NMR Verification of Helical Conformations of Phycocyanobilin in Organic Solvents

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Abstract
Selective NMR decoupling and nuclearOverhausereffect (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 helixMis somewhat more stable thanP. 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.

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