Tyrosine Units Research Articles

Artificial Photosynthesis: From Nanosecond Electron Transfer to Catalytic Water Oxidation

Human society faces a fundamental challenge as energy consumption is projected to increase due to population and economic growth as fossil fuel resources decrease. Therefore the transition to alternative and sustainable energy sources is of the utmost importance. The conversion of solar energy into chemical energy, by splitting H2O to generate molecular O2 and H2, could contribute to solving the global energy problem. Developing such a system will require the combination of several complicated processes, such as light-harvesting, charge separation, electron transfer, H2O oxidation, and reduction of the generated protons. The primary processes of charge separation and catalysis, which occur in the natural photosynthetic machinery, provide us with an excellent blueprint for the design of such systems. This Account describes our efforts to construct supramolecular assemblies capable of carrying out photoinduced electron transfer and to develop artificial water oxidation catalysts (WOCs). Early work in our group focused on linking a ruthenium chromophore to a manganese-based oxidation catalyst. When we incorporated a tyrosine unit into these supramolecular assemblies, we could observe fast intramolecular electron transfer from the manganese centers, via the tyrosine moiety, to the photooxidized ruthenium center, which clearly resembles the processes occurring in the natural system. Although we demonstrated multi-electron transfer in our artificial systems, the bottleneck proved to be the stability of the WOCs. Researchers have developed a number of WOCs, but the majority can only catalyze H2O oxidation in the presence of strong oxidants such as Ce(IV), which is difficult to generate photochemically. By contrast, illumination of ruthenium(II) photosensitizers in the presence of a sacrificial acceptor generates [Ru(bpy)3](3+)-type oxidants. Their oxidation potentials are significantly lower than that of Ce(IV), but our group recently showed that incorporating negatively charged groups into the ligand backbone could decrease the oxidation potential of the catalysts and, at the same time, decrease the potential for H2O oxidation. This permitted us to develop both ruthenium- and manganese-based WOCs that can operate under neutral conditions, driven by the mild oxidant [Ru(bpy)3](3+). Many hurdles to the development of viable systems for the production of solar fuels remain. However, the combination of important features from the natural photosynthetic machinery and novel artificial components adds insights into the complicated catalytic processes that are involved in splitting H2O.

Probing Protein Kinase (CK2) and Alkaline Phosphatase with CdSe/ZnS Quantum Dots

Semiconductor quantum dots (QDs) are used for the optical analysis of casein kinase (CK2) or the hydrolytic activity of alkaline phosphatase (ALP). Two schemes for the analysis of CK2 by a FRET-based mechanism are described. One approach involves the CK2-catalyzed phosphorylation of a serine-containing peptide (1), linked to CdSe/ZnS QDs, with Atto-590-functionalized ATP. The second analytical method involves the specific association of the Atto-590-functionalized antibody to the phosphorylated product. The hydrolytic activity of ALP is followed by the application of phosphotyrosine (4)-modified CdSe/ZnS QDs in the presence of tyrosinase as a secondary reporter biocatalyst. The hydrolysis of (4) yields the tyrosine units that are oxidized by O(2)/tyrosinase to the respective dopaquinone product. The latter quinone units quench the QDs via an electron transfer route, leading to the optical detection of the ALP activity.

Synthesis and conformational analysis of azacyclophanes from l-tyrosine

Abstract The synthesis of a new azacyclophane formed by two l -tyrosine units joined by two methylene bridges is presented. The structural and conformational characteristics are briefly discussed. Spectroscopic and theoretical data reveal a syn structure with two intramolecular hydrogen bonds.

Evidence for CCl/CBr⋅⋅⋅π Interactions as an Important Contribution to Protein–Ligand Binding Affinity

Attractive chlorine: Noncovalent interactions between chlorine or bromine atoms and aromatic rings in proteins open up a new method for the manipulation of molecular recognition. Substitution at distinct positions of two factor Xa inhibitors improves the free energy of binding by interaction with a tyrosine unit. The generality of this motif was underscored by multiple crystal structures as well as high-level quantum chemical calculations (see picture).

