Investigation on the interaction between ilaprazole and bovine serum albumin without or with different C-ring flavonoids from the viewpoint of food-drug interference

The interaction between ilaprazole and bovine serum albumin (BSA) has been investigated in the absence and presence of four popular flavonoids with different C-ring structures, quercetin, luteolin, taxifolin, and (+)-catechin, by means of fluorescence spectroscopy. The results indicated that ilaprazole had a strong ability to quench the intrinsic fluorescence of BSA, and site marker competitive experiments indicated that the binding of ilaprazole to BSA primarily took place in subdomain IIA. The quenching process of ilaprazole with BSA was easily affected by flavonoids,; however, they did not change the quchenching mechanism of ilaprazole with BSA, whereas all of the fluorescence quenching was initiated by a static quenching procedure combining with nonradiative energy transfer. The presence of flavonoids decreased the quenching constants of ilaprazole with BSA from 2.2 to 23.7% and decreased the binding constants from 73.7 to 98.3%, which depended on the different flavonoids' structures. The decreased binding constants and unchangeable spatial distance of ilaprazole with BSA by the introduction of quercetin, luteolin, and taxifolin may result from the competition of flavonoids and ilaprazole binding to BSA, whereas in the presence of (+)-catichin, decreased binding constants and increased spatial distance possibly resulted from the formation of a ternary ilaprazole-BSA-(+)-catechin complex. All of these results may have relevant consequences in rationalizing the interferences of common food to gastric ulcer treatments.     

Interaction of flavonoids with bovine serum albumin: a fluorescence quenching study

The interaction between four flavonoids (catechin, epicatechin, rutin, and quercetin) and bovine serum albumin (BSA) was investigated using tryptophan fluorescence quenching. Quenching constants were determined using the Stern-Volmer equation to provide a measure of the binding affinity between the flavonoids and BSA. The binding affinity was strongest for quercetin and ranked in the order quercetin > rutin > epicatechin = catechin. The pH in the range of 5-7.4 does not affect significantly (p < 0.05) the association of rutin, epicatechin, and catechin with BSA, but quercetin exhibited a stronger affinity at pH 7.4 than at lower pH (p < 0.05). Quercetin has a total quenching effect on BSA tryptophan fluorescence at a molar ratio of 10:1 and rutin at approximately 25:1. However, epicatechin and catechin did not fully quench tryptophan fluorescence over the concentration range studied. Furthermore, the data suggested that the association between flavonoids and BSA did not change molecular conformation of BSA and that hydrogen bonding, ionic, and hydrophobic interaction are equally important driving forces for protein-flavonoid association.     

Screening, identification, and potential interaction of active compounds from Eucommia ulmodies leaves binding with bovine serum albumin

The aqueous extract of Eucommia ulmoides leaves has been commonly known as Du-zhong tea as a functional health food for the treatment of hypertension, hypercholesterolemia, and fatty liver. This study developed a centrifugal ultrafiltration-high-performance liquid chromatography (HPLC) method for screening and identification of bioactive compounds in E. ulmoides leaves binding with bovine serum albumin (BSA). Six active compounds were screened, isolated, and elucidated by their ultraviolet (UV), electrospray ionization-mass spectrometry (ESI-MS), and nuclear magnetic resonance (NMR) data as geniposidic acid (1), caffeic acid (2), chlorogenic acid (3), quercetin-3-O-sambubioside (4), rutin (5), and isoquercitrin (6). The interaction between active compounds and BSA was investigated in the absence and presence of other compounds by quenching the intrinsic BSA fluorescence. The results indicated that the structures significantly affected the binding process. The values of binding constants for compounds 2-6 were in the range of 10(5)-10(6) mol L(-1), while geniposidic acid (1) hardly quenching the BSA intrinsic fluorescence. However, the quenching process of geniposidic acid was easily affected in the presence of other active compounds. The formation of the geniposidic acid-phenylpropanoid (flavonoid) complex could increase the binding affinity of geniposidic acid with BSA; however, the increased steric hindrance of the complex may make phenylpropanoid or flavonoid dissociate from BSA and then decrease their affinities.     

