Chiral separations using various polymerized dipeptide surfactants in electrokinetic capillary chromatography (EKC) are investigated. The two main dipeptide surfactants used in this study were sodium N-undecylenyl-L-valine-L-leucine (L-SUVL), and sodium N-undecylenyl-L-leucine-L-valine (L-SULV). These studies were performed in order to determine if the order of amino acids in dipeptide surfactants is important in terms of chiral recognition and separations. Both the monomer and the polymer of these two surfactants were compared for the separation of two model atropisomers, (+/-)-1,1-bi-2-naphtol (BOH) and (+/-)-1,1'-bi-2-naphthyl-2,2'-diyl hydrogen phosphate (BNP). Some advantages and disadvantages of the polymer relative to the monomer are discussed. Four other surfactants, the polymers of sodium N-undecylenyl-L-leucine-L-leucine (L-SULL), sodium N-undecylenyl-L-valine-L-valine (L-SUVV), sodium N-undecylenyl-L-valine (L-SUV), and sodium N-undecylenyl-L-leucine (L-SUL), were also used in this study, and their performance was compared to that of poly(L-SULV). These data show conclusively that the order of amino acids in dipeptide surfactants has a dramatic effect on chiral recognition. Our investigations indicate that poly-(L-SULV) provides the best enantioselectivity among the four dipeptide and two single amino acid surfactants for the separation of BNP and BOH. The advantages of poly-(L-SULV) are demonstrated via the ultrafast separation of the enantiomers of BNP and BOH in less than 1 min.
In this study, a large number of polymeric chiral surfactants were examined and their performances in terms of enantiomeric resolution compared for a variety of chiral analytes. The surfactants investigated in this study include all possible dipeptide combinations of the L-form of alanine, valine, leucine, and the achiral amino acid glycine (except glycine-glycine). Also included in this study were the single amino acid surfactants of alanine, valine, and leucine as well as the single chiral center dipeptide surfactant poly(sodium undecyl-L-leucine-beta-alanine) (poly L-SULbetaA). Several different aspects of polymeric dipeptide surfactants, as they pertain to chiral separations, are examined. Some of the factors investigated in this report include the effect of position and number of chiral centers, amino acid order, and steric effects.
Poly sodium N-undecyl leucine-leucine (poly SULL) is used as a diagnostic tool to investigate chiral molecular interactions via electrokinetic chromatography (EKC). Poly SULL has two chiral centers which are defined by two asymmetric carbons. Each chiral center of poly SULL can have two possible configurations (D or L). Consequently, four different optical configurations are possible within the surfactant molecule (L-L, D-D, L-D, and D-L). In this study, five chiral analytes of various charge states and hydrophobicities were used to investigate the role of electrostatic interactions and hydrophobicity on chiral recognition with polymeric dipeptide surfactants. These studies lead to a proposed hypothesis for interaction of the analytes with dipeptide surfactants. The hypothesis was tested and the contribution of the double chiral centers to this interaction was evaluated by use of two dipeptide surfactants in which one chiral amino acid is replaced by an achiral amino acid glycine, i.e., poly sodium N-undecyl L-leucine-glycine (poly L-SULG) and poly sodium N-undecyl L-glycine-leucine (poly L-SUGL). The results reported here provide new insights into the mechanism for chiral recognition of select chiral analytes by use of polymeric chiral surfactants.
The effect of amino acid order on chiral selectivity in polymeric dipeptide surfactants, as well as the physical properties of the surfactants, is investigated. An understanding of enantioselectivity of such dipeptide surfactants is crucial to the design of more efficient polymeric surfactants and has implications in other areas of research such as enantioselective interactions of amino acid based compounds (i.e., enzymes, hemoglobin, antibodies, etc.). It should be noted that such polymeric surfactants are not easily crystallized. Therefore, in a manner similar to the study of proteins, fluorescence spectroscopy is a powerful tool used to study the structure-function relationship of these polymeric surfactants. The microenvironments inside the core of 18 polymeric surfactants were characterized using the environmentally sensitive probes pyrene and 6-propionyl-2-(dimethylamino)naphthalene (Prodan). The surfactants examined in this study include all possible dipeptide combinations of the L-form of alanine, valine, and leucine and the achiral amino acid glycine (except glycine-glycine) as well as the single amino acid surfactants of alanine, valine, and leucine. The results of the fluorescent probe studies led to a proposed structure of the polymeric dipeptide surfactants in solution. The implications of the proposed structure for chiral selectivity were tested with two model atropisomers, (+/-)1,1'-bi-2-naphthol and (+/-)1,1'-bi-2-naphthyl-2,2'-diyl hydrogen phosphate, using capillary electrokinetic chromatography.
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