Phycobiliprotein - Bilin Linkage Diversity

The peptides were examined by lH NMR, circular dichroism spectroscopy, and secondary ion mass spectrometry. The ‘H NMR data confirmed that in each case the bilin was attached through a single linkage, a thioether bond between the cysteinyl residue and the tetrapyrrole moiety. Comparison of the ’H NMR spectra of these peptides with those of appropriate model compounds showed that the thioether linkage in a-1 PCB was to the C-3’ position and that in &2T PCB to the C-18’ position on the bilin. Refluxing in neutral methanol under nitrogen led to the release of PCB from 0-1 PCB but did not release the D-ring-linked tetrapyrrole from &2T. The above results together with those of an earlier study (Lagarias, J. C., Glazer, A. N., and Rapoport, H. (1979) J. Am. Chem. SOC. 101, 5030-5037) complete the determination of the mode of linkage of each of the three bilins on C-phycocyanin; two are linked through ring A and one through ring D. This is the first documented report of a singly D-ring-linked bilin.