Truth behind the representation of [supramolecule]: Hofmeister effect in supramolecular chemistry

In 1888, Franz Hofmeister found that different salts have different effects on the solubility of protein, and then sorted the effects of ion concentration on the stability of protein to summarize the far-reaching Hofmeister sequence (Figure 1) In the next more than 100 years, scholars found that the influence of ions, especially anions, in solutions on other properties of ionic macromolecular co solutions, such as colloidal system stability, solution viscosity and surface tension, polymer precipitation and crystallization, followed a similar sequence (chem SOC Rev 2014, 43, 205, Chem SOC Rev 2010, 39, 3916) However, under some special conditions, such as when the pH value of solution is less than the isoelectric point (PI), the effect of low concentration anions on the solubility of protein is contrary to that of Hofmeister (proc Natl Acad SCI U.S.A 2009, 106, 15249) Figure 1 Hofmeister sequence (source: J am Chem SOC., 2018, 140, 4092) although Hofmeister effect has great influence on chemical system and biological system, no matter it is positive or negative Hofmeister effect, the reason why they are produced has not been explained in a general way This may be because the properties of macromolecules are relatively complex, and the determination of different properties will get different results Therefore, it will be helpful for us to understand this phenomenon by choosing a simpler small molecular system Recently, Professor Bruce C Gibb of Duran University in the United States has studied the Hofmeister effect of anions on water-soluble calixarenes In fact, most small molecular chemists know a rule: chloride ions can increase the solubility of compounds in the water phase, while perchlorate ions can promote the dissolution of compounds in the organic phase Based on this, Professor Gibb systematically studied the Hofmeister effect in supramolecular chemistry The main compound (positand) with two binding sites (cavity and crown) was used as the research model It was found that at low concentration (~ 2 When the concentration increases, the two binding sites of the main compound interact with the anions, resulting in the decrease of the overall charge of the compound, which leads to the aggregation and finally the precipitation of positand from the solution This achievement was published in the Journal of the American Chemical Society (DOI: 10.1021 / JACS 8b00196) under the title of "ion − hydrogen and / or ion − ion interactions: direct and reverse Hofmeister effects in a synthetic host" Figure 2 Anionic (1) and cationic (2) water-soluble calixarene hole compounds (source: J am Chem SOC., 2018, 140, 4092) The author first selected 14 anions that can combine with positand from 31 monovalent sodium salts by NMR, and then determined their binding constants with the main compounds by NMR titration (Figure 3) Since positand has two binding sites, the affinity of anions to four trimethylamines in the oral cavity (kcrown) was obtained with the help of adamantane carbamate ion (ada-co2 -, Ka = 7.21x106 m-1), and then the bond constant (kcavity) with holes was indirectly calculated It is found that Cl -, no 3 -, Br -, BF 4 - and I - plasma tend to enter the coronary cavity, while CF 3So 3 -, SCN -, BH 3CN -, REO 4 -, ClO 4 - and PF 6 - tend to enter the cavity, but there is no very good law Fig 3 The binding constants of different anions and different binding sites of positand (source: J am Chem SOC., 2018, 140, 4092) In the titration process, the author found that the higher concentration of anions usually leads to the aggregation of main compounds Therefore, after understanding the binding properties of various anions and main compounds, the author began to study their Hofmeister effect Using the measured minimum precipitation concentration (CPC) and DLS data, they found that the precipitation effect of various anions on the main compounds was basically the same as that of Hofmeister sequence, which was pf 6 - > CF 3So 3 - ∼ REO 4 - > ClO 4 - > BH 3CN - > BF 4 - > SCN - > I - > CCL 3Co 2 + Although the author tries to combine the affinity of anions and host compounds with their Hofmeister effect, there is no obvious correlation between them Nevertheless, the author's study proves that Hofmeister effect can also be applied to small molecular systems, and the study on Hofmeister effect of small molecular systems will deepen the understanding of scholars on the interaction of supramolecules At the same time, understanding the precipitation characteristics of the compounds will contribute to the development of crystallography In addition, the Hofmeister effect of small molecules also provides the basis for the follow-up scholars to study the Hofmeister effect of large molecules All authors: Jacobs h Jordan, Corinne L D Gibb, Anthony Wishard, thu Pham, And Bruce C Gibb * corresponding author: Professor Bruce C Gibb is currently working at Duran University in the United States Professor Gibb studied for his Ph.D at Robert gotton University in 1992 From 1993 to 1996, he successively worked in the Prof John C Sherman research group of the University of British Columbia and Prof James W of the University of New York Canary research group is engaged in postdoctoral research; during 2002-2005, he served as an associate professor at the University of New Orleans; in 2005, he was promoted to a professor; since 2012, he has served as a professor at Duran University Professor Gibb is committed to the study of supramolecular chemistry in aqueous solution So far, he has published many articles in international famous journals such as chem SOC Rev., nature chemistry, J am Chem SOC.