Synthesis of a New Water-Soluble Oligo(phenylenevinylene) Containing a Tyrosine Moiety for Tyrosinase Activity Detection

A new water-soluble oligo(phenylenevinylene) containing a tyrosine unit (OPV-Tyr) was synthesized as a fluorescent probe to optically detect tyrosinase activity. Upon the addition of tyrosinase, the tyrosine moiety was oxidized to quinone, which quenched the fluorescence of OPV-Tyr via intramolecular electron transfer from the phenylenevinylene unit to the quinone site. OPV-Tyr was elaborated to detect tyrosinase activity both in aqueous buffer solution and in agarose gel.

Iodine inhibits antiadhesive effect of PEG: Implications for tissue engineering

The success of a biomaterial implant may be affected by the surface chemistry's impact on protein adsorption. We have developed a series of poly(ethylene glycol) (PEG) containing, tyrosine-derived polycarbonates, which have been rendered radio-opaque by the iodination of tyrosine units in the copolymer backbone for use in resorbable biomedical implants including vascular stents and grafts. We tested the hypothesis that protein adsorption along with seeding, growth, and migration of human aortic smooth muscle cells (SMC) and human aortic endothelial cells (EC) will be modified by the presence of iodine and PEG within the polymer composition. Thin films of these polymers were prepared for the protein-material and cell-material interaction studies. Dot blot, SDS-PAGE, and XPS were used to evaluate relative protein adsorption. Cell adhesion and growth studies were performed using an MTS assay. Cell migration was evaluated using an injury model. The presence of PEG attenuated protein adsorption, cell adhesion, and growth. With the subsequent incorporation of iodine, protein adsorption markedly increased while the antiadhesive effect of PEG was counteracted by iodine for EC and SMC adhesion and SMC growth. Iodine incorporation into the polymer resulted in increased protein adsorption thus counteracting the effect of PEG.

Probing Biocatalytic Transformations with CdSe−ZnS QDs

CdSe/ZnS QDs enable the optical probing of the biocatalytic oxidation of tyrosine derivatives and of the scission of peptides by thrombin. CdSe/ZnS QDs were modified with tyrosine methyl ester or with a tyrosine-containing peptide. The tyrosine units were reacted with tyrosinase/O2 to yield the respective l-DOPA and quinone derivatives. The luminescence of QDs modified by the enzyme-generated quinone units is quenched. The quinone-functionalized peptide associated with the QDs was cleaved by thrombin, a process that restored the luminescence of the QDs.

Interaction of endothelial and smooth muscle cells with radiopaque, resorbable polymers

Introduction: Radiopaque-resorbable polymers are needed for the development of medical implants for which the physician has to rely on fluoroscopic monitoring of device placement and function. We have developed a resorbable/radiopaque polymer being utilized in a cardiovascular stent. For this specific application, a poly(ethylene glycol) (PEG) containing, tyrosine-derived polycarbonate, which has been rendered radiopaque by the iodination of tyrosine units in the polymer backbone. We explored the interaction of smooth muscle cells (SMC) and endothelial cells (EC) with PEG and iodine containing polymers.

Microginin FR1, a linear peptide from a water bloom of Microcystis species

A so far unknown structural variant, called microginin FR1, of cyanobacterial peptides of the microginin type was isolated from a water bloom of a German lake. The bloom consisted almost exclusively of Microcystis sp. cells. The structure of microginin FR1 was identified by amino acid analysis, determination of the relative molecular mass by electrospray mass spectrometry, and by NMR to be a linear pentapeptide with a relative molecular mass of 727 which contains a β-amino-α-hydroxy-decanoic acid (Ahda), alanine, N-methyl-leucine, and two tyrosine units (Ahda-Ala-N-Me-Leu-Tyr-Tyr). Microginin FR1 had angiotensin-converting enzyme inhibitory activity with half-maximal inhibition at 6×10−5 M microginin FR1.