Influence of B-ring hydroxylation on interactions of flavonols with bovine serum albumin

The B-ring substitution pattern of flavonols is a significant structural feature for their function as free radical scavengers and antioxidants. In this paper, four differently substituted B-ring hydroxylation flavonols (galangin, kaempferol, quercetin, and myricetin) and a flavonol glycoside (quercitrin) were studied for their ability to bind BSA by quenching the protein intrinsic fluorescence. From the spectra obtained, the biomolecular quenching constants, the apparent static binding constants, and the binding site values were calculated. The B-ring hydroxylation of flavonols significantly affected the binding/quenching process; in general, the binding affinity increased with the number of hydroxyl groups on the B-ring. The binding constants ( Ka) were determined as myricetin (4.90 x 10(8) L/mol) > quercetin (3.65 x 10(7) L/mol) > kaempferol (2.57 x 10(6) L/mol) > galangin (6.43 x 10(5) L/mol). The glycoside substitute at the C-ring position decreased the binding affinity. The chromatographic retention factor ( K') and logarithms of apparent partition coefficient (log Kow) were linear to the logarithms of apparent binding constants (log Ka) for flavonols with increasing hydroxyl groups on the B-ring. These results showed that the hydrogen bond force play an important role in binding flavonols to BSA. These results are also in agreement with the generally accepted structure-dependent free radical scavenger and antioxidant abilities of flavonols.     

Stability of ferric complexes with 3-hydroxyflavone (flavonol), 5,7-dihydroxyflavone (chrysin), and 3',4'-dihydroxyflavone

The acid dissociation and ferric stability constants for complexation by the flavonoids 3-hydroxyflavone (flavonol), 5,7-dihydroxyflavone (chrysin), and 3',4'-dihydroxyflavone in 50:50 (v/v) ethanol/water are determined by pH potentiometric and spectrophotometric titrations and the linear least-squares curve-fitting program Hyperquad. Over the entire range of pH and reagent concentrations spanning the titration experiments, the stoichiometry for iron-flavonoid complex formation was 1:1 for all three flavonoids examined. The three flavonoids were chosen for their hydroxy substitution pattern, with each possessing one of the three most commonly suggested sites for metal binding by the flavonoids. On the basis of the calculated stability constants, the intraflavonoid-binding site competition is illustrated as a function of pH via speciation curves. The curves indicate that the binding site comprised of the 3',4'-hydroxy substitutions, the catecholic site, is most influential for ferric complexation at the physiological pH of 7.4. The possibility for antioxidant activity by flavonoid chelation of ferric iron in the presence of other competitive physiological complexing agents is demonstrated through additional speciation calculations.     

重金属Cd2+对结缕草叶片光合特性的影响

用水培法研究了重金属Cd2+胁迫对结缕草叶片光合气体交换、叶绿素荧光参数、光能分配及叶绿素含量的影响。结果表明,Cd2+胁迫下,除胞间CO₂浓度（Ci）外,结缕草叶片净光合速率（Pn）、蒸腾速率(Tr)、气孔导度(Gs)均较对照降低,差异显著;最小初始荧光(F0)、最大荧光(Fm)、光下最大荧光(Fm′)、光下最小荧光(F0′)、稳态荧光(Fs)、最大量子产量(Fv/Fm)、实际量子产量(Fv′/Fm′)、实际光化学效率(ΦPSⅡ)、相对电子传递速率(rETR)、光化学猝灭系数(qp)等均较对照显著降低,而20d时处理叶片的非光化学猝灭系数(NPQ)较对照显著升高;在光能分配方面,除PSⅡ反应中心的过剩激发能C值无显著性变化,Cd2+胁迫显著降低了结缕草叶片PSⅡ光化学反应消耗的光能A值,显著提高了通过天线热耗散消耗的光能B值;尽管叶绿素与类胡萝卜素含量无显著变化,但30 d的Cd2+胁迫仍使二者含量降低。     

Flavonoids as mushroom tyrosinase inhibitors: a fluorescence quenching study

Flavonoids, a group of naturally occurring antioxidants and metal chelators, can be used as tyrosinase inhibitors due to their formation of copper-flavonoid complexes. Thus, to investigate the underlying inhibition mechanism, a large group of flavonoids from several major flavones and flavonols were tested using fluorescence quenching spectroscopy. In addition, large differences in the tyrosinase inhibitory activities and chelating capacities according to the location of the hydroxyl group(s) in combination with the A and B rings in the flavonoids were confirmed. Accordingly, the major conclusions from this work are as follows: (i) The tyrosinase inhibitory activity is not only dependent on the number of hydroxyl groups in the flavonoids, (ii) the enzyme is primarily quenched by the hydroxyl group(s) of A and B rings on the ether side of the flavonoids, and (iii) the tyrosinase inhibitory activity of 7,8,3',4'-tetrahydroxyflavone is supported by a virtual model of docking with the mushroom tyrosinase, which depicts the quenching of the enzyme. The results also demonstrated that the dihydroxy substitutions in the A and B rings are crucial for Cu2+-chelate formation, thereby influencing the tyrosinase inhibitory activity.     

