NMR Verification of Helical Conformations of Phycocyanobilin in Organic Solvents

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.