Flame retardancy finish with an organophosphorus retardant on silk fabrics

The paper mainly deals with flame retardancy of silk fabrics treated with a commercial organophosphorus flame retardant [N-hydroxymethyl (3-dimethyl phosphono) propionamide (HDPP), also known as Pyrovatex CP], using the pad-dry-cure-wash method. The structures and properties of the treated and control sample are discussed. The Limiting Oxygen Index (LOI) value of the modified sample is above 30%. After 50 laundry cycles, it still has some flame retardancy left.HDPP and a cross-linking agent (HMM) were bound to silk fabrics which is confirmed by FT-IR spectra and amino analysis. The reaction degree of the flame retardant with silk is also high; almost all the tyrosine units have reacted, which can be confirmed by amino acid analysis. The reaction between flame retardant and silk only occurs in the amorphous region of silk fibre, which is confirmed by X-ray diffraction analysis and amino acid analysis. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis show that the flame retardant causes silk fabrics to decompose below its ignition temperature (600°C) and formed carbonaceous residue or char when exposed to fire. The char behaves as a thermal barrier to fire, so silk fabrics show good flame retardancy.The treatment has a little effect on the whiteness of the silk fabrics and the tensile strength of treated silk fabrics slightly decreased; both effects are negligible.

Development of infrared optical sensor for selective detection of tyrosine in biological fluids

In this paper, a new and simple evanescent wave type of infrared biosensor is described for the selective detection of tyrosine in biological fluids. This sensor is based on the formation of copper complexes between the sensing phase and tyrosine. To demonstrate that this principle was applicable to the selective detection of tyrosine, a proline-modified sensing phase was synthesized on the surface of the internal reflection elements. This sensing phase was saturated with copper ions to allow it to interact with tyrosine units in aqueous solution through the formation of stable proline-Cu2+-tyrosine complexes. Tyrosine exhibits a unique spectral feature in its absorption band at 1515 cm-1. This band significantly differs from those of other amino acids and provides a further method for the discrimination of tyrosine. By investigating the signals from 12 amino acids, only three amino acids, each containing a phenyl group, could be sensed selectively by this sensing phase. Based on the unique absorption of tyrosine located at 1515 cm-1, tyrosine can be selectively detected. To perform quantitative analyses of tyrosine using this sensing phase, a theoretical working equation was developed and correlated with the experimental data. The analytical results indicated that the developed equations do explain and predict the detection behaviors of the proposed sensing scheme. Using the optimal conditions, the regression coefficients for standard curves of tyrosine recorded in the region of concentrations below 600 microM were higher than 0.996 under either equilibrium or non-equilibrium conditions. Detection limit of tyrosine when using this method was ca. 3 microM.

Thermodynamic Measurements and Predictions of the Adsorption of Short-Chain Peptides on Nanothin Polymer Films

This contribution describes experimental measurements of submolecular-level interaction energies involved in the process of peptide adsorption on polymer films. The objective of this study was to use surface plasmon resonance (SPR) spectroscopy to measure the Gibbs energy change on adsorption (DeltaG(ad)) for pairs of various homopeptides on highly uniform, nanothin polymer films and to use these data, along with the principle of additivity, to predict DeltaG(ad) for homologous homopeptides, as well as for a mixed-residue peptide. By using a graft polymerization methodology, a nanothin poly(2-vinylpyridine) film was prepared and adsorption energies were measured first for a homologous series of tyrosine (Y) homopeptides on this film to determine submolecular-level interaction energies. By using SPR, adsorption isotherms were measured for YY and YYY peptides; analysis of these isotherms provided DeltaG(ad) data for a midchain tyrosine unit and a set of chain-end tyrosine units; values were -0.75 +/- 0.07 kcal/mol and -2.12 +/- 0.04 kcal/mol, respectively. Combining the thermodynamic contributions for adsorption of individual tyrosine units allowed a predictive estimate of -5.12 +/- 0.32 kcal/mol for the adsorption energy for YYYYYY; this estimate deviated by only 2.3% from its measured value of -5.24 +/- 0.06 kcal/mol. Similarly, adsorption energies were found for phenylalanine, glycine, and tyrosine-leucine peptides. Combining the thermodynamic contributions for adsorption of individual residue units allowed a predictive estimate of -3.24 +/- 0.38 kcal/mol for a pentapeptide, leucine enkephalin; this estimate deviated by only 3.0% from its measured value of -3.34 +/-0.11 kcal/mol.

