Correlation between the NMR Chemical Shifts and Thiolate Protonation Constants of Cysteamine, Homocysteine, and Penicillamine

1H and 13C NMR measurements were carried out to explore anticipated correlations between chemical shifts versus thiolate basicities and redox potentials of cysteamine, homocysteine, penicillamine, and their homodisulfides. All correlations were analyzed and statistically evaluated. The closest correlation was observed for the αCH nuclei concerning 1H and 13C NMR data. Since neither site-specific basicities nor site-specific redox potentials can be directly measured by any means in peptides and proteins containing several thiol and/or disulfide units, these data provide a simple method and predictive power to estimate the aforementioned site-specific physicochemical parameters for analogous sulfur-containing moieties in related biopolymers

Physicochemical Characterization and Cyclodextrin Complexation of the Anticancer Drug Lapatinib

Lapatinib (LAP), the tyrosine kinase inhibitor drug with moderate bioavailability, was characterized in terms of physicochemical properties: acid-base characteristics, lipophilicity, and solubility. The highly lipophilic nature of the drug and its extremely low water solubility (S0=0.82 nM) limit the development of a parenteral formulation. In order to enhance solubility and bioavailability, inclusion complex formation with cyclodextrins (CDs) is a promising method of choice. Therefore, LAP-CD interactions were also studied by a multianalytical approach. The stability constants of LAP with native cyclodextrins, determined by UV spectroscopy, identified the seven-membered β-CD as the most suitable host. Continuous variation method (Job’s plot) by 1H NMR showed a 1 : 1 stoichiometry for the complexes. The geometry of the complex was elucidated by 2D ROESY NMR measurements and molecular modeling, indicating that the partial molecular encapsulation includes the fluorophenyl ring of LAP. Phase-solubility studies with four CDs, β-CD, (2-hydroxypropyl)-β-cyclodextrin (HP-β-CD), randomly methylated-β- (RAMEB-) cyclodextrin, and sulfobutylether-β-cyclodextrin (SBE-β-CD), show an AL type diagram and highly increased solubility via CD complexation. The results are especially promising with SBE-β-CD, exerting more than 600-fold gain in solubility. The equilibrium and structural information presented herein can offer the molecular basis for an improved drug formulation with enhanced bioavailability

Close correlation between thiolate basicity and certain NMR parameters in cysteine and cystine microspecies

The imbalance between prooxidants and antioxidants in biological systems, known as oxidative stress, can lead to a disruption of redox signaling by the reactive oxygen/nitrogen species and is related to severe diseases. The most vulnerable moiety targeted by oxidant species in the redox signaling pathways is the thiol (SH) group in the cysteine residues, especially in its deprotonated (S�) form. Cysteine, along with its oxidized, disulfide-containing form, cystine, constitute one of the most abundant low molecular weight biological redox couples, providing a significant contribution to the redox homeostasis in living systems. In this work, NMR spectra from cysteine, cystine, and cysteine-containing small peptides were thoroughly studied at the submolecular level, and through the chemical shift data set of their certain atoms it is possible to estimate either thiolate basicity or the also related standard redox potential. Regression analysis demonstrated a strong linear relationship for chemical shift vs thiolate logK of the cysteine microspecies data. The αCH 13C chemical shift is the most promising estimator of the acid-base and redox character

Solution Structure and Acid-Base Properties of Reduced α-Conotoxin MI

The reduced derivative of α-conotoxin MI, a 14 amino acid peptide is characterized by NMR-pH titrations and molecular dynamics simulations to determine the protonation constants of the nine basic moieties, including four cysteine thiolates, and the charge-dependent structural properties. The peptide conformation at various protonation states was determined. The results show that the disulfide motifs in the native globular α-conotoxin MI occur between those cysteine moieties that exhibit the most similar thiolate basicities. Since the basicity of thiolates correlates to its redox potential, this phenomenon can be explained by the higher reactivity of the two thiolates with higher basicities. The folding of the oxidized peptide is further facilitated by the loop-like structure of the reduced form, which brings the thiolate groups into sufficient proximity. The 9 group-specific protonation constants and the related, charge-dependent, species-specific peptide structures are presented

Understanding the pH Dependence of Supersaturation State—A Case Study of Telmisartan

Creating supersaturating drug delivery systems to overcome the poor aqueous solubility of active ingredients became a frequent choice for formulation scientists. Supersaturation as a solution phenomenon is, however, still challenging to understand, and therefore many recent publications focus on this topic. This work aimed to investigate and better understand the pH dependence of supersaturation of telmisartan (TEL) at a molecular level and find a connection between the physicochemical properties of the active pharmaceutical ingredient (API) and the ability to form supersaturated solutions of the API. Therefore, the main focus of the work was the pH-dependent thermodynamic and kinetic solubility of the model API, TEL. Based on kinetic solubility results, TEL was observed to form a supersaturated solution only in the pH range 3–8. The experimental thermodynamic solubility-pH profile shows a slight deviation from the theoretical Henderson–Hasselbalch curve, which indicates the presence of zwitterionic aggregates in the solution. Based on pKa values and the refined solubility constants and distribution of macrospecies, the pH range where high supersaturation-capacity is observed is the same where the zwitterionic form of TEL is present. The existence of zwitterionic aggregation was confirmed experimentally in the pH range of 3 to 8 by mass spectrometry