Fully Symmetric Cyclodextrin Polycarboxylates: How to Determine Reliable Protonation Constants from NMR Titration Data

Abstract

Acid-base properties of cyclodextrins (CDs), persubstituted at C-6 by 3-mercaptopropionic acid, sualphadex (Suα-CD), subetadex (Suβ-CD) and sugammadex (Suγ-CD, the antidote of neuromuscular blocking steroids) were studied by 1H NMR-pH titrations. For each CD, the severe overlap in protonation steps prevented the calculation of macroscopic pKa values using the standard data fitting model. Considering the full symmetry of polycarboxylate structures, we reduced the number of unknown NMR parameters in the “Q-fitting” or the novel “equidistant macroscopic” evaluation approaches. These models already provided pKa values, but some of them proved to be physically unrealistic, deceptively suggesting cooperativity in carboxylate protonations. The latter problem could be circumvented by adapting the microscopic site-binding (cluster expansion) model by Borkovec, which applies pairwise interactivity parameters to quantify the mutual basicity-decreasing effect of carboxylate protonations. Surprisingly, only a single averaged interactivity parameter could be calculated reliably besides the carboxylate ‘core’ microconstant for each CD derivative. The speciation of protonation isomers hence could not be resolved, but the optimized microscopic basicity parameters could be converted to the following sets of macroscopic pKa values: 3.84, 4.35, 4.81, 5.31, 5.78, 6.28 for Suα-CD; 3.82, 4.31, 4.73, 5.18, 5.64, 6.06, 6.54 for Suβ-CD and 3.83, 4.28, 4.65, 5.03, 5.43, 5.81, 6.18, 6.64 for Suγ-CD. The pH-dependent charge of these compounds can now be accurately calculated, in support of designing new analytical methods to exploit their charge-dependent molecular recognition such as in cyclodextrin-aided chiral capillary electrophoresis.

Evaluation of polyanionic cyclodextrins as high affinity binding scaffolds for fentanyl

“…Molecular dynamics (MD) were performed with AMBER 59 (version 12) employing the recent charges and parameters applied for cyclodextrin scaffolds 60 , 61 and the GAFF force field for the various fentanyls 62 in their protonated states 63 . Fentanyl charges were generated by AM1-BCC calculations 55 in the program ANTECHAMBER.…”

Effect of Substitution Degree and Homogeneity on Cyclodextrin-Ligand Complex Stability: Comparison of Fenbufen and Fenoprofen Using CD and NMR Spectroscopy

“…The substituents may be neutral (methyl, hydroxypropyl), positively charged (e.g., amino-, guanidino- [ 16 ]), or negatively charged (carboxymethyl, sulfobutyl-ether, sulfated [ 17 ]) derivatives, depending on the intended purpose. In the case of substituted derivatives with acid–base active functional groups, the average protonation state is pH-dependent and the determination of the p K a values for each substituent is not straightforward [ 18 ]. Substitution can be made on the OH groups 2, 3, and 6 of α-D-glucopyranose units, resulting in mono-, di-, or trisubstituted derivatives.…”

Estimating the Bias of Model Compounds for the Determination of Species-Specific Protonation Constants

“…7 Another approach to minimizing the number of unknown parameters in a microspeciation scheme is the cluster expansion model, 8 successfully implemented recently for symmetric octaprotic compounds. 9 A number of papers and book chapters describe the details of the experimental methods used for the determination of macro-and microconstants. 10−14 There are two fundamental approaches for the determination of microconstants: deductive methods and combined spectroscopic-pH-metric methods.…”

“…Several studies covered the principles of microspeciation (i.e., determination of species-specific protonation constants) based on experimental techniques, such as classical potentiometry or spectroscopy-augmented titrimetric methods. − As the number of basic sites grows, the number of microconstants increases exponentially; therefore for macromolecules only site-specific “group” constants are attainable unless some constraints can be introduced on the microspeciation via symmetry in the compound . Another approach to minimizing the number of unknown parameters in a microspeciation scheme is the cluster expansion model, successfully implemented recently for symmetric octaprotic compounds …”