Hydrolyzable tannin structures influence relative globular and random coil protein binding strengths

Binding parameters for the interactions of pentagalloyl glucose (PGG) and four hydrolyzable tannins (representing gallotannins and ellagitannins) with gelatin and bovine serum albumin (BSA) have been determined from isothermal titration calorimetry data. Equilibrium binding constants determined for the interaction of PGG and isolated mixtures of tara gallotannins and of sumac gallotannins with gelatin and BSA were of the same order of magnitude for each tannin (in the range of 10(4)-10(5) M(-1) for stronger binding sites when using a binding model consisting of two sets of multiple binding sites). In contrast, isolated mixtures of chestnut ellagitannins and of myrabolan ellagitannins exhibited 3-4 orders of magnitude greater equilibrium binding constants for the interaction with gelatin (approximately 2 x 10(6) M(-1)) than for that with BSA (approximately 8 x 10(2) M(-1)). Binding stoichiometries revealed that the stronger binding sites on gelatin outnumbered those on BSA by a ratio of at least approximately 2:1 for all of the hydrolyzable tannins studied. Overall, the data revealed that relative binding constants for the interactions with gelatin and BSA are dependent on the structural flexibility of the tannin molecule.     

Interaction of different polyphenols with bovine serum albumin (BSA) and human salivary alpha-amylase (HSA) by fluorescence quenching

Phenolic compounds are responsible for major organoleptic characteristics of plant-derived food and beverages; these substances have received much attention, given that the major function of these compounds is their antioxidant ability. In the context of this study, our major aim was study the binding of several phenolic compounds such as (+)-catechin, (-)-epicatechin, (-)-epicatechin gallate, malvidin-3-glucoside, tannic acid, procyanidin B4, procyanidin B2 gallate, and procyanidin oligomers to different proteins (bovine serum albumin and human alpha-amylase) by fluorescence quenching of protein intrinsic fluorescence. From the spectra obtained, the Stern-Volmer, the apparent static, and the bimolecular quenching constants were calculated. The structure of polyphenols revealed to significantly affect the binding/quenching process; in general, the binding affinity increased with the molecular weight of polyphenol compounds and in the presence of galloyl groups. For catechin monomer and procyanidin dimer B4, the K(SV) was 14,100 and 13,800 M(-1), respectively, and for galloyl derivatives, the K(SV) was 19,500 and 21,900 M(-1), respectively. Tannic acid was shown to be the major quenching molecule for both proteins. However, comparing different proteins, the same polyphenol showed different quenching effects, which are suggested to be related to the three-dimensional structure of the proteins studied. For (+)-catechin and BSA, the K(SV) was 8700 M(-1), and with alpha-amylase, it was 14,100 M(-1); for tannic acid, the K(SV) was 10,0548 and 11,0674 M(-1), respectively. From the results obtained, besides the main binding analysis performed, we conclude that this technique is more sensitive than thought because we can detect several interactions that have not been proven by other methods, namely, nephelometry. Overall, fluorescence quenching has proven to be a very sensitive technique with many potentialities to analyze the interaction between polyphenols and proteins.     

Effects of Nitrolotriacetic Acid-Copper Complexes on Activated Sludge Process

Effects of nitrilotriacetic acid (NTA), copper ion (Cu2+) and their complexes on the efficiency of activated sludge process were evaluated by determining the changes in physical and chemical parameters in a laboratory-scale unit. Primary effects of these toxicants on microorganisms in the activated sludge process were also evaluated by determining the change of in vivo dehydrogenase activity. The presence of 1 mg L-1 or higher of Cu2+ significantly affected the activated sludge process. Nitrification in the activated sludge process was more sensitive to the inhibition of Cu2+. The presence of NTA, as high as 20 mg L-1, did not significantly affect the activated sludge process. Moreover, in the presence of 20 mg L-1 of NTA, the toxicity of Cu2+ on the activated sludge process was significantly reduced.     

