Subtle Chemical Shifts Explain the NMR Fingerprints of Oligomeric Proanthocyanidins with High Dentin Biomodification Potency

The ability of certain oligomeric
proanthocyanidins (OPACs) to
enhance the biomechanical properties of dentin involves collagen cross-linking
of the 1.3–4.5 nm wide space via protein–polyphenol
interactions. A systematic interdisciplinary search for the bioactive
principles of pine bark has yielded the trimeric PAC, ent-epicatechin-(4β→8)-epicatechin-(2β→O→7,4β→8)-catechin (3),
representing the hitherto most potent single chemical entity capable
of enhancing dentin stiffness. Building the case from two congeneric
PAC dimers, a detailed structural analysis decoded the stereochemistry,
spatial arrangement, and chemical properties of three dentin biomodifiers.
Quantum-mechanics-driven 1H iterative full spin analysis
(QM-HiFSA) of NMR spectra distinguished previously unrecognized details
such as higher order J coupling and provided valuable
information about 3D structure. Detection and quantification of H/D-exchange
effects by QM-HiFSA identified C-8 and C-6 as (re)­active sites, explain
preferences in biosynthetic linkage, and suggest their involvement
in dentin cross-linking activity. Mapping of these molecular properties
underscored the significance of high δ precision in both 1H and 13C NMR spectroscopy. Occurring at low- to
subppb levels, these newly characterized chemical shift differences
in ppb are small but diagnostic measures of dynamic processes inherent
to the OPAC pharmacophores and can help augment our understanding
of nanometer-scale intermolecular interactions in biomodified dentin
macromolecules.