Chiral recognition of macromolecules with cyclodextrins

http://www.beilstein-journals.org/bjoc/single/articleFullText.htm?publicId=1860-5397-7-27

Scheme 1: Synthesis of copolymers 3_D, 3_L and CD-complexes 4_D, 4_L.

The resulting copolymer 3_D, 3_L (1:20) is soluble in water below the critical solution temperature (LCST). However, the presence of D- or L-Phe significantly affects the LCST value of pure poly(NIPAAm) (32 °C), since the hydrophilic/hydrophobic balance of the polymer is changed [17,18]. As noted above, incorporation of the hydrophobic phenylalanine moieties into the polymer reduces the LCST to 25 °C for the 3_D, 3_L (1:20) copolymer, whereas the 3_D, 3_L (1:10) copolymer is relatively hydrophobic and not water-soluble.

Surprisingly, the polymers 3_D, 3_L (1:10) become water-soluble due to host–guest interaction with RAMEB-CD yielding 4_D and 4_L.
To confirm the formation of the proposed inclusion complexes between the copolymers and CD, which was added in excess, the corresponding monomers 2_D and 2_L were used as models to prove complexation.

Thus, 2D ROESY NMR spectroscopy was performed to show the correlation between the protons of the β-CD and the protons of the guest 2_D. Figure 1 indicates as an example the case of 2_D where interactions of protons from the phenyl group with protons of the CD cavity are shown, while no interactions with protons of the outer rim are apparent. Furthermore, there is a second interaction of the protons of the double bond of 2_D with the CD cavity. Therefore, it can be concluded, that the complex composition with 2_D or 2_L has a stoichiometry of 1:2.

Figure 1: 2D NMR ROESY spectrum of monomer 2_D with RAMEB-CD.

To assure for further investigations that the complexation also takes place in the copolymer, additional ROESY experiments were carried out (Figure 2).

Figure 2: 2D NMR ROESY experiment showing the correlation between protons of the phenyl moiety of 4_D with the β-CD protons.

The NMR experiments clearly show that the complexation takes place in monomer and polymer solutions, for both enantiomers (also see Supporting Information File 1).
Furthermore, both a shift of proton signals and the duplication of proton signals can be detected when the spectra of the complex is compared to the pure compounds 2_D and 2_L (Figure 3).

Figure 3: NMR shifts of the complexed monomer 2_D.

Figure 3 shows the specific shifts of signals, which occur on complexation. CDs being chiral molecules themselves effect a downfield shift of 0.1 ppm of the aromatic proton signal since the protons are magnetically shielded by the CD cavity. Additionally, each of the aromatic and double bond proton signals, respectively, change from singlets to doublets on complexation.

Obviously, the supramolecular structure changes and former magnetically identical protons become non-equivalent protons and appear as doublets. This can easily be observed in case of aromatic protons at 7.31–7.14 ppm and the proton signal of the double bond at 5.6 ppm and 6.0 ppm, respectively. The signal of the proton next to the double bond (6.3 ppm) remains unffected. The described shift of proton signals can be detected for both enantiomers, but with different intensities as can be observed in Figure 4.

Figure 4: Comparison of ¹H NMR spectra of 2_D and 2_L complexed with β-CD.

As expected, we found that ¹H NMR spectra of complexed compounds 2_D and 2_L with β-CD show differences in the splitting pattern of the proton signals (Figure 4). Therefore we conclude that the complex stability is different for 2_D and 2_L and 4_D and 4_L, respectively.

To further prove the complexation process of β-CD as host and the Phe derivatives 2_D and 2_L as guests, we visualized the complexation utilising phenolphthalein [19], i.e., the complexation and decomplexation of a dye with β-CD in basic medium can be employed as a method to prove these processes [20]. In basic media, phenolphthalein exhibits its characteristic pink colour, caused by its planar configuration and electron delocalisation.

 On adding cyclodextrin to the solution, phenolphthalein becomes colourless in the complexed state. Hydrogen bond formation is accompanied with a conformational change as the planar state of phenolphthalein is reversed which leads to loss of colour. Taking this into account, the addition of a competitive guest molecule should displace the included phenolphthalein and turn the colour of the solution from colourless to pink. Accordingly, the addition of 2_D followed by stirring for one hour caused the solution to regain its characteristic pink colour as Figure 5 shows.

Figure 5: Complex formation with phenolphthalein and phenylalanine as competitor.

 
 amcrasto@gmail.com

http://newdrugapprovals.org/

DRUG APPROVALS BY DR ANTHONY MELVIN CRASTO

 
 amcrasto@gmail.com

http://newdrugapprovals.org/

DRUG APPROVALS BY DR ANTHONY MELVIN CRASTO