Mannich reaction: an approach for the synthesis of water soluble mulundocandin analogues

Semisynthetic modifications at Hydroxy tyrosine (Htyr) unit of mulundocandin ( 1) were carried out to improve its aqueous solubility. A single step introduction of substituted aminomethyl groups at the ortho position(s) of phenolic hydroxyl of HTyr unit of mulundocandin has been achieved in 7–85% yield. The in vitro screening of Mannich products against Candida albicans and Aspergillus fumigatus, retained the in vivo activity of parent by oral and intraperitoneal route. Compound 20, showed significant improvement in activity over mulundocandin ( 1) and activity compares well with that of fluconazole.

Tyrosine-bearing polyphosphazenes.

Tyrosine-functionalized polyphosphazenes were synthesized, and their hydrolytic stability, pH-sensitive behavior, and hydrogel-forming capabilities were investigated. The physical and chemical properties of the polymers varied with the type of linkage between the tyrosine unit and phosphazene backbone. Poly[(ethyl glycinat-N-yl)(ethyl tyrosinat-N-yl)phophazenes] (linkage via the amino group of tyrosine) were found to be hydrolytically erodible. The rate of hydrolysis was dependent on the ratio of the two side groups, the slowest rate being associated with the highest concentration of tyrosine. The hydrolysis products were identified as phosphates, tyrosine, glycine, ammonia, and ethanol derived from the ester group. The hydrolytically stable phenolic-linked tyrosine derivatives were prepared from N-t-BOC-L-tyrosine methyl ester and alkoxy-based cosubstituents. Polyphosphazenes with both propoxy and phenolic-linked tyrosine side groups showed a pH-sensitive solubility behavior, which was dependent on the ratio and nature of the two side groups. The polymer was soluble in aqueous media below pH 3 and above pH 4. From pH 3-4, the polymer was insoluble. Replacement of propoxy by trifluoroethoxy units yielded a polymer that was insoluble in aqueous media at all pH values. Replacement of propoxy by methoxyethoxyethoxy groups gave a polymer that was soluble at all pH values. Exposure of both the propoxy and methoxyethoxyethoxy polymers to calcium ions in aqueous media caused gel formation due to ionic cross-linking through the carboxylate groups.

Enzymatically degradable prodrugs: A novel methodology for drug linkage

We have synthesized a novel enzymatically degradable prodrug system based on poly(ethylene glycol) (PEG) and tyrosine units by employing a synthetic methodology which eliminated the use of conventional blocking and deblocking methodology used for chemical linkage of drug molecule to the pendant NH2 group of amino acid. A diester of PEG (6 kDa) and tyrosine hydrochloride was synthesized by dicyclohexyl carbodiimide (DCC)-mediated condensation. In the second stage, oligomers were prepared by condensing phenolic OH groups of tyrosine in the diester with sebacic acid, using DCC. Finally, the hydrochloride salt of tyrosine in the oligomer was treated with triethylamine to activate NH2 groups, which were reacted with benzoyl chloride to obtain a model prodrug system. The products synthesized were characterized by IR, 1H-NMR, and GPC. The spectral data were in accordance with the proposed structures of products. Chymotrypsin-catalyzed degradation of the oligomers was characterized by both MW measurements and Ninhydrin assay for free tyrosine. Degradation studies indicated that the rate of main-chain degradation (ester hydrolysis) is higher than that of the side chain (amide hydrolysis). This new, simple methodology should be useful for conjugating a variety of bioactive molecules to enzymatically degradable PEG–amino acid based polymers. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2108–2118, 2002

