Inference on the Mechanism of Retention by Force Field Calculations
Here, we discuss the mechanism of retention in polysaccharide-based chiral stationary phases (CSPs) from the results of molecular force field calculations. A table of the retention factors of phenanthrene (1), anthrecene (2), triphenylene (3), and tetracene (4) is shown below.
<Comparison of Retention>
※Numerical values in the table are retention factors K'.
The table shows that some compounds are specifically and strongly retained on polysaccharide-based CSPs when compared to the ODS-type column.
In particular, CHIRALCEL® OJ-H retained tetracene (4) very strongly compared to triphenylene (3), which isn't retained much at all. This prompted us to examine this phenomena more closely. Professor Ichiyoshi Ueda of Yokohama National University analyzed the separation mechanism of tetracene and triphenylene by CHIRALCEL® OJ-H using molecular force field calculations. (Carbohydrate Research 439, p.35-432017).
The recognition site of CHIRALCEL® OJ-H is presumed to be the polymer structure as shown in Figure 1. When tetracene passes along the polymer structure, the most stable location places it within the gaps of the polymer structure as shown in Figure 2. This gap, which is represented by the white circle in Figure 1, is deeper than it might first appear. It forms a corridor-like structure that has a floor composed of glucose and benzene rings from the polysaccharide backbone, and walls composed of benzene rings from the chiral selectors. This allows for the elongated, more linear tetracene to better fit in this gap without distortion of the polymer. Triphenylene will attempt to enter this same gap. However, because triphenylene is bulkier, the polymer needs to distort from it more stable confirmation to accommodate the structure, as shown in Figure 3. This distortion changes the interaction energies of triphenylene roughly 1 kcal lower than that for tetracene. This lower energy results in a weaker interaction, thus a shorter retention time. In other words, there are molecules that can be more easily accommodated into the interaction space of the polymer and molecules that cannot be as easily accommodated into the interaction space. This is believed to contribute to the difference in retention.
<Yokohama National University Ueda laboratory>
※Abbreviations in the page
|Column name||Abbreviations in the page|