I. Protein design: Reengineering cellular retinoic acid binding protein II into a retinal binding protein. II. Pseudo-caged compounds as chromophoric hosts for the absolute stereochemical determination of chiral compounds

I. The underlying chemical reasons that enable color vision have been long studied, but still not fully understood. Human eye contains four different transmembrane proteins called opsins that upon binding to the same small molecule, 11-cis-retinal, as a protonated imine through a lysine residue, result in both dim and color vision. The difficulties in studying these proteins has prompted us to develop a model system, by using a surrogate protein, that could allow us to determine the mechanism by which color vision is mediated. Cellular Retinoic Acid Binding Protein II (CRABPII), a small cytosolic protein that binds all-trans-retinoic acid, was chosen to act as the opsin mimic. By using rational protein design and through site directed mutagenesis CRABPII has been converted into a retinal binding protein. All-trans-retinal binds to the engineered CRABPII protein as a protonated Schiff base through an engineered lysine residue. The mode of binding has been found to be very similar to what is observed in the native opsin systems.; II. Exciton coupled circular dichroism (ECCD) has emerged as a powerful technique for the determination of the absolute stereochemistry of chiral compounds in an non-empirical manner. The method is based on the through space interaction of neighboring chromophores and has been used for the stereochemical determination of a variety of small organic molecules. Along these lines, we were engaged in designing chromophoric receptors that upon coordination of a metal ion can assume a pseudo-caged conformation. In the absence of external chirality, the formed complex exists as a racemic mixture of the two possible enantiomeric forms and is ECCD silent. Upon coordination of a chiral molecule, in our case chiral monoamines, one of the two diastereomeric forms is favored, resulting into an ECCD active species. The sign of the ECCD couplet directly depends on the absolute stereochemistry of the bound chiral compound.