Interactive Sorption of Metal Ions and Humic Acids onto Mineral Particles

The sorption of metal ions (Pb2+, Zn2+ and Cu2+) and soil humic acids (HA) from aqueous solutions onto mineral particles (sand, calcite and clay) was investigated using a batch equilibrium system. The sorption reactions in two- component systems (heavy metals-mineral particles and humic acids- mineral particles), as well as interactions in three-component system (heavy metals-humic acids-mineral particles) were examined. Results showed that the presence of humic acids, dissolved or bound onto mineral surfaces, considerably influenced the fixation of heavy metals. The various effects, depending on mineral type, humic concentration and specific metal-ion, were observed in three- component system. Sorption of Cu2+-ions on all minerals studied rapidly increased as the concentration of dissolved HA increased. The amount of Pb2+-ions sorbed on sand slightly decreased, while on kaolin increased between 15 and 20%. Sorption of Zn2+-ions on all minerals studied decreased at pH 4. At pH 5.5 the sorption of Zn2+-ions onto calcite decreased, while on kaolin and sand increased as a function of the humic acid concentration giving the curve with maximum at c(HA) = 2.5 mmol C L-1. At pH 6.5 sorption onto kaolin and sand increased. This effect occurs as a result of the conditional stability constant of Zn-HA complexes increasing at higher pH which in turn promotes the chelation of Zn2+-ions to mineral- bound humic substances. The enhanced sorption of metal ions from the aqueous phase in three-component systems is not only the result of mineral sorption of free metals but also the result of chelation with HA sorbed on the mineral surface.     

Die Komplexierung von Cu,Pb und Cd durch heißwasserlösliche Huminstoffe II. Die Stabilität der gebildeten Komplexe

Complexation of Cu, Pb, and Cd by hot water soluble humic substances II. The stability of the complexes Soxhlet extracts of terrestrial and underwater soils were used to determine polarographically the stability of heavy metal complexes. Stability constants were calculated using the law of mass action. They were independent of concentration at levels higher than 2 mg Cd or 5 mg Pb/g humic substance, respectively. For Cu, constants increased below 8 mg Cu/g. At pH values in the polarographic medium between 4 and 8, the stability of the Pb complexes increased markedly with increasing pH. In most cases this was also valid for the copper complexes, cadmium complexes were either not influenced by pH or stability maximum was obtained at about pH 5. Drying of the samples before extraction did not alter significantly complex stabilities. Complex stabilities increased with increasing degree of humification for oligotrophic, underwater soils and nutrient poor, low moor peat, but reduced horizons of eutrophic gyttjas exhibited lower stability values.     

Microbial degradation of hydrolysable and condensed tannin polyphenol-protein complexes in soils from different land-use histories

Polyphenols are capable of binding to proteins and form polyphenol-protein complexes thus reducing the release of N from decomposing plant materials. The objective of this work was to test if under polyphenol-rich vegetations adapted microbial communities had developed capable of breaking down recalcitrant polyphenol-protein complexes. Soils used for this investigation were from different 10-year-old tropical agricultural systems (maize, sugarcane plots and Gliricidia sepium or Peltophorum dasyrrachis woodlots) and natural systems (secondary forest and Imperata cylindrica grassland). TA (tannic acid, hydrolysable tannin), QUE (quebracho, condensed tannin), BSA (bovine serum albumin, protein) or TA/BSA and QUE/BSA polyphenol-protein complexes were incubated at 28 °C in these soils. CO₂-C and ¹³C evolution were periodically monitored and mineral N release, microbial biomass N and phospholipid fatty acid (PLFA) profiles measured at the end.QUE was able to bind about 25% more protein than TA. In all systems the individual uncomplexed substrates were more easily degraded than the complexes. On average, net cumulative CO₂-C evolution from TA/BSA complexes was more than 5 times higher than from QUE/BSA complexes, indicating higher C availability and/or lower protection capability of TA compared to QUE. However, net N release was higher from QUE/BSA than from TA/BSA probably due to their higher protein-binding capacity and associated larger degradation of partly unprotected protein as suggested by ¹³C-CO₂ signatures. Microbial respiration patterns indicated that polyphenol complexes were initially degraded more quickly in the maize cropping system than in soils from under polyphenol-rich communities (Peltophorum and natural forest) but this pattern reversed with time. Long-term incubation of QUE/BSA complexes even caused a negative effect on microbial respiration in agricultural soils with low polyphenol contents (e.g. maize and sugarcane).Incubation of polyphenol complexes in soil depressed microbial biomass N in maize, sugarcane, Imperata and forest systems and led to reduced soil pH. However, microbial biomass was increased under the polyphenol-rich vegetation of Peltophorum. The PLFA group 18:2w6,9 was highly enhanced by condensed tannin-protein complexes additions as compared to control and hydrolysable polyphenol-protein complexes in soils with high polyphenol contents. Polyphenol complexes increased the fungi:bacteria ratio in systems with a high polyphenol content, particularly with condensed tannin complexes. The results indicated that systems with a high polyphenol content favoured development of fungal communities that are highly adaptable to phenol-rich soil conditions and high acidity, particularly with regards to the more recalcitrant condensed tannin-protein complexes.     

