New Optical Probes of Chiral Molecules

The physical chemistry of chirality

Chirality is found at all physical length scales: from interactions mediated by the weak force to the anatomy of living organisms. It plays a particularly important role in biological systems, as many organic molecules are chiral. For example, all but one amino acid are chiral, and shown below is a generic amino acid that exists in two mirror-image forms (enantiomers), if the side-chain R is not a hydrogen (i.e. not glycine). In the D, L-nomenclature introduced by Emil Fischer, natural amino-acids on earth are almost all of the L-form.

(see http://biomedia.bio.purdue.edu/IML/Amino/html/stereoisomers.html)

Some D-amino acids can occur in nature due to racemization of L-amino acids. However, they are the exception and just as amino acids are found predominately in one configuration, so are the natural sugars, but this time in the D-form. The chirality of the amino acids and sugars in turn cause proteins and DNA to be chiral (handed), and thus are often the cause for highly specific biological function. Described by Emil Fischer as a "lock and key" mechanism, the stereospecific action between an enzyme and its substrate is the reason for the homochiral biochemstry of living organisms. Starting with sugars themselves, of which organisms can only metabolize the D-form, there are many examples of the different biological and physiological action of enantiomers. Despite the clear differences in biological action, the two mirror image forms of a chiral molecule have in many respects identical physical properties. Only under a chiral influence, such as another chiral molecule or circularly polarized light, do the interactions become enantio-specific. Optical methods are often the only practical physical means to distinguish between enantiomers, and much of our work concerns new optical effects that can probe chiral molecules in solution.

Conventional optical activity phenomena, such as optical rotation and circular dichroism, are based on the interference of induced oscillating electric- and magnetic (and electric-quadrupole) moments, and arise from a differential response to left and right circularly polarized light. Circularly polarized light is clearly chiral, and so it is remarkable that the action of three linearly polarized electric fields can also be a chiral probe in a homogeneous and isotropic liquid. This is indeed the case, when two optical fields of different frequency interact coherently to generate light at their sum- or difference frequency. Some of our recent work examines sum-frequency generation (SFG) in liquids as a nonlinear chiral spectroscopy [1-4]. SFG is in general a purely electric-dipolar nonlinear-optical process and the signal photons from an optically active liquid are only generated, if the molecules are chiral.

In the study of SFG from liquids, we have recently observed a new electro-optic effect, i.e. the influence of a static electric field on the intensity of SFG. The effect arises when a static electric field is applied to coherent sum-frequency generation in an optically active liquid (R-(+)-1,1'-binaphthol in tetrahydrofuran). The static field does not change the phase matching conditions of the sum-frequency process, but it gives rise to an electric-field induced contribution to the signal. The beat between chirality-sensitive SFG (a second-order process) and achiral electric-field induced sum-frequency generation (a third order process) yields a contribution to the intensity that is linear in the static electric field and that changes sign with the enantiomer. The effect can therefore be used to determine the absolute sign of the isotropic part of the sum-frequency hyperpolarizability and hence the handedness of chiral molecules in solution [5,6].

Selected Publications:

General bibliography