Dendritic encapsulation of amino acid–metal complexes. Synthesis and studies of dendron-functionalized l-tyrosine–metal (ZnII, CoII) complexes

This paper describes the synthesis, characterization and studies of dendrimers possessing an amino acid–metal complex as the core. Using Fréchet-type polyaryl ether dendrons, L-tyrosine–metal (ZnII and CoII) complex cored dendrimers of 0–4 generations were synthesized. The metal complexation of the tyrosine unit at the focal point of these dendrons took place smoothly, in excellent yields, even though the sizes of the dendrons increase as the generations advance. Spectrophotometric titrations with CoII metal ion confirmed the formation of a 2 ∶ 1 dendritic ligand to metal complex and the existence of a pseudotetrahedral geometry at the metal centre is also inferred. Cyclic voltammetric studies of dendrimer–CoII complexes showed that while the electron transfer of CoII to CoI was facile for generations 0–2, such a process was difficult with generations 3 and 4, indicating a rigid encapsulation of the metal ion centre by proximal dendron groups. Further reduction of CoI to Co0 and the corresponding oxidation processes appear to be limited by adsorption at the surfaces of the electrodes.

Compositions of Highly Dispersed Silica with Trypsin and a Cationic Surfactant Obtained by a Mechanosorption Method

Powdered drug preparations usually appear as mechanical mixtures of parent compounds with inert fillers, ground down to micron particle size. The process of dispersing solid substances is one of the major processing stages in drug fabrication technology. This processing significantly modifies the physicochemical properties of substances and, as a rule, considerably increases their specific surface areas. The process of joint milling usually not only distributes the parent compound between filler particles but leads to their mutual adsorption as well. Being simple and technologically flexible, the method of adsorption immobilization is a promising approach to creating combined drug preparations. An additional advantage is the absence of foreign substances, which considerably simplifies toxicological testing of the products. Previously [1, 2] it was established that the adsorption of the proteolytic enzyme trypsin on the surface of a highly dispersed silica (Aerosil) from an aqueous medium leads to partial loss of the catalytic activity of the enzyme. At the same time, it was shown [3] that the cationic surfactants ethonium and decamethoxine lead to a significant stabilization of trypsin and chymotrypsin in solution, but the mechanism of this effect is still incompletely clear. The purpose of this work was to study the effect of cationic surfactants on the stability of trypsin comminuted jointly with a highly dispersed silica in a ball mill, with a view to the development of a technology for enzyme-containing drug compositions. The experiments were performed with commercial crystalline trypsin (Rudikom, Kiev) possessing a specific activity of not less than 10 TUg (TU = tyrosine unit of activity); a highly dispersed silica in the form of Aerosil A-300 grade (Khlorvinyl Corporation, Kalush, Ukraine); and pharmacopoeial preparations of ethonium and decamethoxine. The mixtures were mechanically treated in a ball mill with a drum volume of 0.9 dm 3 operating at a drum rotation speed

Towards artificial photosynthesis: ruthenium–manganese chemistry for energy production

The synthesis and characterisation of supramolecular model systems mimicking the light reactions on the donor side of Photosystem II (PSII) in green plants have been reviewed. In these systems, manganese complexes and tyrosine are electron donors, modelling the manganese cluster and tyrosineZ in PSII. The donors have been covalently linked to a photosensitizer, a ruthenium(II) tris-bipyridyl complex, that plays the role of the P680 chlorophylls in PSII. It has been demonstrated that, in the presence of an external electron acceptor in solution, the model systems can undergo an intermolecular electron transfer from the photoexcited state of RuII to an acceptor, followed by an intramolecular electron transfer from the coordinated Mn complexes or the tyrosine unit to the photogenerated RuIII. This leads to regeneration of the RuII and oxidation of the Mn complexes or generation of a tyrosine radical. The process closely mimics the primary reaction steps on the donor side of PSII.

