Structure and theoretical NMR chemical shifts of modified cyclodextrins†

Structural, Vibrational, and Electronic Properties of the Glucoalkaloid Strictosidine: A Combined Experimental and Theoretical Study

A detailed structural analysis and spectral behavior of the glucoalkaloid strictosidine, a precursor of all monoterpene indole alkaloids, are discussed. The experimental NMR, FTIR, and UV results were compared to the theoretical DFT spectra calculated by Becke using the three-parameter Lee-Yang-Parr (B3LYP) function with 6-31G(d) and 6-311++G(2d,p) basis sets. The theoretical geometry optimization data were compared with the X-ray data for precursors and similar structures in the associated literature. The similarity between the theoretical and experimental coupling constants values made it possible to affirm the values of dihedral angles and their configuration, reinforcing findings from previous stereochemical studies. Theoretical UV analysis agreed well with the measured experimental data, with bands assigned. Calculated HOMO/LUMO gaps show low excitation energy for strictosidine, justifying its stability and reaction kinetics. The molecular electrostatic potential map shows opposite potentials regions that form hydrogen bonds that stabilize the dimeric form, which were confirmed by excellent agreement of the dimeric form theoretical wavenumbers with the experimental IR spectrum. ESI-MS/MS data revealed patterns for the fragmentation of the protonated strictosidine molecule outlined by an NBO study.

Intramolecular hydrogen bond energy and cooperative interactions in α-, β-, and γ-cyclodextrin conformers

Accurate estimation of individual intramolecular hydrogen bond (H-bond) energies is an intricate task for multiply H-bonded systems. In such cases, the hydrogen bond strengths could be highly influenced by the cooperative interactions, for example, those between hydroxyl groups in sugars. In this work, we use the recently proposed molecular tailoring approach-based quantification (Deshmukh, Gadre, and Bartolotti, J Phys Chem A 2006, 110, 12519) to the extended systems of cyclodextrins (CDs). Further, the structure and stability of different conformers of α-, β-, and γ-CDs are explained based on the energetics and cooperative contribution to the strength of these H-bonds. The estimated O-H···O H-bond energies in the various CD conformers are found to vary widely from 1.1 to 8.3 kcal mol(-1). The calculated energy contributions to cooperativity toward the H-bond strengths fall in the range of 0.25-2.75 kcal mol(-1).

Theoretical studies on hydrogen bonding, NMR chemical shifts and electron density topography in alpha, beta and gamma-cyclodextrin conformers

Hydrogen-bonded interactions in alpha-, beta-, and gamma-CD conformers are investigated from the molecular electron density topography and chemical shift in the nuclear magnetic resonance (NMR) spectra calculated by using the Gauge Invariant Atomic Orbital (GIAO) method within the framework of density functional theory. For the lowest-energy CD conformers in the gas phase, the O3-H...O2' hydrogen-bonding interactions are present. Calculated 1H NMR chemical shifts (delta H) correlate well with the hydrogen-bond distance as well as electron density at the bond critical point in the molecular electron density (MED) topography. The conformers of beta- and gamma-CD comprised of relatively strong secondary hydroxyl interactions are stabilized by solvation from polar solvents.

Chelate effect in cyclodextrin dimers: a computational (MD, MM/PBSA, and MM/GBSA) study

The complexation of an adamantyl-phosphate derivative with one beta-cyclodextrin, with two beta-cyclodextrins, and with two beta-cyclodextrins dimerized with a disulfide bridge was studied by computational methods (MD, MM/PBSA, and MM/GBSA) to analyze and rationalize the chelate effect. Although this effect is usually explained by invoking favorable entropy contribution due to the preorganization of the ligand, it has been determined experimentally that in this case it is enthalpy-driven. The computational results are in accord with this finding, although the entropy contribution due to the solvent structural organization around the complex is crucial for the final estimates of the free energy of complexation.