Thiourea derivatives as chelating agents for bioconjugation of rhenium and technetium.

Potential tetradentate thiocarbamoylbenzamidine derivatives H4L have been synthesized from the corresponding benzimidoyl chlorides and triglycine. They are suitable chelating agents for the oxidotechnetium(v) and oxidorhenium(v) cores and form stable, neutral [MO(HL)] complexes with an equatorial SN3 coordination sphere and an additional, uncoordinated carboxylic group, which can be used for bioconjugation. Representatives of the rhenium and 99Tc products have been isolated and analyzed with spectroscopic methods and X-ray diffraction. Bioconjugates of these complexes with angiotensin-II have been synthesized and structurally characterized. Analogous 99mTc complexes have been produced and tested in vitro and in vivo. The experiments confirm a considerable stability for the [99mTc(HL)] product as well as for its bioconjugate and recommend this class of compounds for further bioconjugation studies towards clinical applications.

Exploiting Oligo(amido amine) Backbones for the Multivalent Presentation of Coiled-Coil Peptides.

The investigation of coiled coil formation for one mono- and two divalent peptide-polymer conjugates is presented. Through the assembly of the full conjugates on solid support, monodisperse sequence-defined conjugates are obtained with defined positions and distances between the peptide side chains along the polymeric backbone. A heteromeric peptide design was chosen, where peptide K is attached to the polymer backbone, and coiled-coil formation is only expected through complexation with the complementary peptide E. Indeed, the monovalent peptide K-polymer conjugate displays rapid coiled-coil formation when mixed with the complementary peptide E sequence. The divalent systems show intramolecular homomeric coiled-coil formation on the polymer backbone despite the peptide design. Interestingly, this intramolecular assembly undergoes a conformational rearrangement by the addition of the complementary peptide E leading to the formation of heteromeric coiled coil-polymer aggregates. The polymer backbone acts as a template bringing the covalently bound peptide strands in close proximity to each other, increasing the local concentration and inducing the otherwise nonfavorable formation of intramolecular helical assemblies.

Influence of the hydrophobic interface and transition metal ions on the conformation of amyloidogenic model peptides.

The transition of alpha-helical or unfolded peptides and proteins to beta-sheets and the subsequent amyloid formation are characteristic for neurodegenerative diseases like Alzheimer's or Parkinson's disease. The interactions of amyloidogenic peptides with surfaces such as biological membranes are considered to play an important role regarding the onset of secondary structure changes. In our project, we used a peptide designed to have specific secondary structure propensities in order to investigate the driving forces and conditions which lead to the beta-sheet formation. The model peptide is able to adopt the coiled coil conformation, alpha-helical peptide strands that wind around each other in a superhelical structure. In addition to building principles stabilizing this alpha-helical conformation it also has beta-sheet stabilizing features. We focused on the interactions of the peptide with the hydrophobic air-water interface. Infrared reflection absorption spectroscopy was used as a surface sensitive method and complemented with grazing incidence X-ray diffraction and reflectivity. Furthermore, the model peptide provides metal binding sites. The binding of transition metal ions leads to a local preference of certain secondary structure elements, depending on the metal ion and the geometry of metal ion binding sites. The interplay and competition of the two trigger mechanisms (1) interaction with surfaces and (2) metal ion complexation were investigated. We found that the secondary structure of the peptide strongly depends on the interactions with the hydrophobic air-water interface and the orientation imposed by it. The metal ions Zn(2+) and Cu(2+) were used for complexation. The structure of the peptide surface layer differs according to the bound metal ion.

Thyronamines are isozyme-specific substrates of deiodinases.

3-Iodothyronamine (3-T 1 AM) and thyronamine (T AM) are novel endogenous signaling molecules that exhibit great structural similarity to thyroid hormones but apparently antagonize classical thyroid hormone (T(3)) actions. Their proposed biosynthesis from thyroid hormones would require decarboxylation and more or less extensive deiodination. Deiodinases (Dio1, Dio2, and Dio3) catalyze the removal of iodine from their substrates. Because a role of deiodinases in thyronamine biosynthesis requires their ability to accept thyronamines as substrates, we investigated whether thyronamines are converted by deiodinases. Thyronamines were incubated with isozyme-specific deiodinase preparations. Deiodination products were analyzed using a newly established method applying liquid chromatography and tandem mass spectrometry (LC-MS/MS). Phenolic ring deiodinations of 3,3',5'-triiodothyronamine (rT3AM), 3',5'-diiodothyronamine (3',5'-T2AM), and 3,3'-diiodothyronamine (3,3'-T2AM) as well as tyrosyl ring deiodinations of 3,5,3'-triiodothyronamine (T3AM) and 3,5-diiodothyronamine (3,5-T2AM) were observed with Dio1. These reactions were completely inhibited by the Dio1-specific inhibitor 6n-propyl-2-thiouracil (PTU). Dio2 containing preparations also deiodinated rT(3)AM and 3',5'-T2AM at the phenolic rings but in a PTU-insensitive fashion. All thyronamines with tyrosyl ring iodine atoms were 5(3)-deiodinated by Dio3-containing preparations. In functional competition assays, the newly identified thyronamine substrates inhibited an established iodothyronine deiodination reaction. By contrast, thyronamines that had been excluded as deiodinase substrates in LC-MS/MS experiments failed to show any effect in the competition assays, thus verifying the former results. These data support a role for deiodinases in thyronamine biosynthesis and contribute to confining the biosynthetic pathways for 3-T 1 AM and T 0 AM.

