Tyrosine properties

As a p hydroxy derivative of phenylalanine tyrosine has properties similar to those of phenylalanine plus the ability to engage m hydrogen bonding via its —OH group Asparagine and glutamine are not amines they are amides The side chains of both O... 

Kojima, S., et al. (1998). Fluorescent properties of model chromophores of tyrosine-66 substitute mutants of Aequorea green fluorescent protein (GFP). Tetrahedron Lett. 39 5239-5242. 

S-acylated proteins include many GTP-binding regulatory proteins (G proteins), including most a subunits of heterotrimeric G-proteins and also many members of the Ras superfamily of monomeric G proteins, a number of G protein-coupled receptors, several nonreceptor tyrosine kinases, and a number of other signaling molecules, -acylation is posttranslational and reversible, a property that allows the cell to control... 

Besides all the sensory and texturizing properties, GA has interesting antioxidant properties such as an efficient capacity for deactivation of excited electronic states and moderated radical scavenging capacity. There is increasing experimental evidence that associate the antioxidant function with its protein fraction, mainly by amino acid residues such as histidine, tyrosine and lysine, which are generally considered as antioxidants molecules (Marcuse, 1960,1962 Park et al., 2005). 

Several such polymers have by now been prepared and were found to possess a variety of interesting material properties. Tyrosine-derived poly(iminocarbonates) (see Sec. IV) would be a specific example. These polymers were synthesized by means of a polymerization reaction involving the two phenolic hydroxyl groups located on the side chains of a protected tyrosine dipeptide (12). 

FIGURE 6 Molecular structures of poIy(CTTE), poly(CTTH), and poly(CTTP), a homologous series of tyrosine-derived polymers used in a study of the effect of the C-terminus protecting group on the materials properties of the resulting polymers. Cbz" stands for the benzyloxycarbonyl group (47). 

FIGURE 7 Tyrosine-derived poly(iminocarbonates) used to evaluate the effect of various side chain configurations on the physicomechan-ical properties of the resulting polymers. 

In order to test the influence of the C-terminus protecting groups on the properties of the resulting polymer, the ethyl, hexyl, and palmityl esters of N-benzyloxycarbonyl-L-tyrosyl-L-tyrosine were synthesized and the corresponding polymers (poly(CTTE),... 

The thermal properties of tyrosine-derived poly(iminocarbonates) were also investigated. Based on analysis by DSC and thermogravi-metric analysis, all poly(iminocarbonates) decompose between 140 and 220 C. The thermal decomposition is due to the inherent instability of the iminocarbonate bond above 150°C and is not related to the presence of tyrosine derivatives in the polymer backbone. The molecular structure of the monomer has no significant influence on the degradation temperature as indicated by the fact that poly(BPA.-iminocarbonate) also decomposed at about 170 C, while the structurally analogous poly(BPA-carbonate) is thermally stable up to 350 C. 

The mechanical properties of tyrosine-derived poly(iminocarbon-ates) were investigated using the procedures described in ASTM standard D882-83 (Table 2). Solvent-cast, thin polymer films were prepared, cut into the required shape, and tested in an Instron stress strain tester. Since the films were unoriented, noncrystalUne samples, the results are representative of the bulk properties of the polymers. In order to put these results into perspective, several commercial polymers were tested under identical conditions. In addition, some literature values were included in Table 2. 

These results open the exciting possibility of using degradable, tyrosine-derived polymers as "custom-designed" antigen delivery devices. On the other hand, our results indicate that the immunological properties of tyrosine-derived polymers will have to be carefully evaluated before such polymers can be considered for use as drug delivery systems or medical implants. 

Furthermore, our results on the characterization of the physico-mechanical properties of tyrosine-derived poly(iminocarbonates) provide preliminary evidence for the soundness of the underlying experimental rationale The incorporation of tyrosine into the backbone of poly(iminocarbonates) did indeed result in the formation of mechanically strong yet apparently tissue-compatible polymers. 

The reinforeing properties of psychomotor stimulants have also been linked to the aetivation of eentral dopamine neurons and their postsynaptie reeep-tors. When the synthesis of eatecholamines is inhibited by administering alpha-methyl-para-tyrosine, an attenuation of the subjective effeets of euphoria assoeiated with psyehomotor stimulants oeeurs in man (Jonsson et al. 1971), and a bloekade of the reinforeing effects of methamphetamine occurs in animals (Pickens et al. 1968). Furthermore, low doses of dopamine antagonists will increase response rates for intravenous injections of h-amphetamine (Risner and Jones 1976 Yokel and Wise 1975 Yokel and Wise 1976). 

The oxidative polymerization of 5,6-dihydroxyindole (1) and related tyrosine-derived metabolites is a central, most elusive process in the biosynthesis of eumelanins, which are the characteristic pigments responsible for the dark color of human skin, hair, and eyes. Despite the intense experimental research for more than a century,36 the eumelanin structure remains uncharacterized because of the lack of defined physicochemical properties and the low solubility, which often prevents successful investigations by modem spectroscopic techniques. The starting step of the oxidative process is a one-electron oxidation of 5,6-dihydroxyindole generating the semiquinone 1-SQ (Scheme 2.7). 

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