolar environment stabilizes bacitracin A(1) in a conformational structure with the lateral chain of apolar amino acids exposed on the external surface. This conformation of the macrocyclic dodecapeptide is ready to interact with Zn(2+) ion, as pointed out by the strong increase of the association constant measured in water/2,2,2-trifluoroethanol with respect to the value obtained in aqueous solution. In addition, whereas Zn(2+) ion binding in aqueous solution provides a sensible reduction of peptide Stokes radius, no sensible variations following to ion binding were observed in hydro-organic solution. The present results suggest that the apolar environment, rather than the metal ion binding, could be responsible for the conformational transition that brings bacitracin A(1) towards its biologically active structure.*     

Perturbation of the NH2 pKa value of adenine in platinum(II) complexes: distinct stereochemical internucleobase effects

The degree of acidification of the exocyclic N6 amino group of the model nucleobase 9-methyladenine (9MeA) in relation to the number and site(s) of Pt(II) binding has been studied in detail. It is found that twofold Pt(II) binding to N1 and N7 lowers the pK(a) value from 16.7 in the free base to 12-8. The lowest pK(a) values are observed when the resulting N6H(-) amide group is intramolecularly stabilized by an H-bond donor such as the N6H(2) group of a suitably positioned second 9MeA ligand. Deprotonation of the N6 amino group facilitates Pt migration from N1 to N6, and subsequent reprotonation of the N1 position yields a twofold N7,N6-metalated form of the rare imino tautomer of 9MeA, which has a pK(a) value of 5.03. These findings demonstrate a principle that is of potential relevance to the topic of "shifted pK(a)" values of adenine nucleobases, which is believed to be important with regard to acid-base catalysis of RNAs at physiological pH values. The principle states that a nucleobase pK(a) value can be sufficiently lowered to reach near-neutral values and that the pK(a) value of the protonated base does not necessarily have to be increased to accomplish this effect.     

Physical and kinetic analysis of the cooperative role of metal ions in catalysis of phosphodiester cleavage by a dinuclear Zn(II) complex

A dinuclear metal ion complex Zn(2)()(L2O) and its mononuclear analogue Zn(L1OH) were synthesized and studied as catalysts of the cleavage of the phosphate diester 2-hydroxypropyl-4-nitrophenyl phosphate (HPNP). X-ray crystal structure data, potentiometric titrations, and (1)H NMR spectra obtained over a wide range of pH values provide strong evidence that the alcohol linker in the complex Zn(2)()(L2O) is ionized below pH 6.0, while the alcohol group in the complex Zn(L1OH) remains protonated even at high pH. The ionizations observed at high pH correspond to the formation of the monohydroxo complexes, Zn(2)(L2O)(OH) and Zn(L1OH)(OH), with pK(a)'s of 8.0 and 9.2, respectively. The pH-rate profiles of second-order rate constants for metal-ion complex-catalyzed cleavage of HPNP are reported. These show downward curvature centered at the pK(a)'s for the respective zinc-bound waters, and limiting second-order rate constants at high pH of k(c) = 0.71 M(-)(1) s(-)(1) for Zn(2)()(L2O) and 0.061 M(-)(1) s(-)(1) for Zn(L1OH). The larger catalytic activity of Zn(2)()(L2O) compared with Zn(L1OH) is due to the cooperative role of the metal ions in facilitating the formation of the ionized zinc-bound water at close to neutral pH and in providing additional stabilization of the rate-limiting transition state for phosphodiester cleavage. Zn(2)()(L2O) complex (1 M) at pH 7.6 stabilizes the transition state for the uncatalyzed reaction by 9.3 kcal/mol. Assuming that the dissociation constant determined for a diethyl phosphate inhibitor is similar to that for substrate, then ca. 2.4 kcal/mol of these stabilizing interactions are expressed in the ground-state Michaelis complex, while the bulk of these interactions are only expressed as the reaction approaches the transition state for phosphodiester cleavage.     