Synthesis of alpha-trifluoromethyl alpha-amino acids with aromatic, heteroaromatic and ferrocenyl subunits in the side chain.

5-Benzyloxy-4-trifluoromethyl-1,3-oxazoles, obtained from 5-fluoro-4-trifluoromethyloxazoles and benzyl alcohols, are capable for rearrangements. A 1,3 shift of a benzyl group is the key step of a new general route toward alpha-trifluoromethyl substituted aromatic and heteroaromatic amino acids, demonstrating that 5-fluoro-4-trifluoromethyl-1,3-oxazole is a synthetic Tfm-Gly equivalent. On reaction with benzpinacol partially fluorinated oxazoles are transformed into bis(trifluoromethyl) substituted 2,5-diamino adipic acid and N-benzoyl-2-benzhydryl-3,3,3-trifluoroalanine.

Domino reactions with fluorinated five-membered heterocycles. alpha-Trifluoromethyl alpha-amino acids with unsaturated side-chains.

Alpha-trifluoromethyl alpha-amino acids with unsaturated side-chains have been prepared from 5-fluoro-4-trifluoromethyloxazole and allyl, propargyl as well as terpene alcohols in a one-pot procedure.

First synthesis of totally orthogonal protected alpha-(trifluoromethyl)- and alpha-(difluoromethyl)arginines.

The first synthesis of a series of totally orthogonal protected racemic alpha-(trifluoromethyl)- and alpha-(difluoromethyl)arginines is described. The key steps of the synthesis are the mild guanidinylation procedure and the selective hydrogenation of a CC triple bond in the presence of a Cbz-group.

Proteolytically stable peptides by incorporation of alpha-Tfm amino acids.

A series of model peptides containing alpha-trifluoromethyl-substituted amino acids in five different positions relative to the predominant cleavage site of the serine protease alpha-chymotrypsin was synthesized by solution methods to investigate the influence of alpha-Tfm substitution on the proteolytic stability of peptides. Proteolysis studies demonstrated absolute stability of peptides substituted to the P1 position and still considerable proteolytic stability for peptides substituted at the P2 and P'2 positions compared with the corresponding unsubstituted model peptide. Comparison with peptides containing the fluorine-free disubstituted amino acid alpha-aminoisobutyric acid allowed to separate electronic from steric effects. Furthermore, the absolute configuration of the alpha-Tfm-substituted amino acid was found to exert considerable effects on the proteolytic stability, especially in P'1 substituted peptides. Investigations of this phenomenon using empirical force field calculations revealed that in the (S,R,S)-diasteromer the steric constraints exhibited by the alpha-Tfm group can be outweighed by an advantageous interaction of the flourine atoms with the serine side chain of the enzyme. In contrast, a favourable interaction between substrate and enzyme is impossible for the (S,S,S)-diastereomer.

Peptide modification by incorporation ofα-trifluoromethyl substituted amino acids.

Metabolic stabilization of pharmacologically active peptides can be achieved by incorporation of sterically hindered non-natural amino acids, e.g. C (α,α) -disubstituted amino acids.α-Trifluoromethyl substituted amino acids, a subclass of C (α,α) -disubstituted amino acids, also fulfil this requirement while featuring additional properties based on the electronic influence of the fluorine substituents.This review summarizes the results concerning the stability of peptides containingα-TFM amino acids towards proteolysis byα-chymotrypsin. Furthermore, configurational effects ofα-TFMAla on the proteolytic stability of peptides are explained using empirical force field calculations. The influence ofα-TFMAla incorporation on the secondary structure of selected tripeptide amides is compared to the effects exerted by its fluorine-free analogue, aminoisobutyric acid.Finally, results on metabolic stabilization and biological activity of modified thyrotropin releasing hormone are interpreted.

Peptide modification by introduction ofα-trifluoromethyl substituted amino acids.

Methodology for the synthesis and incorporation ofα-trifluoromethyl substituted amino acids into N- and C-terminal position of peptides is described. The incorporation ofα-trifluoromethyl substituted amino acids into strategical positions of peptides enhances proteolytic stability and lipophilicity. Furthermore, it improves transport rates in vivo and permeability through certain body barriers.