Solid-Phase Synthesis of Chiral Modular Ferrocene-Based Peptides

Crystal structures of 1'-amino-cobaltocenium-1-carb-oxy-lic acid chloride monohydrate and of its azo dye 1'-[2-(1-amino-2,6-dimethylphenyl)diazen-1-yl]cobaltocenium-1-carb-oxy-lic acid hexa-fluorido-phosphate monohydrate

1'-Amino-cobaltocenium-1-carb-oxy-lic acid chloride, [Co(C5H6N)(C6H5O2)]Cl·H2O, (3), and its azo derivative 1'-[2-(1-amino-2,6-dimethylphenyl)diazen-1-yl]cobaltocenium-1-carb-oxy-lic acid hexa-fluorido-phosphate, [Co(C13H14N3)(C6H5O2)]PF6·H2O (5) were obtained from cobaltocenium-1,1'-di-carb-oxy-lic acid hexa-fluorido-phosphate by converting one carboxyl group to its chloro-carboxyl derivative followed by chloride/azide exchange, Curtius rearrangement, diazo-tiation and azo coupling with 2,6-di-methyl-aniline. Both title compounds crystallize as their monohydrates. In the crystal structure of 3, both functional groups lie in the same direction, with the Cp rings being nearly eclipsed, and participate in an extended hydrogen-bonded supra-molecular network including the counter-ion and the water mol-ecule of crystallization. Although the functional groups in 5 are somewhat further apart, bearing a greater torsion angle with the Cp rings now staggered, a similar supra-molecular network is observed with not only the carb-oxy-lic acid and azo groups, but also with the more remote amino group participating in a hydrogen-bonded network, again including the counter-ion and the water mol-ecule. The hexa-fluorido-phosphate ion shows positional disorder. Compound 3 was refined as an inversion twin. In 5, each of the six F atoms is disordered over two sets of sites in a 1:1 ratio.

Application of the Curtius rearrangement to the synthesis of 1′-aminoferrocene-1-carboxylic acid derivatives

The shortest synthesis of N-protected 1′-aminoferrocene-1-carboxylic acid from readily available ferrocene-1,1′-dicarboxylic acid is reported.

Force-Field Development and Molecular Dynamics Simulations of Ferrocene–Peptide Conjugates as a Scaffold for Hydrogenase Mimics

The increasing importance of hydrogenase enzymes in the new energy research field has led us to examine the structure and dynamics of potential hydrogenase mimics, based on a ferrocene-peptide scaffold, using molecular dynamics (MD) simulations. To enable this MD study, a molecular mechanics force field for ferrocene-bearing peptides was developed and implemented in the CHARMM simulation package, thus extending the usefulness of the package into peptide-bioorganometallic chemistry. Using the automated frequency-matching method (AFMM), optimized intramolecular force-field parameters were generated through quantum chemical reference normal modes. The partial charges for ferrocene were derived by fitting point charges to quantum-chemically computed electrostatic potentials. The force field was tested against experimental X-ray crystal structures of dipeptide derivatives of ferrocene-1,1'-dicarboxylic acid. The calculations reproduce accurately the molecular geometries, including the characteristic C2-symmetrical intramolecular hydrogen-bonding pattern, that were stable over 0.1 micros MD simulations. The crystal packing properties of ferrocene-1-(D)alanine-(D)proline-1'-(D)alanine-(D)proline were also accurately reproduced. The lattice parameters of this crystal were conserved during a 0.1 micros MD simulation and match the experimental values almost exactly. Simulations of the peptides in dichloromethane are also in good agreement with experimental NMR and circular dichroism (CD) data in solution. The developed force field was used to perform MD simulations on novel, as yet unsynthesized peptide fragments that surround the active site of [Ni-Fe] hydrogenase. The results of this simulation lead us to propose an improved design for synthetic peptide-based hydrogenase models. The presented MD simulation results of metallocenes thereby provide a convincing validation of our proposal to use ferrocene-peptides as minimal enzyme mimics.

A Bioorganometallic Approach for the Electrochemical Detection of Proteins: A Study on the Interaction of Ferrocene–Peptide Conjugates with Papain in Solution and on Au Surfaces

In this paper, a new bioorganometallic approach for the detection of proteins using surface-bound ferrocene-peptide conjugates is presented. Specifically, a series of peptide conjugates of 1'-aminoferrocene-1-carboxylic acid (ferrocene amino acid, Fca) is synthesized: Boc-Fca-Gly-Gly-Tyr(Bzl)-Arg(NO2)-OMe (2), Thc-Fca-Gly-Gly-Tyr(Bzl)-Arg(NO2)-OMe (3), Thc-Fca-Gly-Gly-Tyr(Bzl)-Arg(NO2)-OH (4), Boc-Fca-Gly-Gly-Arg(Mtr)-Tyr-OMe (7), Thc-Fca-Gly-Gly-Arg(Mtr)-Tyr-OMe (8), Thc-Fca-Gly-Gly-Arg(Mtr)-Tyr-OH (9), Thc-Fca-Gly-Gly-Arg-Tyr-OH (10). The peptide is conjugated to the C-terminal side of Fca and compounds 4, 7-10 possess a thiostic acid linked to the N-terminal side of Fca in order to facilitate formation of thin films on gold substrates. Competitive inhibition towards papain was determined for Thc-Fca-Gly-Gly-Tyr(Bzl)-Arg(NO2)-OH (4), Thc-Fca-Gly-Gly-Arg(Mtr)-Tyr-OH (9) and Thc-Fca-Gly-Gly-Arg-Tyr-OH (10). The binding interaction between the peptide modified substrates and papain enzyme was studied using real-time surface plasmon resonance (SPR) imaging. Peptide 10 showed the strongest binding affinity to papain. Adsorption/desorption rate constants were ka = 1.75+/-0.05 x 10(5) M(-1) s(-1) and kd = 2.90 +/- 0.05 x 10(-2) s(-1). Interactions of papain with Fca-peptide 10 were investigated by cyclic voltammetry. The interaction results were also verified by measuring the electrochemical response of the peptide-papain interaction as function of increasing enzyme concentration. A linear relationship was observed for papain concentration of up to 80 nM. Shifting to higher potentials as well as decrease in the overall signal intensity was observed. The detection limit was 4 x 10(-9) M.