The inorganic perspective of nerve growth factor: interactions of Cu2+ and Zn2+ with the N-terminus fragment of nerve growth factor encompassing the recognition domain of the TrkA receptor

There is a significant overlap between brain areas with Zn(2+) and Cu(2+) pathological dys-homeostasis and those in which the nerve growth factor (NGF) performs its biological role. The protein NGF is necessary for the development and maintenance of the sympathetic and sensory nervous systems. Its flexible N-terminal region has been shown to be a critical domain for TrkA receptor binding and activation. Computational analyses show that Zn(2+) and Cu(2+) form pentacoordinate complexes involving both the His4 and His8 residues of the N-terminal domain of one monomeric unit and the His84 and Asp105 residues of the other monomeric unit of the NGF active dimer. To date, neither experimental data on the coordination features have been reported, nor has one of the hypotheses according to which Zn(2+) and Cu(2+) may have different binding environments or the Ser1 α-amino group could be involved in coordination been supported. The peptide fragment, encompassing the 1-14 sequence of the human NGF amino-terminal domain (NGF(1-14)), blocked at the C terminus, was synthesised and its Cu(2+) and Zn(2+) complexes characterized by means of potentiometric and spectroscopic (UV/Vis, CD, NMR, and EPR) techniques. The N-terminus-acetylated form of NGF(1-14) was also investigated to evaluate the involvement of the Ser1 α-amino group in metal-ion coordination. Our results demonstrate that the amino group is the first anchoring site for Cu(2+) and is involved in Zn(2+) coordination at physiological pH. Finally, a synergic proliferative activity of both NGF(1-14) and the whole protein on SHSY5Y neuroblastoma cell line was found after treatment in the presence of Cu(2+). This effect was not observed after treatment with the N-acetylated peptide fragment, demonstrating a functional involvement of the N-terminal amino group in metal binding and peptide activity.     

Helical hairpin structure of influenza hemagglutinin fusion peptide stabilized by charge-dipole interactions between the N-terminal amino group and the second helix

The fusion domain of the influenza coat protein hemagglutinin HA2, bound to dodecyl phosphocholine micelles, was recently shown to adopt a structure consisting of two antiparallel α-helices, packed in an exceptionally tight hairpin configuration. Four interhelical H(α) to C═O aliphatic H-bonds were identified as factors stabilizing this fold. Here, we report evidence for an additional stabilizing force: a strong charge-dipole interaction between the N-terminal Gly(1) amino group and the dipole moment of helix 2. pH titration of the amino-terminal (15)N resonance, using a methylene-TROSY-based 3D NMR experiment, and observation of Gly(1 13)C' show a strongly elevated pK = 8.8, considerably higher than expected for an N-terminal amino group in a lipophilic environment. Chemical shifts of three C-terminal carbonyl carbons of helix 2 titrate with the protonation state of Gly(1)-N, indicative of a close proximity between the N-terminal amino group and the axis of helix 2, providing an optimal charge-dipole stabilization of the antiparallel hairpin fold. pK values of the side-chain carboxylate groups of Glu(11) and Asp(19) are higher by about 1 and 0.5 unit, respectively, than commonly seen for solvent-exposed side chains in water-soluble proteins, indicative of dielectric constants of ε = ～30 (Glu(11)) and ～60 (Asp(19)), placing these groups in the headgroup region of the phospholipid micelle.     

Determination of pK(a) values of diastereomers of phosphinic pseudopeptides by CZE

A CE method was used for the determination of acidity constants (pK(a)) of a series of ten phosphinic pseudopeptides, which varied in number and type of ionogenic groups. Effective electrophoretic mobilities were measured in the 1.8-12.0 pH range in the BGEs of constant ionic strength of 25 mM. Effective electrophoretic mobilities, corrected to standard temperature of 25 degrees C, were subjected to non-linear regression analysis and the obtained apparent pK(a) values were recalculated to thermodynamic pK(a)'s by extrapolation to zero ionic strength according to the extended Debye-Hückel model. The pK(a) values of the phosphinic acid group fell typically in the 1.5-2.25 interval, C-terminal carboxylic groups in the 2.94-3.50 interval, carboxylic groups of the lateral chain of glutamate and aspartate in the 4.68-4.97 interval, imidazolyl moiety of histidine in the 6.55-8.32 interval, N-terminal amino groups in the 7.65-8.28 interval and epsilon-amino group of the lateral chain of lysine in the 10.46-10.61 interval. Further, separation of diastereomers of the phosphinic pseudopeptides was investigated in achiral BGEs. Evaluation of the resolution of the diastereomers as a function of pH of the BGE revealed that most suitable pH region for separation of the diastereomers is around the pK(a) values of the central phosphinic acid group of the pseudopeptides. Successful separation of some diastereomers was, however, achieved in the neutral and alkaline BGEs as well.     

