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Centerband‐Only Detection of Exchange NMR with Natural‐Abundance Correction Reveals an Expanded Unit Cell in Phenylalanine Crystals
Author(s) -
Xue Kai,
Dervisoglu Riza,
Sowa Heidrun,
Andreas Loren B.
Publication year - 2020
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.202000517
Subject(s) - spins , chemistry , spin diffusion , phenylalanine , pulse sequence , magnetization , crystallography , diffusion , spin (aerodynamics) , crystal (programming language) , molecule , chemical physics , nuclear magnetic resonance , amino acid , physics , condensed matter physics , magnetic field , organic chemistry , thermodynamics , biochemistry , quantum mechanics , computer science , programming language
The NMR pulse sequence CODEX (centerband‐only detection of exchange) is a widely used method to report on the number of magnetically inequivalent spins that exchange magnetization via spin diffusion. For crystals, this rules out certain symmetries, and the rate of equilibration is sensitive to distances. Here we show that for 13 C CODEX, consideration of natural abundance spins is necessary for crystals of high complexity, demonstrated here with the amino acid phenylalanine. The NMR data rule out the C 2 space group that was originally reported for phenylalanine, and are only consistent with a larger unit cell containing eight magnetically inequivalent molecules. Such an expanded cell was recently described based on single crystal data. The large unit cell dictates the use of long spin diffusion times of more than 200 seconds, in order to equilibrate over the entire unit cell volume of 1622 Å 3 .