Optimal Environment for Maximal Molecular Recognition against Temperature. Three Distinct Patterns in Temperature–Recognition Profiles as a Function of Composition of Aqueous Binary Solvents

• Published in: Bulletin of the Chemical Society of Japan 2004-12, Vol.77 (12), p.2213-2218
• Main Authors: Endo, Tadashi, Sugio, Keitaro, Kogure, Sadahiro, Horikosi, Suguru, Tojima, Motohiro, Aono, Yosihiro, Ito, Masato M
• Format: Article
• Language: English
• ISSN: 0009-2673; 1348-0634
• DOI: 10.1246/bcsj.77.2213

Oxidation of a pair of associating thiols 1 and 2, each having a binding site [–C(=O)NHC(=O)NH–] and a recognition site (R1 or R2), is examined at various temperatures as a function of mole fraction of water (xw) in aqueous binary solvents with EtOH, CH3CN, and i-PrOH. The selectivity (r) — a measure of the degree of molecular recognition in the oxidation — is represented by the logarithmic ratio of the yield of the unsymmetrical disulfide 4 to twice that of the symmetrical one 3. It is found that three distinct patterns are present in temperature–recognition profiles, depending on composition of the aqueous binary solvents: (1) “progressive decrease” in the lower xw region; (2) “presence of a maximum” in the medium xw region; (3) “progressive increase” in the higher xw region. It is also clarified that aqueous mixed solvents in the medium xw region are indispensable for maximal recognition to occur against temperature (the presence of a maximum in the temperature–selectivity relationship). Dependence of the r on the xw at lower and higher temperatures, presence of three distinct patterns in the temperature–recognition profiles, and so on are discussed in relation to the maximal recognition.