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

Abstract

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 (13)C (carbonyl and Cα) and (15)N (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.

Solid-State NMR Characterization of the Structure of Self-Assembled Ile–Phe–OH

“…Another discrepancy was also found in the atoms of Cγ2 and Cδ1 at Ile1 and Cδ, Cε, and Cζ at Phe2, which was probably caused by the lack of intermolecular side-chain interactions because of the existence of these atoms respectively, although the S/N ratio was rather low due to the extremely low 15 N natural abundance (0.37%) and resonance frequency (60.81 MHz). Reportedly, the 15 N chemical shifts of amides are dependent on the intermolecular hydrogen bond distance [14,21]. Hence, in the present study, the NMR signal of the amide indicates that the peptide formed an intermolecular hydrogen bond during the self-assembly process.…”

“…Hence, in the present study, the NMR signal of the amide indicates that the peptide formed an intermolecular hydrogen bond during the self-assembly process. Furthermore, 15 N chemical shifts of amines are also affected by multiple hydrogen bonds [14]. Therefore, herein, the signal position at 16.0 ppm was identified with the formation of the hydrogen bond.…”

“…The morphology of the self-assembled peptide appeared as a straight fiber with an average width of around 80 nm, which was easily dissolved in an aqueous solution. Generally, solid-state NMR spectroscopy can provide structural insights not only on insoluble molecules, such as membrane proteins and amyloid fibers [12,13], but also on crystalline dipeptides [14,15]. Previously, the molecular structures of the self-assembled Phe-Phe-OH and cyclic dipeptides have been characterized by 13 C solid-state NMR experiments [16,17].…”

The Effects of pH on the Supramolecular Structure of Amino Amides

“…On the other hand, 15 N NMR has been mentioned as an excellent tool to determine the conformation of peptides and the presence of hydrogen bonding interactions in their crystal structure. [24][25][26][27] However, the 15 N NMR study of 1-8 in DMSO solutions did not correlate with the conformation obtained in the crystalline structures (Table 2). This behavior is normal if it is taken into account that the amino amides do not have a rigid structure in solution due to the low energy rotation of the CÀ C and CÀ N single bonds.…”

“…On the other hand, 15 N NMR has been mentioned as an excellent tool to determine the conformation of peptides and the presence of hydrogen bonding interactions in their crystal structure [24–27] . However, the 15 N NMR study of 1 – 8 in DMSO solutions did not correlate with the conformation obtained in the crystalline structures (Table 2).…”