EPR and optical spectroscopic studies of Met80X mutants of yeast ferricytochrome c. Models for intermediates in the alkaline transition

The ferric forms of Met80X mutants of yeast iso-1-cytochrome c (X = Ala, Ser, Asp, and Glu) display EPR and optical spectra that are strongly pH dependent. At low pH values (pH approximately 5) the sixth coordination sites are filled by H(2)O that, on elevating the pH, is replaced by OH(-) in the cases of Met80Ala and -Ser (pK approximately 5.6 and 5.9, respectively) and by a lysine amino group in the cases of Met80Asp and -Glu (pK approximately 9.3 and 11.6, respectively). The ligand sets and the pK values of the transitions are rationalized in terms of the structure of the heme pocket, and a possible mechanism of the "trigger" in the alkaline transition of the native protein is suggested.     

Determination of acid-base dissociation constants of very weak zwitterionic heterocyclic bases by capillary zone electrophoresis

Thermodynamic acid-base dissociation (ionization) constants (pK(a)) of seven zwitterionic heterocyclic bases, first representatives of new heterocyclic family (2,3,5,7,8,9-hexahydro-1H-diimidazo[1,2-c:2',1'-f][1,3,2]diazaphosphinin-4-ium-5-olate 5-oxides), originally designed as chiral Lewis base catalysts for enantioselective reactions, were determined by capillary zone electrophoresis (CZE). The pK(a) values of the above very weak zwitterionic bases were determined from the dependence of their effective electrophoretic mobility on pH in strongly acidic background electrolytes (pH 0.85-2.80). Prior to pK(a) calculation by non-linear regression analysis, the CZE measured effective mobilities were corrected to reference temperature, 25°C, and constant ionic strength, 25 mM. Thermodynamic pK(a) values of the analyzed zwitterionic heterocyclic bases were found to be particularly low, in the range 0.04-0.32. Moreover, from the pH dependence of effective mobility of the bases, some other relevant characteristics, such as actual and absolute ionic mobilities and hydrodynamic radii of the acidic cationic forms of the bases were determined.     

In situ particle film ATR FTIR spectroscopy of carboxymethyl cellulose adsorption on talc: binding mechanism, pH effects, and adsorption kinetics

Carboxymethyl cellulose (CMC), in solution and adsorbed on the surface of talc, has been studied with ATR FTIR spectroscopy as a function of the solution pH. The solution spectra enable the calculation of the extent of ionization of the polymer (due to protonation and deprotonation of the carboxyl group) at various pH values, yielding a value of 3.50 for the pK(app)(1/2) (pH at which half of all carboxyl groups are ionized) in a simple electrolyte solution and a value of 3.37 for the pK(app)(1/2) in solutions containing magnesium ions (3.33 x 10(-4) M). The spectra of the adsorbed layer reveal that CMC interacts with the talc surface through a chemical complexation mechanism, via the carboxyl groups substituted on the polymer backbone. The binding mechanism is active at all pH values down to pH 2 and up to pH 11. The adsorbed layer spectra reveal that protonation and deprotonation of the polymer are affected by adsorption, with an increase in the pK(app)(1/2) to a value of 4.80. Spectra of the adsorbed polymer were also acquired as a function of the adsorption time. Adsorption kinetic data reveal that the polymer most likely has two different interactions with the talc surface, with a stronger interaction with the talc edge through chemical complexation and a weaker interaction with the talc basal plane presumably through the hydrophobic interaction.     

