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The Stereostructure of Porphyra‐334: An Experimental and Calculational NMR Investigation. Evidence for an Efficient ‘Proton Sponge’
Author(s) -
Klisch Manfred,
Richter Peter,
Puchta Ralph,
Häder DonatP.,
Bauer Walter
Publication year - 2007
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.200790052
Subject(s) - chemistry , chemical shift , stereocenter , protonation , proton , proton nmr , moiety , tautomer , density functional theory , two dimensional nuclear magnetic resonance spectroscopy , carbon 13 nmr , yield (engineering) , absolute configuration , computational chemistry , nmr spectra database , stereochemistry , crystallography , spectral line , organic chemistry , ion , physics , materials science , quantum mechanics , astronomy , metallurgy , enantioselective synthesis , catalysis
The mycosporine‐like amino acid (MAA) porphyra‐334 ( 1 ) is subjected to extensive 1 H‐ and 13 C‐NMR analysis as well as to density‐functional‐theory (DFT) calculations. All 1 H‐ and 13 C‐NMR signals of 1 are assigned, as well as the resonances of prochiral proton pairs. This is achieved by 500‐MHz standard COSY, HMQC, and HMBC experiments, as well as by one‐dimensional (DPFGSE‐NOE) and two‐dimensional (NOESY) NOE experiments. Diffusion measurements (DOSY) confirm that 1 is monomeric in D 2 O solution. DFT Calculations yield 13 C‐NMR chemical shifts which are in good agreement for species 6 which is the imino N‐protonated form of 1 . An exceptionally high proton affinity of 265.7 kcal/mol is calculated for 1 , indicating that 1 may behave as a very powerful ‘proton sponge’ of comparable strength as synthetic systems studied so far. Predictions of 13 C‐NMR chemical shifts by the ‘NMRPredict’ software are in agreement with the DFT data. The absolute configuration at the ring stereogenic center of 1 is concluded to be ( S ) from NOE data as well as from similarities with the absolute configuration ( S ) found in mycosporine‐glycine 16 . This supports the assumption that 1 is biochemically derived from 3,3‐ O ‐didehydroquinic acid ( 17 ). The data obtained question the results recently published by a different research group claiming that the configuration at the imino moiety of 1 is ( Z ), rather than ( E ) as established by the here presented study.