Theoretical speciation of ethylenediamine-N-(o-hydroxyphenylacetic)-N'-(p-hydroxyphenylacetic) acid (o,p-EDDHA) in agronomic conditions

The presence of ethylenediamine-N-(o-hydroxyphenylacetic)-N'-(p-hydroxyphenylacetic) acid (o,p-EDDHA) as the second largest component in commercial EDDHA iron chelates has recently been demonstrated. Here is reported the speciation of o,p-EDDHA by the application of a novel methodology through the determination of the complexing capacity, protonation, and Ca(2+), Mg(2+), Cu(2+), and Fe(3+) stability constants. The pM values and species distribution in solution, hydroponic, and soil conditions were obtained. Due to the para position of one phenol group in o,p-EDDHA, the protonation constants and Ca and Mg stability constants have different values from those of o,o-EDDHA and p,p-EDDHA regioisomers. o,p-EDDHA/Fe(3+) stability constants are higher than those of EDTA/Fe(3+) but lower than those of o,o-EDDHA/Fe(3+). The sequence obtained for pFe is o,o-EDDHA/Fe(3+) >/= o,p-EDDHA/Fe(3+) > EDTA/Fe(3+). o,p-EDDHA/Fe(3+) can be used as an iron chelate in hydroponic conditions. Also, it can be used in soils with limited Cu availability.     

A Study of Diffusive Gradients in Thin Films for the Chemical Speciation of Zn(II), Cd(II), Pb(II), and Cu(II): The Role of Kinetics

The lability and mobility of Zn(II)–, Cd(II)–, Pb(II)–, and Cu(II)–humic acid complexes were studied using diffusive gradients in thin films (DGT). A unique feature of this research was (1) the use of DGTs with diffusive layer thicknesses ranging from 0.4 to 2.0 mm to study lability and mobility of Zn(II)–, Cd(II)–, Pb(II)–, and Cu(II)–humic acid complexes, combined with (2) the application of a competing ligand exchange (CLE) method using Chelex 100, the same chelating resin that is used in DGT, to study the kinetic speciation. The CLE experiments were run immediately after the completion of the DGT experiments, thereby allowing effects of the competing ligand to be separated from the effects introduced by the use of the polyacrylamide gel that is used in DGT. The results indicate that Zn(II) and Cd(II) tend to form more labile and more mobile complexes with humic acid than Pb(II) or Cu(II). The dissociation rate constants of Zn(II), Cd(II), and Pb(II) were found to increase with the ionic potential of the metal, suggesting that the binding between some trace metals and humic acid has a significant covalent component. Furthermore, the results suggest that the Eigen mechanism may not be strictly obeyed for metals such as Cu(II) which have high rate constants of water exchange, k _w. Consequently, the markedly slow kinetics of Cu(II)-HA species suggests that the usual equilibrium assumption may not be valid in freshwaters.     

Noncovalent interaction of dietary polyphenols with common human plasma proteins

Common human plasma proteins (CHPP), also called blood proteins, are proteins found in blood plasma. The molecular structure/property-affinity relationships of dietary polyphenols noncovalently binding to CHPP were investigated by comparing the binding constants obtained from the fluorescence titration method. An additional methoxy group in flavonoids increased their binding affinities for CHPP by 1.05 to 72.27 times. The hydroxylation on the 4' position (ring B) of flavones and flavonols and the 5 position (ring A) of isoflavones weakened the binding affinities; however, the hydroxylation on other positions of flavonoids slightly enhanced or little affected the binding affinities for CHPP. The glycosylation of flavonoids weakened or slightly affected the affinities for CHPP by 1 order of magnitude. The hydrogenation of the C2═C3 double bond of flavone, 6-hydroxyflavone, 6-methoxyflavone and myricetin decreased the binding affinities about 10.02 to 17.82 times. The galloylation of catechins significantly improved the binding affinities with CHPP about 10 to 1000 times. The esterification of gallic acid increased its binding affinity. The binding affinities with CHPP were strongly influenced by the structural differences of dietary polyphenols. Polyphenols with higher affinities for purified HSA also showed stronger affinities with CHPP. The hydrophobic force played an important role in binding interaction between polyphenols and CHPP.     