Tuning the activity of Zn(II) complexes in DNA cleavage: clues for design of new efficient metallo-hydrolases

The hydrolytic ability toward plasmid DNA of a mononuclear and a binuclear Zn(II) complex with two macrocyclic ligands, containing respectively a phenanthroline (L1) and a dipyridine moiety (L2), was analyzed at different pH values and compared with their activity in bis( p-nitrophenyl)phosphate (BNPP) cleavage. Only the most nucleophilic species [ZnL1(OH)]+ and [Zn2L2(OH)2]2+, present in solution at alkaline pH values, are active in BNPP cleavage, and the dinuclear L2 complex is remarkably more active than the mononuclear L1 one. Circular dichroism and unwinding experiments show that both complexes interact with DNA in a nonintercalative mode. Experiments with supercoiled plasmid DNA show that both complexes can cleave DNA at neutral pH, where the L1 and L2 complexes display a similar reactivity. Conversely, the pH-dependence of their cleavage ability is remarkably different. The reactivity of the mononuclear complex, in fact, decreases with pH while that of the dinuclear one is enhanced at alkaline pH values. The efficiency of the two complexes in DNA cleavage at different pH values was elucidated by means of a quantum mechanics/molecular mechanics (QM/MM) study on the adducts between DNA and the different complexed species present in solution.     

Pyrene-appended alpha-cyclodextrin as a fluorescent pH probe responding to a wide range.

Pyrene-appended alpha-cyclodextrin (3) in which a trimethylenediamine linker connected the pyrene residue to the alpha-cyclodextrin moiety showed pH-dependent fluorescence intensity changes. The fluorescence intensity was almost linearly changed within the pH range of 5 - 10. The unique fluorescence response of 3 to the pH was due not only to the favorable pK(a) values (pK(a1) = 6.4 and pK(a2) = 8.8), but also to the almost equal contributions of the amino groups to the pyrene's fluorescence quenching.     

Formation and stability of peptide enolates in aqueous solution

Second-order rate constants k(DO) (M(-1) s(-1)) were determined in D(2)O for deprotonation of the N-terminal alpha-amino carbon of glycylglycine and glycylglycylglycine zwitterions, the internal alpha-amino carbon of the glycylglycylglycine anion, and the acetyl methyl group and the alpha-amino carbon of the N-acetylglycine anion and N-acetylglycinamide by deuterioxide ion. The data were used to estimate values of k(HO) (M(-1) s(-1)) for proton transfer from these carbon acids to hydroxide ion in H(2)O. Values of the pK(a) for these carbon acids ranging from 23.9 to 30.8 were obtained by interpolation or extrapolation of good linear correlations between log k(HO) and carbon acid pK(a) established in earlier work for deprotonation of related neutral and cationic alpha-carbonyl carbon acids. The alpha-amino carbon at a N-protonated N-terminus of a peptide or protein is estimated to undergo deprotonation about 130-fold faster than the alpha-amino carbon at the corresponding internal amino acid residue. The value of k(HO) for deprotonation of the N-terminal alpha-amino carbon of the glycylglycylglycine zwitterion (pK(a) = 25.1) is similar to that for deprotonation of the more acidic ketone acetone (pK(a) = 19.3), as a result of a lower Marcus intrinsic barrier to deprotonation of cationic alpha-carbonyl carbon acids. The cationic NH(3)(+) group is generally more strongly electron-withdrawing than the neutral NHAc group, but the alpha-NH(3)(+) and the alpha-NHAc substituents result in very similar decreases in the pK(a) of several alpha-carbonyl carbon acids.     

Synthesis of new Cu(II)-chelating ligand amphiphiles and their esterolytic properties in cationic micelles

Four new tetradentate 2,6-disubstituted pyridine and tridentate 2-substituted pyridine ligands were synthesized. Two of these compounds possessed a metal ion binding subunit in the form of a 2,6-disubstituted-4-N,N'-dimethylamine pyridine moiety. Cu(2+)-complexes of these ligands incorporated in cetyltrimethylammonium bromide (CTABr) micelles speeded the cleavage of p-nitrophenyldiphenyl phosphate and p-nitrophenyl hexanoate at pH 7.6. On the basis of a kinetic version of Job plot analysis, a 1:1 ligand/Cu(2+) stoichiometry was found to be the most active species. In CTABr micelles, the pK(a) values for the Cu(2+)-coordinated hydroxyl or pendant -CH(2)OH in these ligands were between 7.8 and 7.9. The metallomicellar systems displayed catalytic (turnover) behavior in the presence of excess substrates.     

