Synthesis and Characterization of Dicobalthexacarbonyl-Alkyne Derivatives of Amino Acids, Peptides, and Peptide Nucleic Acid (PNA) Monomers

The reaction of CO2(CO)(8) with alkyne-containing amino acids [1a: phenylalanine (Phe) and 1b: methionine (Met)], two suitably alkyne-functionalized derivatives of the neuropeptide enkephalin (Enk) [3: Ac-Enk-Prop and 5: Ac-Enk(Pgl)-NH2 (Ac - Acetyl; Pgl - propargylglycine; Prop - propargylamine)], a thymine Peptide Nucleic Acid (T-PNA) monomer (7), and a PNA-like monomer (9) derivative gave the respective dicobalthexacarbonyl bioconjugates in very good yields. Two different sites for labeling of the biomolecules were successfully used: The organometallic moiety was reacted with the C-terminus of alkyne-containing amino acids, peptide or PNA thymine monomers, and alternatively the organometallic compound was complexed to an internal site in the peptide or PNA. To this end, a simple glycine was replaced by propargylglycine in peptides, and a new alkyne-containing PNA-like monomer, in which an alkyne chain replaces the nucleobase, was used for PNA chemistry. For the synthesis of the two alkyne-containing enkephalin derivatives 3 and 5, two different resins, namely sulfamylbutyryl and Rink amid, were used as they allow to selectively insert, on the solid phase, an alkyne moiety at the C-terminus and on a side-chain of a peptide sequence, respectively. The identity and constitution of all cobalt complexes were confirmed by different analytical methods (IR, FAB, ESI-MS, and NMR). Most notably, IR spectroscopy shows intensive bands in the 2100-2000 cm(-1) region because of the CO2(CO)(6) Moiety. In both H-1 NMR spectra of the dicobalthexacarbonyl PNA monomer derivatives 8 and 10, all signals are doubled because of the cis-trans isomerism about the central amide bond. The X-ray structure of a dicobalthexacarbonyl phenylalanine derivative (2a) confirms the proposed composition of the bioconjugates and shows that, as anticipated, the alkyne group of 2a is no longer linear upon complexation in comparison to the alkyne group of the bioconjugate precursor la, as indicated by a C-C equivalent to C angle of about 143 degrees in 2a. Moreover, the C equivalent to C bond of la was elongated by about 0.15 angstrom upon CO2 coordination.