Effects of Structural Differences on the NMR Chemical Shifts in Isostructural Dipeptides

Porous
crystalline dipeptides have gained recent attention for
their potential as gas-storage materials. Within this large class
is a group of dipeptides containing alanine, valine, and isoleucine
with very similar crystal structures. We report the 13C
(carbonyl and Cα) and 15N (amine and amide) solid-state
NMR isotropic chemical shifts in a series of seven such isostructural
porous dipeptides as well as shift tensor data for the carbonyl and
amide sites. Using their known crystal structures and aided by ab
initio quantum chemical calculations for the resonance assignments,
we elucidate trends relating local structure, hydrogen-bonding patterns,
and chemical shift. We find good correlation between the backbone
dihedral angles and the Cα1 and Cα2 shifts. For the C1
shift tensor, the δ11 value shifts downfield as the
hydrogen-bond distance increases, δ22 shifts upfield,
and δ33 shows little variation. The C2 shift tensor
shows no appreciable correlation with structural parameters. For the
N2 tensor, δ11 shows little dependence on the hydrogen-bond
length, whereas δ22 and δ33 both
show a decrease in shielding as the hydrogen bond shortens. Our analysis
teases apart some, but not all, structural contributors to the observed
differences the solid-state NMR chemical shifts.