Intrinsic surface reaction equilibrium constants of structurally charged amphoteric hydrotalcite-like compounds

The relative equations among intrinsic surface reaction equilibrium constants (K in 1-pK model, K(a1)(int) and K(a2)(int) in 2-pK model, and *K(Na)(int) and *K(Cl)(int) in inert electrolyte chemical binding model), points of zero charge (PZC), and structural charge density (sigma(st)) for amphoteric solids with structural charge were established to investigate the effects of sigma(st) on intrinsic equilibrium constants and PZC. The intrinsic equilibrium constants of HTlc with general formulas [(Zn,Mg)(1-x)Al(x)(OH)(2)](Cl,OH)(x) and [Mg(1-x)(Fe,Al)(x)(OH)(2)](Cl,OH)(x) were evaluated. The following main conclusions were obtained. For amphoteric solids with structural charge, a point of zero net charge (PZNC) independent of electrolyte concentration (c) exists. A common intersection point (CIP) should appear among the acid-base titration curves at different c, and the pH at the CIP is pH(PZNC). The pK, pK(a1)(int), and pK(a2)(int) may be expressed as a function of pH(PZNC) and sigma(st), and these intrinsic equilibrium constants can be directly calculated from pH(PZNC) and sigma(st). The inert electrolyte chemical binding does not exist for amphoteric surfaces with structural charge. PZNC is not equal to the point of zero net proton charge (PZNPC) when sigma(st) not equal 0. pH(PZNC) > pH(PZNPC) when sigma(st)>0; pH(PZNC) < pH(PZNPC) when sigma(st)<0; and pH(PZNC) = pH(PZNPC) only when sigma(st)=0. With increasing c, the difference between pH(PZNC) and pH(PZNPC) decreases; i.e., pH(PZNPC) moves forward to pH(PZNC) with increasing c. For the HTlc samples studied, with increasing x, the pH(PZNC) and the pK(a1)(int) and pK(a2)(int) decrease, and the pK increases. These results can be explained on the basis of the affinity of metal cations for H(+) or OH(-) and the electrostatic interaction between the charging surface and H(+) or OH(-).     

Zn(II) coordination to polyamine macrocycles containing dipyridine units. New insights into the activity of dinuclear Zn(II) complexes in phosphate ester hydrolysis

Zn(II) binding by the dipyridine-containing macrocycles L1-L3 has been analyzed by means of potentiometric measurements in aqueous solutions. These ligands contain one (L1, L2) or two (L3) 2,2'-dipyridine units as an integral part of a polyamine macrocyclic framework having different dimensions and numbers of nitrogen donors. Depending on the number of donors, L1-L3 can form stable mono- and/or dinuclear Zn(II) complexes in a wide pH range. Facile deprotonation of Zn(II)-coordinated water molecules gives mono- and dihydroxo-complexes from neutral to alkaline pH values. The ability of these complexes as nucleophilic agents in hydrolytic processes has been tested by using bis(p-nitrophenyl) phosphate (BNPP) as a substrate. In the dinuclear complexes the two metals play a cooperative role in BNPP cleavage. In the case of the L2 dinuclear complex [Zn(2)L2(OH)(2)](2+), the two metals act cooperatively through a hydrolytic process involving a bridging interaction of the substrate with the two Zn(II) ions and a simultaneous nucleophilic attack of a Zn-OH function at phosphorus; in the case of the dizinc complex with the largest macrocycle L3, only the monohydroxo complex [Zn(2)L3(OH)](3+) promotes BNPP hydrolysis. BNPP interacts with a single metal, while the hydroxide anion may operate a nucleophilic attack. Both complexes display high rate enhancements in BNPP cleavage with respect to previously reported dizinc complexes, due to hydrophobic and pi-stacking interactions between the nitrophenyl groups of BNPP and the dipyridine units of the complexes.     

