Tuning the cavity of cyclodextrins: altered sugar adaptors in protein pores

Cyclodextrins (CDs) have been widely used in host-guest molecular recognition because of their chiral and hydrophobic cavities. For example, β-cyclodextrin (βCD) lodged as a molecular adaptor in protein pores such as α-hemolysin (αHL) is used for stochastic sensing. Here, we have tuned the cavity and overall size of βCD by replacing a single oxygen atom in its ring skeleton by a disulfide unit in two different configurations to both expand our ability to detect analytes and understand the interactions of βCD with protein pores. The three-dimensional structures of the two stereoisomeric CDs have been determined by the combined application of NMR spectroscopy and molecular simulation and show distorted conformations as compared to natural βCD. The interactions of these synthetic βCD analogues with mutant αHL protein pores and guest molecules were studied by single-channel electrical recording. The dissociation rate constants for both disulfide CDs from the mutant pores show ∼1000-fold increase as compared to those of unaltered βCD, but are ∼10-fold lower than the dissociation rate constants for βCD from wild-type αHL. Both of the skeleton-modified CDs show altered selectivity toward guest molecules. Our approach expands the breadth in sensitivity and diversity of sensing with protein pores and suggests structural parameters useful for CD design, particularly in the creation of asymmetric cavities.

Cyclodextrin effect on solvolysis of substituted benzoyl chlorides

A kinetic study was carried out on the solvolysis of substituted benzoyl chlorides in the presence of alpha-, beta- and gamma-CD. Combination of the substituent dependent mechanism for solvolysis of benzoyl chlorides and the complexation ability of the cyclodextrin yields the following experimental behavior: (i) catalysis by beta- and gamma-CD for solvolysis of electron-attracting substituted benzoyl chlorides due to the reaction with its hydroxyl group C(6); (ii) absence of alpha-CD influence on solvolysis of benzoyl chlorides with electron withdrawing substituents; (iii) inhibition of solvolysis of benzoyl chlorides with electron-donating groups. This behavior is observed for solvolysis of meta/para substituted substrates in the presence of beta-CD, solvolysis of meta-substituted benzoyl chlorides in the presence of alpha-CD and solvolysis of para-substituted benzoyl chlorides in the presence of gamma-CD. This decrease in the rate constant is a consequence of the complexation of the substrate in the cyclodextrin cavity and its low solvation ability, causing the rate of solvolysis in its interior to be negligible. (iv) The solvolysis of meta-substituted benzoyl chlorides in the presence of gamma-CD yields a new behavior where the reaction of the complexed substrate is not negligible in the interior of the cyclodextrin cavity, which has been interpreted as a consequence of incomplete expulsion of hydration water from its cavity when the complexation takes place. (v) The experimental results obtained in the presence of alpha-CD show that meta-substituted benzoyl chlorides give rise to host : guest complexes with 1 : 1 stoichiometries, whereas those which are para-substituted cause a 2 : 1 stoichiometry to be formed. This difference in behavior has been interpreted taking into account the size of the different benzoyl chlorides and their accommodation in the alpha-CD cavity.

Further syntheses employing phosphorylase

Maltopentaose was immobilized on silica gel and used as a recyclable primer in the reaction of glycogen phosphorylase with D-glucal. An improved method to obtain 2-deoxy-alpha-D-arabino-hexopyranosyl phosphate (4) by using this catalyst as well as the specific synthesis of low molecular weight, water-soluble 2-deoxy-maltooligosaccharides (12, 13, 14 and 15) are described. Further investigations with modified phosphorylase substrates showed that mannosyl phosphate (16) can be slowly transferred to the primer maltotetraose. alpha-1,4-Mannosyl-maltotetraose (17) and its degradation product alpha-1,4-mannosyl-maltose (18) were identified.

Synthesis of 2-deoxy-alpha-D-arabino-hexopyranosyl phosphate and 2-deoxy-maltooligosaccharides with phosphorylase

A convenient one-step synthesis of 2-deoxy-alpha-D-arabino-hexopyranosyl phosphate on a millimolar scale is described by reaction of potato phosphorylase with D-glucal at equimolar phosphate concentration. Furthermore, in the presence of catalytic amounts of phosphate, a 2-deoxy-maltooligosaccharide is obtained from maltotetraose and D-glucal. The water-insoluble oligosaccharide was isolated and characterized by 1H and 13C NMR spectroscopy. An average dp of 20 was thus determined.