Towards an artificial model for Photosystem II: a manganese(II,II) dimer covalently linked to ruthenium(II) tris-bipyridine via a tyrosine derivative

In order to model the individual electron transfer steps from the manganese cluster to the photooxidized sensitizer P 680 + in Photosystem II (PS II) in green plants, the supramolecular complex 4 has been synthesized. In this complex, a ruthenium(II) tris-bipyridine type photosensitizer has been linked to a manganese(II) dimer via a substituted l-tyrosine, which bridges the manganese ions. The trinuclear complex 4 was characterized by electron paramagnetic resonance (EPR) and electrospray ionization mass spectrometry (ESI-MS). The excited state lifetime of the the ruthenium tris-bipyridine moiety in 4 was found to be about 110 ns in acetonitrile. Using flash photolysis in the presence of an electron acceptor (methylviologen), it was demonstrated that in the supramolecular complex 4 an electron was transferred from the excited state of the ruthenium tris-bipyridine moiety to methylviologen, forming a methylviologen radical and a ruthenium(III) tris-bipyridine moiety. Next, the Ru(III) species retrieved the electron from the manganese(II/II) dimer in an intramolecular electron transfer reaction with a rate constant k ET>1.0×10 7 s −1, generating a manganese(II/III) oxidation state and regenerating the ruthenium(II) photosensitizer. This is the first example of intramolecular electron transfer in a supramolecular complex, in which a manganese dimer is covalently linked to a photosensitizer via a tyrosine unit, in a process which mimics the electron transfer on the donor side of PS II.

Hydrogen-Bond Promoted Intramolecular Electron Transfer to Photogenerated Ru(III): A Functional Mimic of TyrosineZ and Histidine 190 in Photosystem II

As a model for redox components on the donor side of photosystem II (PS II) in green plants, a supramolecular complex 4 has been prepared. In this, a ruthenium(II) tris-bipyridyl complex which mimics the function of P680 in PS II, has been covalently linked to a tyrosine unit which bears two hydrogen-bonding substituents, dipicolylamine (dpa) ligands. Our aim is to mimic the interaction between tyrosineZ and a basic histidine residue, namely His190 in PSII, and also to use the dpa ligands for coordination of manganese. Two different routes for the synthesis of the compound 4 are presented. Its structure was fully characterized by 1H NMR, COSY, NOESY, 13C NMR, IR, and mass spectrometry. 1H NMR and NOESY gave evidence for the existence of intramolecular hydrogen bonding in 4. The interaction between the ruthenium and the substituted tyrosine unit was probed by steady-state and time-resolved emission measurements as well as by chemical oxidation. Flash photolysis and EPR measurements on 4 in the presence of an electron acceptor (methylviologen, MV2+, or cobalt pentaminechloride, Co3+) showed that an intermolecular electron transfer from the excited state of Ru(II) in 4 to the electron acceptor took place, forming Ru(III) and the methylviologen radical MV+ or Co2+. This was followed by intramolecular electron transfer from the substituted tyrosine moiety to the photogenerated Ru(III), regenerating Ru(II) and forming a tyrosyl radical. In water, the radical has a g value of 2.0044, indicative of a deprotonated tyrosyl radical. In acetonitrile, a radical with a g value of 2.0029 was formed, which can be assigned to the tyrosine radical cation. In both solvents the electron transfer is intramolecular with a rate constant kET > 1 × 107 s-1. This is 2 orders of magnitude greater than the one for a similar compound 3, in which no dpa arm is attached to the tyrosine unit. Therefore the hydrogen bonding between the substituted tyrosine and the dpa arms in 4 is proposed to be responsible for the fast electron transfer. This interaction mimics the proposed His190 and tyrosineZ interaction in the donor side of PS II. (Less)