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1983 (1)
1Author    Harald Lehner, Hugo ScheerRequires cookie*
 Title    Circulardichroism of C-Phycocyanin: Origin of Optical Activity in Denatured Biliproteins and Evidence for an Intermediate during Unfolding  
 Abstract    The circulardichroism spectra (cd) o f native and (partially) urea-denaturated C-phycocyanin (PC) in the spectral range 7 0 0 -2 1 0 nm are presented. The large ellipticities observed in the chrom ophoric region of native PC are retained in the fully denatured state (8 M urea). This is sim ilar to the behavior of the red form of phytochrome (Pc), but in contrast to C-phycoerythrin (PE). These differences are rationalized in terms of epim eric equilibria betw een P-and M-helix shaped chromophores. D epending on the num ber and location o f the chirality centers present in the tetrapyrrol moities a priori, the excess populations of the inherently chiral P-and M-helices differ, thus accounting for large (PC, Pr) or small (PE) ellipticities in the denatured pigments. Hence, the large optical activity observed for the form er is generated by an excess population of the P-helix induced by the asymmetric C-2, C-3 and C-3'. In PE the additional chirality center at C -l6 counteracts the influence of the others. The excess population o f the inherently chiral species is therefore lowered, in agreem ent with the nearly vanishing cd reported for denatured PE. The cd has also been studied at interm ediate urea concentrations. U nfolding of PC with urea can be interpreted from these data as a stepwise process. M onitoring the urea induced unfolding of PC by cd at different wavelengths (A = 220, 345, and 610 nm); the "melting po in t" o f the apoprotein (4 -5 M urea) coincides with the extrema of the titration curves obtained in the chrom ophore region (345, 610 nm). These results give direct evidence for the existence of an interm ediate species whose population reaches a m aximum at 4 — 5 M urea. 
  Reference    Z. Naturforsch. 38c, 353 (1983); received January 17 1983 
  Published    1983 
  Keywords    Phycocyanin, Phycoerythrin, Phytochrom, C irculardichroism, D enaturation 
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 TEI-XML for    default:Reihe_C/38/ZNC-1983-38c-0353.pdf 
 Identifier    ZNC-1983-38c-0353 
 Volume    38