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

Abstract

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.

Vital significance of addition of “shape similarity” between solutes to shape complementarity for more precise molecular recognition in aqueous binary solvents

“…Figure 8 shows a schematic drawing of the three distinct patterns in the temperature—recognition profiles as a function of the x w in aqueous binary solvents, based on the data shown in Figs. 6 and 7 57. In the lower x w regions, the r decreased progressively with temperature.…”

“…The following finding mentioned above should be noted. The pattern changes occurred by the very slight x w change of 0.02 in the aqueous binary solvents 57. This might be considered to have resulted from alterations in solvent structures of aqueous binary solvents.…”

“…Figure 6 shows temperature dependence of the r for R 1 = R 2 = n ‐C 5 H 11 as a function of x w of a H 2 O—EtOH system in lower (A), medium (B), and higher regions (C) 57. When the x w s were in lower regions (Fig.…”

“…Figure 9 illustrates temperature dependence of the r for R 1 = R 2 = n C 5 H 11 in nonaqueous binary solvents containing EtOH ( x EtOH = 0.25) 57. When the other component was not water but MeOH or 1,4‐dioxane, the r simply decreased with temperature.…”