Entropy/Enthalpy Compensation in Anion Binding: Biotin[6]uril and Biotin-l-sulfoxide[6]uril Reveal Strong Solvent Dependency

Abstract

Binding of anions using macrocyclic structures with a nonpolar interior using the CH···anion interaction as the recognition motif has gained popularity in the past few years, and such receptors often rely on a subtle interplay between enthalpic and entropic factors. For these types of receptors solvation of both the anion and the binding pocket of the macrocyclic host play important roles in the overall energetic picture of the binding event. Systematic chemical modifications of synthetic receptors that are able to bind anions in a variety of solvents is an important tool to gain understanding of the factors that determine the supramolecular chemistry of anions. Here we present the chiral macrocyclic structure biotin-l-sulfoxide­[6]­uril as a host molecule that binds anions in both water and in organic solvents. Biotin-l-sulfoxide­[6]­uril is prepared in a highly diastereoselective one-pot synthesis from the macrocycle biotin[6]­uril. We compare the binding properties with that of biotin[6]­uril, also studied in acetonitrile and in aqueous buffer at neutral pH. The biotin-l-sulfoxide­[6]­uril generally exhibits stronger recognition of anions in acetonitrile, but weaker binding in water as compared to the biotin[6]­uril macrocycle. We have studied the binding events using a combination of NMR spectroscopy, isothermal titration calorimetry (ITC), and computational methods.

Cyclohexanohemicucurbit[8]uril Inclusion Complexes With Heterocycles and Selective Extraction of Sulfur Compounds From Water

“…Hemicucurbiturils, formed in templated single-step oligomerization reactions ( Kaabel and Aav, 2018 ), are single-bridged cucurbituril-type macrocycles ( Andersen et al, 2018 ; Lizal and Sindelar, 2018 ; Xi et al, 2018 ) that bear an electron-deficient hydrophobic cavity. The latter grants these macrocycles the ability to encapsulate anions ( Buschmann et al, 2005 ; Cucolea et al, 2016 ; Kaabel et al, 2017 ; Assaf and Nau, 2018 ; Reany et al, 2018 ; Vázquez and Sindelar, 2018 ; Andersen et al, 2019 ; Kandrnálová et al, 2019 ; Valkenier et al, 2019 ; Maršálek and Šindelář, 2020 ); in addition, the formation of complexes with acids and some neutral species has been reported in the previous work. In particular, unsubstituted hemicucurbit[ n ]urils ( n = 6, 12) bind phenol derivatives ( Jin et al, 2016a ; 2016b ) and ferrocene ( Jin et al, 2017 ), and cyclohexanohemicucurbit[ n ]urils cycHC [ n ] ( n = 6, 8, 12) ( Li et al, 2009 ; Aav et al, 2013 ; Prigorchenko et al, 2015 , 2019 ; Mishra et al, 2020 ) form external complexes with both inorganic and organic acids ( Öeren et al, 2014 ; Ustrnul et al, 2019 , 2021 ).…”

Supramolecular Gold Stripping from Activated Carbon Using α-Cyclodextrin

“…There are only three reported molecular receptors for Au(CN) 2 − using metal−organic cages 69,70 or biotin [6]uril. 72 There are no reports of molecular receptors for Au(CN) 2 − , to our knowledge, within the context of commercial gold recovery. The first aim of the present research was to identify molecular receptors for Au(CN) 2 − in aqueous solution with the ultimate goal of achieving efficient gold stripping from the surface of activated carbon at room temperature.…”

“…Anion recognition − has witnessed a blossoming during the past two decades, where strong binding affinities and high selectivities have been achieved for a wide range of anions. Molecular recognition of Au­(CN) 2 – , the most relevant anion in the gold-mining industry, however, has been explored − hardly if at all. There are only three reported molecular receptors for Au­(CN) 2 – using metal–organic cages , or biotin[6]­uril .…”

Cyclohexanohemicucurbit[8]uril forms inclusion complexes with heterocycles and removes sulfuric guests from water via solid-phase extraction

“…Hemicucurbiturils formed in templated single step oligomerization reactions, 18 are single-bridged cucurbituril-type macrocycles [19][20][21] that bear an electron-deficient hydrophobic cavity. The latter grants these macrocycles the ability to encapsulate anions, 19,18,[21][22][23][24][25][26][27][28][29][30][31][32] , and also the formation of complexes with acids and some neutral species has been reported. In particular, unsubstituted hemicucurbit[n]urils (n = 6, 12) bind phenol derivatives 33,34 and ferrocene, 35 and cyclohexanohemicucurbit [n]urils (cycHC[n]) (n = 6, 8, 12) 36-40 form external complexes with inorganic and organic acids.…”