Viktor Farkas

On the Mechanism of Bifunctional Squaramide-Catalyzed Organocatalytic Michael Addition: A Protonated Catalyst as an Oxyanion Hole

A joint experimental-theoretical study of a bifunctional squaramide-amine-catalyzed Michael addition reaction between 1,3-dioxo nucleophiles and nitrostyrene has been undertaken to gain insight into the nature of bifunctional organocatalytic activation. For this highly stereoselective reaction, three previously proposed mechanistic scenarios for the critical CC bond-formation step were examined. Accordingly, the formation of the major stereoisomeric products is most plausible by one of the bifunctional pathways that involve electrophile activation by the protonated amine group of the catalyst. However, some of the minor product isomers are also accessible through alternative reaction routes. Structural analysis of transition states points to the structural invariance of certain fragments of the transition state, such as the protonated catalyst and the anionic fragment of approaching reactants. Our topological analysis provides deeper insight and a more general understanding of bifunctional noncovalent organocatalysis.

Cooperativity network of Trp-cage miniproteins: probing salt-bridges

Trp‐cage miniprotein was used to investigate the role of a salt‐bridge (Asp9–Arg16) in protein formation, by mutating residues at both sides, we mapped its contribution to overall stability and its role in folding mechanism. We found that both of the above side‐chains are also part of a dense interaction network composed of electrostatic, H‐bonding, hydrophobic, etc. components. To elucidate the fold stabilizing effects, we compared and contrasted electronic circular dichroism and NMR data of miniproteins equipped with a salt‐bridge with those of the salt‐bridge deleted mutants. Data were acquired both in neutral and in acidic aqueous solutions to decipher the pH dependency of both fully and partially charged partners. Our results indicate that the folding of Trp‐cage miniproteins is more complex than a simple two‐state process as we detected an intermediate state that differs significantly from the native fold. The intermediate formation is related to the salt‐bridge stabilization; in the miniprotein variants equipped with salt‐bridge the population of the intermediate state at acidic pH is significantly higher than it is for the salt‐bridge deleted mutants. In this molecular framework Arg16 stabilizes more than Asp9 does, because of its higher degree of 3D‐fold cooperation. In conclusion, the Xxx

Structure, enzymatic stability and antitumor activity of sea lamprey GnRH-III and its dimer derivatives.

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Chiral Recognition Studies of α-(Nonafluoro- tert -butoxy)carboxylic Acids by NMR Spectroscopy

Yyy16 salt‐bridge is not an isolated entity of this fold; rather it is an integrated part of a complex interaction network. Copyright

Tryptophan to phenylalanine substitutions allow differentiation of short- and long-range conformational changes during denaturation of goat α-lactalbumin

Direct antitumor activity of sea lamprey (Petromyzon marinus) gonadotropin-releasing hormone III (Glp-His-Trp-Ser-His-Asp-Trp-Lys-Pro-Gly-NH(2); lGnRH-III) was described on several tumor cells. To improve the selectivity of antitumor effects without increasing the hormone releasing activity and to enhance the enzymatic stability, lGnRH-III dimers were prepared via disulfide bond formation. Our results demonstrate that the lGnRH-III dimer derivatives exhibited higher antiproliferative effect and enzymatic stability in comparison with the native lGnRH-III, while lower LH-releasing potency was determined. In order to find a correlation between the biological and structural features of these compounds, the conformation of lGnRH-III and its dimer derivatives was determined by ECD, VCD, FT-IR and (1)H NMR.

Rational design of α-helix-stabilized exendin-4 analogues.

Three chiral α-(nonafluoro-tert-butoxy)carboxylic acids (R)-1, (RS)-2, (R)-3 were synthesized to examine their application as chiral solvating agents with amines. As a model compound, first (S)- and/or (RS)-α-phenylethylamine was used, and their diastereomeric salts were investigated by (1)H and (19)F NMR and ECD spectroscopy. The NMR spectroscopic studies were carried out at room temperature using the slightly polar CDCl3 and apolar C6D6 as solvents in 5 mM and 54 mM concentrations. The difference of the chemical shifts (Δδ) in the diastereomeric complexes is comparable with other, well-known chiral derivatizing and solvating agents (e.g., Moshers acid, Pirkles alcohol). Diastereomeric salts of racemic acids (RS)-1 and (RS)-2 with biologically active amines (1R,2S)-ephedrine and (S)-dapoxetine were also investigated by (19)F NMR spectroscopy.