Uptake of Cu2+ by starch granules as affected by counterions

Potato and wheat starch granules were soaked in 1% aqueous solutions of copper(II) salts: acetate, chloride, and sulfate. Such treatment caused sorption of Cu(2+) ions at the granule surface and their penetration into the granule interior as was proven, for sectioned granules of investigated starch, by scanning electron microscopy combined with an X-ray microanalysis system (energy dispersive spectroscopy). Copper ions incorporated into the granules influenced the starch thermal stability. Uptake of Cu(2+) by potato, determined by flame atomic absorption spectrometry, was much higher than obtained for the wheat starch. Moreover, it was dependent on copper counteranions present in the solution. In all investigated granules, the most effective sorption occurred in the acetate solution. Starch dehydration or/and freezing and thawing, affecting the water-dependent inner structure of the granules, also influenced the amount of Cu(2+) taken from the solutions. Thus, compared to that in native starch, this value was considerably higher in Cu(CH 3COO)2, almost unchanged in CuSO4, and significantly lower in the case of CuCl2 solution. The influence of chloride and sulfate anions seemed to correlate with their water structure-making and structure-breaking ability, affecting the migration of Cu(2+) in the amorphous parts of the granules. However, high Cu uptake observed for acetate solution could be explained on the basis of acetate anion hydrolysis activating the polysaccharide matrix for cation binding. The obtained results provide new information about interactions of starch granules with salt solution and therefore support our understanding of starch properties.     

Metal binding with root exudates of low molecular weight

Root exudates were collected from maize plants ( Zea mays L .), grown under sterile and hydroponic conditions. Exudates of low molecular weight were isolated by filtration and dialysis (MW 1000) of the nutrient solution and analysed. Metal binding properties of the exudates were investigated at 25°C in 0.1 m NaCIO₄ ionic medium, using a potentiometric method. The total acidity of the exudates and the successive stability constants of complexes formed with Cu²⁺, Pb²⁺ and Zn²⁺ were determined.
Sugars and organic acids were the major components of the exudates. Measured total acidity was resolved into strong (5.85meq g⁻¹C), weak (2.10meq g⁻¹C) and very weak (2.20 meq g⁻¹ C) acidities. Mono-complexes were formed with Cu, Pb and Zn and bis-complexes with Cu. At pH 5, stability constants followed the order Cu>Pb>Zn.     

Evidence for aliphatic mixed mode coordination in copper(II)–fulvic acid complexes

A comparative approach based on ion–selective electrode measurements is presented for studying Cu(II) complexing by fulvic acids. Metal binding curves (% free metal vs pH) and complexation capacity curves (% free metal vs total metal at fixed pH) for copper(II) fulvic acid systems are compared with those for appropriate model ligands.
It is inferred that citrate, malonate and amino acid moieties could dominate complexing to Cu(II) at pH 3 to 7. The involvement of amino acid moieties is supported by fluorescence quenching and by anodic stripping voltammetry (ASV) studies. The proportion of ASV–non labile complexes is reported for Cu(II) and Pb(II) fulvic acid systems at pH 4.8.
A mixed mode of coordination is proposed, with the dominant binding sites varying with pH and metal:ligand ratio.     

Computer simulation of the interactions of glyphosate with metal ions in phloem

Essential nutrients such as trace metal ions, amino acids, and sugars are transported in the phloem from leaves to other parts of the plant. The major chelating agents in phloem include nicotianamine, histidine, cysteine, glutamic acid, and citrate. A computer model for the speciation of metal ions in phloem has been used to assess the degree to which the widely used herbicide glyphosate binds to Fe(3+), Fe(2+), Cu(2+), Zn(2+), Mn(2+), Ca(2+), and Mg(2+) in this fluid over the pH range of 8 to 6.5. The calculations show that glyphosate is largely unable to compete effectively with the biological chelating agents in phloem. At a typical phloem pH of 8, 1.5 mM glyphosate binds 8.4% of the total Fe(3+), 3.4% of the total Mn(2+), and 2.3% of the total Mg(2+) but has almost no effect on the speciation of Ca(2+), Cu(2+), Zn(2+), and Fe(2+). As the pH decreases to 6.5, there are some major shifts of the metal ions among the biological chelators, but only modest increases in glyphosate binding to 6% for Fe(2+) and 2% for Zn(2+). The calculations also indicate that over 90% of the glyphosate in phloem is not bound to any metal ion and that none of the metal-glyphosate complexes exceed their solubility limits.