Preparation of tetrapyrrole-amino acid covalent complexes

The presented synthetic approach towards chemical modifications of chlorophylls (Chls) provides a perspective to construct model systems, where tetrapyrrole-amino acid and tetrapyrrole-peptide interactions could be studied in covalent model compounds. The approach relies on the fact that in Chls the 17(2) propionic acid side chain does not participate in the tetrapyrrole pi-electron system. It makes use of a plant enzyme chlorophyllase (EC 3.1.1.14). which in vivo and in vitro catalyses reactions at this side function. The transesterification and hydrolysis enzymatic reactions are useful on a preparative scale. In the transesterification reaction, a desired amino acid residue possessing primary hydroxyl group can be directly attached to the propionic acid side chain Chl. This method allows to replace the phytyl moiety in Chls with serine. The other reaction, enzymatic hydrolysis of Chls, yields chlorophyllides and opens a convenient route for further modifications. If sufficiently mild synthetic methods are used, such as catalysis with 4-dimethyl amino pyridine or activation with N-hydroxysuccinimide, an amino acid or peptide residue can be covalently bound to chlorophyllides' carboxylic group, leaving the essential electronic structure of Chl intact. The activation with N-hydroxysuccinimide allows for the coupling even in aqueous media. Following these two methods, the chlorophyllides were linked e.g. to tyrosine or melanocyte stimulating hormone (alpha-4,7-MSH). The spectral features of these model compounds indicate a formation of a ground state charge transfer complex between the tetrapyrrole and amino acid moieties. Thanks to the high stereospecificity of chlorophyllase, the described model compounds are the non-prime diastereisomers. They have chemical features of both Chl and amino acid and thus can be used as modules to build more complicated model systems.