Introduction

       Since Pasteur’s seminal observations with enantiomers of para-tartrate over one hundred and fifty years ago, chirality has been recognized to play a critical role in living systems. Whereas natural chemical reactions rarely show chiral preference, chirality is commonplace in biological systems. Therefore, one would expect that the detection of chiral selectivity would be an excellent indicator for signs of life and potentially provide critical diagnostic information during space flight. Unfortunately, current instrumentation, such as polarimetry and circular dichroism, lack sensitivity and are unsuited for the rigors of space instrumentation. Recent advances in our lab with a magneto-optical technique coined Magneto-Optical Enantiomeric Detector (MOPED) aims to address these shortcomings and provide a device uniquely able to sensitively detect chiral signatures. In addition to providing a novel detector for space exploration, providing such a device would greatly aid the pharmaceutical industry where current analytical instruments are lacking the required speed/sensitivity and two thirds of the drugs on the market are chiral and recent rulings by the FDA ensure that all drugs possessing stereogenic centers will be developed as pure enantiomers.
 
         MOPED utilizes several experimentally simple, but scientifically sophisticated techniques from state-of-the-art optics research, first developed for nonlinear-optical spectroscopy, but which we now apply to simple polarimetry. These techniques include lock-in detection, the Faraday effect using a sinusoidally varying magnetic field, and optical heterodyne detection (OHD). It also involves the recent remarkable discovery that the second-order Faraday effect is correlated to the chirality of a substance. While large dc magnetic fields were used to confirm this equivalence, we propose to use OHD and lock-in detection, allowing us to much more sensitively detect the second-order Faraday effect, and hence also a substance’s chirality. This involves applying a sinusoidally varying a magnetic field and lock-in detecting at the second harmonic of the applied magnetic field oscillation frequency. We not only find that these techniques yield much more information than simple polarimetry, but, even in preliminary measurements, they also provide dramatically increased sensitivity and decreased analysis time. Furthermore, the device is relatively simple, light, easy to use, and robust—suitable for space travel.

Fig. 1: Apparatus for MOPED - The magnetic field is varied sinusoidally. The polarizers are slightly uncrossed to allow some input light to propagate into the detector and add coherently with the polarization-rotated beam. The lock-in detector can be set to detect at various frequencies, but the best case is the second harmonic of the frequency of the magnetic-field sinusoid. In this manner, the second-order Faraday effect, which highly yields the chirality, can be measured with all the advantages of lock-in detection and heterodyne detection. Also, insertion of a quarter-wave plate after the first polarizer yields the analogous quantity for circular dichroism.

Fig. 2: Signal waveforms that result in MOPED for various cases. In all four figures, the black curve is the magnetic field vs. time, and the red curve is the output light intensity (in the absence of any polarizer leakage). Upper left: constant signal results in the absence of the Faraday effect, even if chirality is present. Upper right: in the absence of chirality, the linear Faraday effect induces sinusoidal output light. Lower left: The presence of both chirality and the Faraday effect yields a phase-shifted signal intensity. Lower right: the presence of higher-order Faraday effects introduces distortions in the intensity vs. time curve. In this case, detection at higher lock-in frequencies yields the higher-order terms. The second-order Faraday term is a manifestation of the chirality and can be more sensitively detected than the simple chirality, which doesn’t lend itself to lock-in detection.

       We are also considering the molecules likely to be detected. Necessary components in the search for extraterrestrial life include liquid water, organic homochiral molecules, and biopolymers acting as replicators and catalysts. Search for chiral molecules and/or catalytic biomolecules thus serve as suitable benchmarks for determining the presence of life. One of the simplest reactions involving change of optical rotation to test for chirality is racemization, a first-order process changing optical rotation from an initial value towards zero for the racemate.

       Mandelate racemase, an excellent test case for this scenario, is a protein with an ancestral template that catalyzes the racemization of mandelate, one of the simplest chiral molecules, with virtually no chemical background reaction but a huge specific optical rotation ([a_D²⁰] = -152° in water), which makes it enticing for use in a polarimetric assay.  We have successfully cloned, over-expressed, and purified this enzyme.

       Conditions for life on other planets or moons, most specifically Mars, Titan, and Europa, differ from those on Earth, however, and tend to involve much more extreme temperatures, pressures, and/or compositions of the immediate environment.  Temperatures on all mentioned celestial objects are much lower than on Earth, from 0 to –40°C on Mars down to –180°C on the surface of Titan.  Recently on Titan, lakes of hydrocarbons were found next to traces of solid water.  For these reasons, we embarked on an exploration of reactivity of mandelate racemase at low temperatures (down to –40°C and lower) and in partially or mostly organic solvent systems mimicking conditions on surfaces of bodies such as Titan.

       We have found mandelate racemase at ambient temperature to be very active in reversed micelles formed of nano-sized aqueous pools surrounded by surfactants in a hydrocarbon continuum. Both anionic and non-ionic surfactant systems, Aerosol OT/n-heptane/water and Triton X-100/hexanol/n-heptane/water, respectively, show similar results.  Trials at lower temperatures, –40°C, are encouraging: the protein seems to stay active.  We intend to submit a manuscript to the journal Astrobiology within a month.