The chemically stabilized somatostatin-derived cyclic octapeptide octreotate has a number of interesting applications in medicinal chemistry. Here, a number of different organometallic derivatives of octreotate were prepared, and their properties were investigated. Specifically, we report the synthesis and characterization of ruthenocene, ferrocene, and cobaltocenium octreotate derivatives and their fluorophore-labeled conjugates as well as a dicobalt hexacarbonyl alkyne functionalized octreotate. To provide further insights into their characteristics, the log P values and electrochemical properties of the novel metal conjugates were compared. For biological activity, we determined their toxicity in three different cell lines. Cellular uptake and colocalization of selected compounds were studied by fluorescence microscopy with particular focus on efficiency and specificity of their uptake through the somatostatin receptor SSTR to elucidate the value of the metallocene head group for its potential use as a nontoxic and universal peptide label.

Gold-surface grafted peptide nucleic acid (PNA) strands, which carry a redox-active ferrocene tag, present unique tools to electrochemically investigate their mechanical bending elasticity based on the kinetics of electron-transfer (ET) processes. A comparative study of the mechanical bending properties and the thermodynamic stability of a series of 12-mer Fc-PNA⋅DNA duplexes was carried out. A single basepair mismatch was integrated at all possible strand positions to provide nanoscopic insights into the physicochemical changes provoked by the presence of a single basepair mismatch with regard to its position within the strand. The ET processes at single mismatch Fc-PNA⋅DNA modified surfaces were found to proceed with increasing diffusion limitation and decreasing standard ET rate constants k⁰ when the single basepair mismatch was dislocated along the strand towards its free-dangling Fc-modified end. The observed ET characteristics are considered to be due to a punctual increase in the strand elasticity at the mismatch position. The kinetic mismatch discrimination with respect to the fully-complementary duplex presents a basis for an electrochemical DNA sensing strategy based on the Fc-PNA⋅DNA bending dynamics for loosely packed monolayers. In a general sense, the strand elasticity presents a further physicochemical property which is affected by a single basepair mismatch which may possibly be used as a basis for future DNA sensing concepts for the specific detection of single basepair mismatches.

N-Terminally ferrocenylated and C-terminally gold-surface-grafted peptide nucleic acid (PNA) strands were exploited as unique tools for the electrochemical investigation of the strand dynamics of short PNA(⋅DNA) duplexes. On the basis of the quantitative analysis of the kinetics and the diffusional characteristics of the electron-transfer process, a nanoscopic view of the Fc-PNA(⋅DNA) surface dynamics was obtained. Loosely packed, surface-confined Fc-PNA single strands were found to render the charge-transfer process of the tethered Fc moiety diffusion-limited, whereas surfaces modified with Fc-PNA⋅DNA duplexes exhibited a charge-transfer process with characteristics between the two extremes of diffusion and surface limitation. The interplay between the inherent strand elasticity and effects exerted by the electric field are supposed to dictate the probability of a sufficient approach of the Fc head group to the electrode surface, as reflected in the measured values of the electron-transfer rate constant, k⁰. An in-depth understanding of the dynamics of surface-bound PNA and PNA⋅DNA strands is of utmost importance for the development of DNA biosensors using (Fc-)PNA recognition layers.

A Fc–PNA biosensor (Fc: ferrocenyl, C₁₀H₉Fe) was designed by using two electrochemically distinguishable recognition elements with different molecular information at a single electrode. Two Fc–PNA capture probes were therefore synthesized by N-terminal labeling different dodecamer PNA sequences with different ferrocene derivatives by click chemistry. Each of the two strands was thereby tethered with one specific ferrocene derivative. The two capture probes revealed quasi-reversible redox processes of the Fc^0/+ redox couple with a significant difference in their electrochemical half-wave potentials of ΔE_1/2=160 mV. A carefully designed biosensor interface, consisting of a ternary self-assembled monolayer (SAM) of the two C-terminal cysteine-tethered Fc–PNA capture probes and 6-mercaptohexanol, was electrochemically investigated by square wave (SWV) and cyclic voltammetry (CV). The biosensor properties of this interface were analyzed by studying the interaction with DNA sequences that were complementary to either of the two capture probes by SWV. Based on distinct changes in both peak current and potential, a parallel identification of these two DNA sequences was successful with one interface design. Moreover, the primary electrochemical response could be converted by a simple mathematical analysis into a clear-cut electrochemical signal about the hybridization event. The discrimination of single-nucleotide polymorphism (SNP) was proven with a chosen single-mismatch DNA sequence. Furthermore, experiments with crude bacterial RNA confirm the principal suitability of this dual-potential sensor under real-life conditions.

Enkephalin peptides are thought to be suitable vectors for the passage of the blood–brain barrier (BBB). Modifications that do not alter the amino acid sequence are often used to improve the permeation through living membrane systems. As a new type of modification we introduce organometallic compounds, in particular ferrocene carboxylic acid. Derivatives of [Leu⁵]enkephalin were synthesised and labelled with organometallic compounds by using solid-phase synthesis techniques. All new metal–peptide bioconjugates were comprehensively characterised by HPLC, NMR spectroscopy and mass spectrometry and found to be at least 95 % pure. For the first time, permeation coefficients in a BBB model for organometal–peptide derivatives were determined in this work. The uptake and localisation of fluorescein-labelled enkephalins was monitored by fluorescence microscopy on three cancer cell lines. Octanol/H₂O partition coefficients of the compounds were measured by HPLC. The introduction of the organometallic moiety enhances the uptake into cells and the permeation coefficient of [Leu⁵]-enkephalin. This could be due to an increase in lipophilicity caused by the organometallic label. The metal–peptide conjugates were found to be nontoxic up to mM concentrations. The low cytotoxicity encourages further experiments that could take advantage of the selectivity of enkephalin derivatives for opioid receptors.