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Apparent and true enantioselectivity in enantioseparations
Author(s) -
Götmar Gustaf,
Fornstedt Torgny,
Guiochon Georges
Publication year - 2000
Publication title -
chirality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.43
H-Index - 77
eISSN - 1520-636X
pISSN - 0899-0042
DOI - 10.1002/1520-636x(2000)12:7<558::aid-chir2>3.0.co;2-2
Subject(s) - chemistry , enantioselective synthesis , astrobiology , organic chemistry , catalysis , physics
The separation factor of two compounds in chromatography is the ratio of their equilibrium constants or retention factors. This parameter is universally employed to investigate their resolution and to optimize the experimental conditions of their analysis. In enantioseparations, the situation is more complex because there is a mixed retention mechanism. The retention factor is the sum of two contributions, one enantioselective, the other nonselective. Although both contribute to retention, the latter being identical for the two enantiomers and does not contribute to their separation. We show how these two contributions can be measured and how it becomes necessary to distinguish between the apparent, α app , and the true, α true , separation factors. The existence of nonselective sites is responsible for α app being less than α true . Depending on the difference between these two factors, the more effective approach to improve a separation is either to increase the enantioselectivity or to reduce the nonselective interactions. Practical applications to separations of different β‐blockers on cellobiohydrolase are discussed. The apparent enantioselectivity of alprenolol is larger and increases faster with increasing pH than that of the more hydrophobic propranolol, in spite of the importance of hydrophobic interactions in the enantioselective mechanism. These two unexpected properties are discussed and explained. Chirality 12:558–564, 2000. © 2000 Wiley‐Liss, Inc.