C-3 epimers of sugar amino acids as foldameric building blocks: improved synthesis, useful derivatives, coupling strategies

To test the occurrence of local particularities during the unfolding of Ca2+‐loaded goat α‐lactalbumin (GLA) we replaced Trp60 and ‐118, either one or both, by Phe. In contrast with alternative studies, our recombinant α‐lactalbumins are expressed in Pichia pastoris and do not contain the extra N‐terminal methionine. The substitution of Trp60 leads to a reduction of the global stability. The effect of the Trp118Phe substitution on the conformation and stability of the mutant, however, is negligible. Comparison of the fluorescence spectra of these mutants makes clear that Trp60 and ‐118 are strongly quenched in the native state. They both contribute to the quenching of Trp26 and ‐104 emission. By the interplay of these quenching effects, the fluorescence intensity changes upon thermal unfolding of the mutants behave very differently. This is the reason for a discrepancy of the apparent transition temperatures derived from the shift of the emission maxima (Tm,Fl λ) and those derived from DSC (Tm,DSC). However, the transition temperatures derived from fluorescence intensity (Tm,Fl int) and from DSC (Tm,DSC), respectively, are quite similar, and thus, no local rearrangements are observed upon heat‐induced unfolding. At room temperature, the occurrence of specific local rearrangements upon GdnHCl‐induced denaturation of the different mutants is deduced from the apparent free energies of their transition state obtained from stopped‐flow fluorescence measurements. By ϕ‡‐value analysis it appears that, while the surroundings of Trp118 are exposed in the kinetic transition state, the surroundings of Trp60 remain native. Proteins 2005.

Origin of problems related to Staudinger reduction in carbopeptoid syntheses

Exendin-4 (Ex4) is a potent glucagon-like peptide-1 receptor agonist, a drug regulating the plasma glucose level of patients suffering from type 2 diabetes. The molecules poor solubility and its readiness to form aggregates increase the likelihood of unwanted side effects. Therefore, we designed Ex4 analogues with improved structural characteristics and better water solubility. Rational design was started from the parent 20-amino acid, well-folded Trp cage (TC) miniprotein and involved the step-by-step N-terminal elongation of the TC head, resulting in the 39-amino acid Ex4 analogue, E19. Helical propensity coupled to tertiary structure compactness was monitored and quantitatively analyzed by electronic circular dichroism and nuclear magnetic resonance (NMR) spectroscopy for the 14 peptides of different lengths. Both (15)N relaxation- and diffusion-ordered NMR measurements were established to investigate the inherent mobility and self-association propensity of Ex4 and E19. Our designed E19 molecule has the same tertiary structure as Ex4 but is more helical than Ex4 under all studied conditions; it is less prone to oligomerization and has preserved biological activity. These conditions make E19 a perfect lead compound for further drug discovery. We believe that this structural study improves our understanding of the relationship between local molecular features and global physicochemical properties such as water solubility and could help in the development of more potent Ex4 analogues with improved pharmacokinetic properties.

Synthesis and in vitro biochemical evaluation of oxime bond-linked daunorubicin–GnRH-III conjugates developed for targeted drug delivery

To obtain key sugar derivatives for making homooligomeric foldamers or α/β-chimera peptides, economic and multigram scale synthetic methods were to be developed. Though described in the literature, the cost-effective making of both 3-amino-3-deoxy-ribofuranuronic acid (H–tX–OH) and its C-3 epimeric stereoisomer, the 3-amino-3-deoxy-xylofuranuronic acid (H–cX–OH) from d-glucose is described here. The present synthetic route elaborated is (1) appropriate for large-scale synthesis; (2) reagent costs reduced (e.g. by a factor of 400); (3) yields optimized are ~80% or higher for all six consecutive steps concluding –tX– or –cX– and (4) reaction times shortened. Thus, a new synthetic route step-by-step optimized for yield, cost, time and purification is given both for d-xylo and d-ribo-amino-furanuronic acids using sustainable chemistry (e.g. less chromatography with organic solvents; using continuous-flow reactor). Our study encompasses necessary building blocks (e.g. –X–OMe, –X–OiPr, –X–NHMe, Fmoc–X–OH) and key coupling reactions making –Aaa–tX–Aaa– or –Aaa–tX–tX–Aaa– type “inserts”. Completed for both stereoisomers of X, including the newly synthesized Fmoc–cX–OH, producing longer oligomers for drug design and discovery is more of a reality than a wish.

Cooperation between a salt bridge and the hydrophobic core triggers fold stabilization in a Trp-cage miniprotein.

We report the solid phase synthesis of –GG-X-GG– type α/β-carbopeptoids incorporating RibAFU(ip) (1a, tX) or XylAFU(ip) (2a, cX) sugar amino acids. Though coupling efficacy is moderate, both the lengthier synthetic route using Fmoc derivative (e.g., Fmoc-RibAFU(ip)-OH) and the azido derivative (e.g., N3-RibAFU(ip)-OH) via Staudinger reaction with nBu3P can be successfully applied. Both X-ray diffraction, 1H- and 31P-NMR, and theoretical (QM) data support and explain why the application of Ph3P as Staudinger reagent is “ineffective” in the case of a cis stereoisomer, if cX is attached to the preceding residue with a peptide (–CONH–) bond. The failure of the polypeptide chain elongation with N3-cX originates from the “coincidence” of a steric crowdedness and an electronic effect disabling the mandatory nucleophilic attack during the hydrolysis of a quasi penta-coordinated triphenylphosphinimine. Nevertheless, the synthesis of the above α/β-chimera peptides as completed now by a new pathway via 1,2-O-isopropylidene-3-azido-3-deoxy-ribo- and -xylo-furanuronic acid (H-RibAFU(ip)-OH 1a and H-XylAFU(ip)-OH 2a) coupled with N-protected α-amino acids on solid phase could serve as useful examples and starting points of further synthetic efforts.