PREPARATION OF TETRAPYRROLE-AMINO ACID COVALENT COMPLEXES

The presented synthetic approach towards chemical modifications of chl orophylls (Chls) provides a perspective to construct model systems, wh ere tetrapyrrole-amino acid and tetrapyrrole-peptide interactions coul d be studied in covalent model compounds. The approach relies on the f act that in Chls the 17(2) propionic acid side chain does not particip ate in the tetrapyrrole pi-electron system. It makes use of a plant en zyme chlorophyllase (EC 3.1.1.14). which in vivo and in vitro catalyse s reactions at this side function. The transesterification and hydroly sis enzymatic reactions are useful on a preparative scale. In the tran sesterification reaction, a desired amino acid residue possessing prim ary hydroxyl group can be directly attached to the propionic acid side chain Chl. This method allows to replace the phytyl moiety in Chls wi th serine. The other reaction, enzymatic hydrolysis of Chls, yields ch lorophyllides and opens a convenient route for further modifications. If sufficiently mild synthetic methods are used, such as catalysis wit h 4-dimethyl amino pyridine or activation with N-hydroxysuccinimide, a n amino acid or peptide residue can be covalently bound to chlorophyll ides' carboxylic group, leaving the essential electronic structure of Chl intact. The activation with N-hydroxysuccinimide allows for the co upling even in aqueous media. Following these two methods, the chlorop hyllides were linked e.g. to tyrosine or melanocyte stimulating hormon e (alpha-4,7-MSH). The spectral features of these model compounds indi cate a formation of a ground state charge transfer complex between the tetrapyrrole and amino acid moieties. Thanks to the high stereospecif icity of chlorophyllase, the described model compounds are the non-pri me diastereisomers. They have chemical features of both Chl and amino acid and thus can be used as modules to build more complicated model s ystems.