Synthesis of Selectively 13 C‐Labelled Bilin Compounds

A series of open‐chain tetrapyrroles, each carrying one or more 13 C‐labels in its three ring‐bridging methine groups, were synthesized. These compounds – [5‐, [10‐, and [15‐ 13 C]‐phycocyanobilins (PCB), [10‐ 13 C]‐phytochromobilin, and [10,15‐ 13 C 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 13 C‐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 13 C‐isotope is demonstrated in the FT‐IR spectra of phytochrome‐incorporated [10‐ 13 C]‐phytochromobilin. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)