Spatial arrangements of cyclodextrin host–guest complexes in solution studied by ¹³C NMR and molecular modelling

Introduction

Complexation of organic and inorganic compounds with α-, β-, or γ-cyclodextrins and their derivatives [1] is an established tool used in medicine for drug delivery [2-4], in analytical and preparative chemistry for compound separation [5] and in materials science for small molecule detection [6,7]. Association (binding) constants between the host and guest molecules [8-10] are typically measured by ¹H NMR titration [11,12] or isothermal titration calorimetry [13]. Single crystals for many host–guest complexes have been prepared, and their structure elucidated by X-ray crystallography [14,15]. Conformations of host–guest complexes in solution have been studied by 2D NMR experiments [11] (NOESY, ROESY) or proposed computationally [16,17] based on dispersion forces and hydrogen bonding between the cyclodextrin (CD) unit and the guest molecule. Determination of the ee of chiral guests was achieved by observing the splitting of ¹H NMR signals of the achiral host upon formation of diastereomeric inclusion complexes [18,19]. Shifting the H-3 and H-5 proton signals of CDs in ¹H NMR or 2D NMR ROESY or NOESY experiments can indicate the spatial distances between host and guest atoms and distinguish between enantiomeric guests. However, the evaluation of such interactions is often hampered by overlapping signals [20].

Results and Discussion

In this work, we reveal the conformation (spatial arrangement) of the host–guest complex in solution spectroscopically by measuring the ¹³C NMR spectra of a suitable guest molecule. We decided to take C_s symmetric guest molecules, CD as a host and measure the ¹³C NMR spectra of these complexes. We expect that the anisotropy of the chiral cavity is expressed by differences in the magnetic shielding of prochiral atoms, resulting in signal splitting of the prochiral carbons of the guest molecule in ¹³C NMR spectra. Adamantan-2-amine hydrochloride was used as a model guest molecule containing three sets of prochiral carbons (Figure 1).

The degree of the signal split correlates with the distance of prochiral atoms (carbons) to the host cavity and thus gives information about the conformation of the guest molecule in the host cavity. First, we measured simple 1D ¹³C NMR spectra and indeed observed the split of the peaks of all prochiral carbon signals of the guest upon complexation with α-CD. The biggest split, 22.5 Hz, was observed for prochiral carbons 8 and 10, depicted in Figure 1 in green color, followed by a split of carbons 3 and 9 (in blue color) and the smallest difference in the magnetic field shielding was detected for carbons 1 and 3 (in red color).

The same 1D ¹³C NMR spectra were measured for guest molecule 1 in host cavities of β-CD and γ-CD, showing almost no split of prochiral carbon peaks and suggesting a higher degree of conformational flexibility of the host–guest complexes than for the complex of 1 with α-CD. Interatomic distances within the host–guest complex were measured using rotating-frame nuclear Overhauser effect spectroscopy (ROESY) measurements. For the complex of 1 with α-CD, the cross-peak in the 2D ROESY spectrum between proton 5 of the glucose moiety of the host α-CD and the protons 6 of molecule 1 reveals deep penetration of the guest into the cavity of α-cyclodextrin (Figure 2). All protons of 1 show ROESY cross-peaks with proton 3 of the glucose unit (see page S73 in Supporting Information File 1).

For complexes of ligands with α-CD, we always performed a series of ten classical molecular dynamics (MD) simulations [16] (each lasting 100 ns, Figure 3). Then, we superimposed α-CD structures from different snapshots of each MD run. Further, the 3D densities, showing the spatial distribution of prochiral atoms of ligands (that rotate and wobble towards α-CD), were calculated. If two atoms have the same spatial density, then the splitting of their signals in ¹³C NMR spectra cannot occur. On the contrary, if these densities, which belong to a pair of prochiral atoms, are well separated, then NMR splitting can be expected due to the anisotropic environment of the chiral α-CD cavity.

The NMR splitting is generally larger if the prochiral guest atoms are located closer to the cavity of the α-CD. In other words, the splitting is larger, if the radius of a density is bigger or the density runs deeper into the α-CD cavity. In Figure 3, the green and blue densities with well-separated light and dark clouds (belonging to different prochiral carbons from a pair) have larger radii and run deeper into the α-CD cavity than the red density with mixed light and dark clouds. Accordingly, our NMR experiment only showed splitting for the green and blue atoms (see Figure 1).

Having established the method, we chose various cyclic compounds (noradamantane, adamantane, cyclohexane derivatives) with desired symmetry and measured NMR spectra of these guest molecules (2–8) in α-cyclodextrin and cyclodextrins with a larger cavity (β-CD and γ-CD, Figure 4).

In all cases, the ¹H NMR spectra indicated the interaction of the guest molecule with the host. In cases where the split of prochiral carbons was observed, ¹³C NMR spectra showed which pair of prochiral carbons in the guest molecule is close to the wall of the host’s chiral cavity, creating an anisotropic environment. For compound 4, we were thus able to select representative conformations of the guest molecule in all types of cyclodextrins (Figure 5) using the spatial densities gained from classical MD simulations.

Compounds 1–8 (as hydrochloride salts or free bases), were attempted to co-crystallize with α-, β- and γ-cyclodextrins. Compounds 1, 6 and 8 are commercial, compounds 2–5 [21] and 7 [22] were prepared according to published procedures. Compound 2 (as a free amine) crystallized in the supramolecular capsule of two α-cyclodextrins, and the mono-crystal was subjected to X-ray analysis. This experiment was performed to compare the solid-state structure of the guest 2–α-CD host complex with its proposed solution state conformations (Figure 6).

Conclusion

We have demonstrated that simple ¹³C NMR analyses of properly chosen C_s symmetric compounds varying in size can be used to estimate the host–guest spatial arrangement in solution and extrapolated for compounds with similar size and bonding nature.