journalof MEEEE Journal of Membrane Science 93 ( 1994) 157- 164 ELSEVIER Optical resolution of amino acids by ultrafiltration membranes containing serum albumin Akon Higuchi”p*,Mariko Haraa, T&u Horiuchib, Tsutomu Nakagawab zyxwvutsrqponmlkj ‘Department ofIndustrial Chemistry, Seikei University, Kichvoji Kitamachi, Musashino, Tokyo 180, Japan bDepartment of Industrial Chemistry, Meiji University, Higashimita, Tama- ku. Kaw asaki, Kanagaw a 214, Japan Received 12 August 1993; accepted in revised form 2 1 March 1994 Abstract Ultrafiltration experiments for optical resolution of racemic phenylalanine and leucine were performed in a solution system containing bovine serum albumin (BSA) and by using immobilized BSA membranes. It was found that Dphenylalanine preferentially existed in the permeate at pH 7.0 due to the binding of BSA to L-phenylalanine and that the concentration ratio of D-isomer to L-isomer in the permeate increased with a decrease in the feed concentration of the racemate in the solution system. This observation is explained by a site saturation mecha- nism. A minimum point was observed in the plot of the separation factor vs. feed concentration of leucine for the optical resolution of leucine in the solution system. It is suggested that BSA in the solution has two binding sites to leucine (i.e., binding sites to Dleucine and L-leucine). It was found that the immobilized BSA membranes effi- ciently demonstrated optical resolution of racemic amino acids, although the racemic amino acids were optically resolved less effectively in the ultrafiltration using the immobilized BSA membranes than in the BSA solution. Keywords: Ultrafiltration; Serum albumin; Optical resolution; Amino acids zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON 1. Introduction The resolution of optical isomers from the racemate is potentially in demand for the pro- duction of pharmaceuticals and food products. Column chromatography or crystallization has been used for the optical resolution of race- mates, but there are several potential advantages to the use of membrane processes instead of the chromatographic method [ l-3 ] and the prefer- ential crystallization method [ 41. The advan- tages are, for example, large-scale separation, en- ergy-saving technique and easy operation of the *Corresponding author. membranes. Although there are several studies on liquid membranes [ 5-7 1, there are only a few studies [8-l 3 ] on the separation of optical iso- mers using polymeric (solid) membranes and these studies are mainly focussed on the optical resolution of hydrophobic amino acids (i.e., tryptophan and phenylalanine ) . Serum albumin is known to have a high-affin- ity binding site for Ltryptophan. Its binding con- stant is reported to be 4.4~ lo4 M-i by Kragh- Hansen [ 141, and it is reported that weakly bound Dtryptophan displaces L-tryptophan [ 151. This evidence prompted us to think that optical resolution of amino acids might be pos- sible by means of ultrafiltration of solutions of 0376-7388/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO376-7388(94)00081-9