Synthesis of selectively 13C-labelled bilin compounds

A series of open-chain tetrapyrroles, each carrying one or more C-13-labels in its three ring-bridging methine groups, were synthesized. These compounds - [5-, [10-, and [15-C-13]-phycocyanobilins (PCB), [10-C-13]-phytochromobilin, and [10,15-C-13(2)]-PCB - were each obtained by a convergent route, starting with the four pyrrole building blocks, with the initial formation of the left and the right halves of the tetrapyrrole separately, followed by a final condensation of the two dipyrrole units to yield the target bilin compound. The C-13-labels were all inserted as C-1- or C-2-units prior to the appropriate condensation. The substitution pattern of these bilins on ring A (ethylidene substituent at position 3) allows covalent attachment to the apoprotein of the plant photoreceptor phytochrome. The isotope shift produced by insertion of a C-13-isotope is demonstrated in the FT-IR spectra of phytochrome-incorporated [10-C-13]-phytochromobilin. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).