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Histidine C–H bonds observed in protein structures include (clockwise from top left): myoglobin, β-lactamase, and photoactive yellow protein; calculations indicate that tautomeric/protonation state influences H-bonding ability (bottom left).

Three direct complexometric titration methods for bismuth subsalicylate determination in over-the-counter stomach relief caplets and liquids are presented, as well as a UV–vis assay, for use in student laboratories. The main difference between titration methods is the choice of indicator; either xylenol orange (XO), pyrocatechol violet (PV), or potassium iodide (KI) is used. These methods are simple and safe, making this experiment desirable in a general chemistry or analytical laboratory to relate real-world sample analysis to theoretical titrimetric concepts. Learning outcomes, student feedback, and student data from Wichita State University (WSU) Chemistry II Honors lab, Bethel College Introduction to Chemistry lab, and University of Missouri (Mizzou) Quantitative Analysis lab are presented. The majority of students enjoyed this real-world sample analysis; however, titration results are more precise for more advanced students. The titration with EDTA using XO as an indicator is the method of choice for caplet analysis by the authors, but methods utilizing PV or KI as indicator are also acceptable, although the KI method may generate large amounts of iodide waste depending on class size. All titration methods presented are selective at low pH (∼1.5–1.6) and do not suffer interferences from commonly occurring ions such as magnesium and calcium. Using the titrimetric procedures presented in this work, all tested caplet and liquid brands appear to meet United States Pharmacopeia (USP) standards, where the allowable range of active ingredient is within 90–110% of label value.

Supramolecular macrocyclic forces have been used to trap phytate, myo‐inositol‐1,2,3,4,5,6‐hexakisphosphate, a key bioanion with multiple roles in metabolic processes. Due to the complex chemistry of six multivalent phosphates surrounding the small, cyclic inositol framework, crystallographic information of simple phytate salts has been elusive. This report represents a combined crystallographic, theoretical, and solution binding investigation of a supramolecular macrocyclic complex of phytate.  Together the results provide significant insight to phytate’s intramolecular and intermolecular interactions at the microenvironment level. The macrocycle‐phytate aggregates consist of phytate anionic pairs, each partly sandwiched by two 24‐membered, amide/amine‐based cationic macrocycles.  The phytate ion pairs hold the tetramer macrocyclic array together by six strong intermolecular hydrogen bonds. Both phytates crystallize in 1a5e phosphate conformations (one axial (P2) and five equatorial phosphates). Solution NMR binding studies in 1:1 DMSO‐d6:D2O indicate 2:1 macrocycle:phytate associations, suggesting that the sandwich‐like nature of the complex holds together in solution. DFT studies indicate the likely occurrence of dynamic intramolecular interchange of phosphate protons, as well as important roles for the axial (P2) phosphate in both intramolecular and intermolecular hydrogen bonding interactions.

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