| Chirality, or handedness, is pervasive in the natural world. An object, like the human hand, which has a non-superposible minor image, is called chiral, and the two forms are enantiomers of each other. Nearly all proteins in all living organisms are chiral, and as a consequence, both enantiomers of a chiral pharmaceutical compound may not have the same properties in biological systems. The market for enantiomerically pure compounds is over ;A method for studying the enantioselective chemisorption of chiral molecules on chirally modified Pd(111) surfaces in ultrahigh vacuum is presented; the modified surface is exposed to both chiralities of a probe molecule, and the coverage of the probe is measured. The ratio of probe coverage for the two experiments is defined as the enantioselective ratio (ER), and is a measurement of the effective amount of space available to each enantiomer on the modified surface. A surface modified with 2-butanol has a maximum ER of ∼1.9 towards the adsorption of propylene oxide, and hydrogen-bonding interactions are found to be critically important to the expression of the modifier chirality. A series of a-amino acids show a trend of decreasing enantioselectivity towards propylene oxide with increasing alpha-side-group length, which was suppressed with branching in the beta-position. The modifier alpha-(1-naphthyl)ethylamine (NEA) is selective for both 2-butanol and methyl lactate over a very narrow range of modifier coverage, and H-D exchange in the tautomerization of methyl pyruvate is enhanced in the presence of NEA. |
