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NMR Verification of Helical Conformations of Phycocyanobilin in Organic Solvents
Author(s) -
Knipp Bernd,
Müller Martin,
MetzlerNolte Nils,
Balaban Teodor S.,
Braslavsky Silvia E.,
Schaffner Kurt
Publication year - 1998
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19980810509
Subject(s) - chemistry , phycocyanobilin , nuclear overhauser effect , diastereomer , steric effects , proton nmr , substituent , circular dichroism , stereochemistry , solvent , proton , crystallography , two dimensional nuclear magnetic resonance spectroscopy , dimethylformamide , nuclear magnetic resonance spectroscopy , organic chemistry , cyanobacteria , genetics , physics , quantum mechanics , phycocyanin , bacteria , biology
Selective NMR decoupling and nuclear Overhauser effect (NOE) experiments with phycocyanobilin (PCB) show proton‐proton interactions between the terminal rings A and D, viz . the chiral C(2) methine center and the ethyl substituent at C(18), as a result of the helical conformation of this open‐chain tetrapyrrole in solution. Quantitative NOE measurements and a combination of force‐field and semiempirical calculations (FSC) afford inter‐proton distances across the helical gap of 4.2–4.6 (NOE) and 3.2–4.2 A° (FSC). The NOE and FSC, in conjuction with a qualitative evaluation of the steric interactions in the two optimized helices, suggest furthermore that, in solution, the helix M is somewhat more stable than P . The coexistence of at least two diastereoisomers is corroborated also by the circular dichroism (CD) spectra of PCB in MeOH/EtOH which point to a temperature‐dependent equilibrium in solution, and by a considerable increase of this CD upon changing the solvent from the achiral alcohols to ethyl (−)‐( S )‐lactate which reflects a selective solvent‐induced CD differentiating between diastereoisomers.