Tunable pK of amino acid residues at the air-water interface gives an L-zyme (langmuir enzyme)

Various amino acid-carrying amphiphiles were synthesized, and the pK values of the attached amino acid residues were investigated at the air-water interface and in aqueous vesicles using pi-A isotherm measurements, (1)H NMR titration, and IR spectroscopy in reflection-adsorption mode. The epsilon-amino group of the Lys residue embedded at the air-water interface displays a significant pK shift (4 or 5 unit) compared with that observed in bulk water, while the pK shift in aqueous vesicles was not prominent (ca. 1 unit). Moreover, pK values of the amino acids at the air-water interface can be tuned simply by control of the subphase ionic strength as well as by molecular design of the amphiphiles. A simple equation based on the dominant contribution by the electrostatic energy to the pK shift reproduces well the surface pressure difference between protonated and unprotonated species, suggesting a reduction in the apparent dielectric constant at the air-water interface. Hydrolysis of a p-nitrophenyl ester derivative was used as a model reaction to demonstrate the use of the Lys-functionalized monolayer. Efficient hydrolysis was observed, even at neutral pH, after tuning of pK for the Lys residue in the monolayer, which is a similar case to that occurring in biological catalysis.     

Analysis of host-assisted guest protonation exemplified for p-sulfonatocalix[4]arene--towards enzyme-mimetic pKa shifts

The pD dependence of the complexation of p-sulfonatocalix[4]arene (CX4) with the azoalkanes 2,3-diazabicyclo[2.2.1]hept-2-ene (1), 2,3-diazabicyclo[2.2.2]oct-2-ene (2), 2,3-diazabicyclo[2.2.3]non-2-ene (3), and 1-methyl-4-isopropyl-2,3-diazabicyclo[2.2.2]oct-2-ene (4) in D(2)O has been studied. The pD-dependent binding constants, determined by (1)H NMR spectroscopy, were analyzed according to a seven-state model, which included the CX4 tetra- and penta-anions, the protonated and unprotonated forms of the azoalkanes, the corresponding complexes, as well as the complex formed between CX4 and the deuteriated hydronium ion. The variation of the UV absorption spectra, namely the hypsochromic shift in the near-UV band of the azo chromophore upon protonation, was analyzed according to a four-state model. Measurements by independent methods demonstrated that complexation by CX4 shifts the pK(a) values of the guest molecules by around 2 units, thereby establishing a case of host-assisted guest protonation. The pK(a) shift can be translated into improved binding (factor of 100) of the protonated guest relative to its unprotonated form as a result of the cation-receptor properties of CX4. The results are discussed in the context of supramolecular catalytic activity and the pK(a) shifts induced by different types of macrocyclic hosts are compared.     

一个具有大Stokes位移的苯并噻唑类pH荧光探针

本文通过乙烯基将作为荧光团的苯并噻唑与作为H+受体的4-吡啶基桥联构筑了一个基于分子内电荷转移机制的pH荧光探针BTP2。研究表明该探针的Stokes位移为237 nm,远大于相应2-吡啶基类似物BTP1。滴定实验表明该探针的荧光在pH3.80至5.50之间随pH值增大而增强,且不受其他金属离子的干扰,具有检测胞内酸性细胞器pH的良好前景。探针pKa为4.72,略高于BTP1。4-吡啶基连接导致的更大的Stokes位移表明调节吡啶连接位置可以实现对该类探针分子Stokes位移的调控。     

Effect of pH on the reaction of tris(2,2'-bipyridyl)ruthenium(III) with amino-acids: Implications for their detection

A new technique for the detection of amino-acids is described, which is based on their chemiluminescence reaction with tris(2,2'-bipyridyl)ruthenium(III). The pH-dependence of this reaction has been investigated and found to be the key experimental parameter in application of this reaction as a detection technique. The chemiluminescence emission obtained is maximal at pH values higher than the N-terminal amino group pK(a) of the amino-acid. The background reaction between the ruthenium reagent and hydroxide ion does not occur with the same efficiency as the amino-acid reaction and the optimum signal to noise ratio is obtained at pH 10. A limit of detection of 30 picomole was found for valine and the response was shown to be linear over two orders of magnitude.