G.L.J.A. Rikken and E. Raupach
In biology, chemistry and physics one often deals with systems that can occur in two forms that are each others mirror image. This phenomenon of chirality (from the Greek cheiros = hand) plays a particularly important role in biochemistry, since in most biochem-ical reactions only one enantiomer (mirror image) of the molecules involved can partici-pate. In physical terms, chirality is equivalent to breaking of parity. It is clear on symmetry grounds that in order to obtain a chiral result from any reaction, it has to contain a chiral component or it has to be subjected to some external chiral influence. A well-known ex-ample of this principle is the use of circularly polarized light as a chiral agent in certain photochemical reactions, where the handedness of the light determines the handedness of the reaction product. It has often been attempted to use magnetic fields to induce chirality in chemical reactions, but so far in vain. Recently we have observed a new magneto-optical effect, called magneto-chiral anisotropy (MChA) that establishes a coupling between chi-rality and magnetic fields and that can discriminate between the two enantiomers of chiral systems. Here we show experimentally that through this effect, a magnetic field can lead to an enantiomeric excess in a photochemical reaction with unpolarized light.
Natural optical activity, which occurs exclusively in chiral media, and magnetic optical activity, which is induced by a longitudinal magnetic field, show a strong phenomenolog-ical resemblance. In both cases, the polarization of light is rotated during propagation through the medium. Interpreting magnetic optical activity as a sign of magnetically in-duced chirality, Pasteur was the first to search for an enantioselective effect of magnetic fields, followed by many others. These searches were in particular motivated by the hope of finding an explanation for the homochirality of life. They were however unsuccessful as symmetry forbids a magnetic field per se to induce chirality.
The recently discovered MChA effect can be regarded as a cross-effect between natural optical activity and magnetic optical activity, with its own characteristic symmetry proper-ties. It is described by an extra term in the dielectric constant; for chiral isotropic media
like gases, liquids, or cubic crystals, and light propagating with wave vector k in a mag-netic field B, this term is proportional to kB, and of opposite sign for the two enantiomers, i. e. it is enantioselective. Amongst others, MChA leads to a difference in absorption co-efficient for unpolarized light in a magnetic field for the two enantiomers, proportional to kB. As a photochemical reaction rate is proportional to the absorption coefficient, this will lead to different rates for the two enantiomers, and therefore in general could lead to an excess of one enantiomer over the other.
We have studied the implementation of this principle on the Cr(III)tris-oxalato complex (see inset Fig. 15), as this is expected to show a relatively large MChA. This complex, that exists in a right and a left handed version, is unstable in solution and spontaneously dissociates and re-associates. In equilibrium, one has equal concentrations of right and left handed complex. The dissociation is accelerated by the absorption of light, so under irradiation with unpolarized light in a magnetic field, one enantiomer will dissociate more often, whereas the re-association is random. This leads to an excess of the less absorbing enantiomer, the handedness of which depends on the relative orientation of k and B. If this process, called photoresolution, is much faster than the thermal racemization (the return to a 1 : 1 mixture), the size of the excess in dynamic equilibrium should be given by gMChA=2, where gMChA is the MChA asymmetry factor which equals
2f(k""B) (k"#B)g=f(k""B)+(k"#B)g, being the optical extinction coeffi-cient.
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P E M
T i-sa pphire
G re N e
B
Figure 15: Experimental setup for determining photochemistry of the Cr(III)tris-oxalato com-plex. Monochromatic irradiation is performed with a Ti : sapphire laser around 696 nm, with a po-larization state that can be se-lected between linear, circular and unpolarized. The latter was obtained after passing the light through 10 meters of 1 mm dia-meter optical fiber. The inset shows the molecular model of the Cr(III)tris-oxalato complex.
(LA: lock-in amplifier, PEM:
photoelastic modulator, GreNe:
helium-neon laser at 543 nm).
Figure 15 shows the experimental setup to generate and detect the enantiomeric excess (ee).
Irradiation is done by a Ti-sapphire laser around the spin-forbidden transition4A2g!2Eg of the Cr3+ion. Detection of the excess is done by measuring the natural circular dichroism at the4A2g!4T2g transition, the value of which known for the enantiopure compound.
Figure 16 shows the obtained excess as a function of magnetic field. For fields parallel to the irradiation direction k, we observe a strictly linear relation, for perpendicular fields no significant excess can be detected, in agreement with the prediction.
Figure 16: Enantiomeric excess obtained after irradiation with unpolarized light during 25 min-utes at = 695.5 nm, as a func-tion of magnetic field, with an ir-radiation direction k either par-allel or perpendicular to the magnetic field B.
Figure 17 gives the obtained excess as a function of irradiation wavelength, which shows a characteristic derivative-type line shape, indicative of so-called A-terms, implying that the magnetic field influence on the optical properties is through the Zeeman effect. This behavior was predicted for the MChA of the Co(III)tris-oxalato complex, but no detailed prediction for the Cr(III) complex is available. We have found earlier that for a given elec-tronic transition, the order of magnitude of gMChAcan be estimated by gNCDgMCD, where gNCDand gMCDare the asymmetry factors for natural and magnetic circular dichroism. At
= 701 nm this yields an estimate of ee/B = 1.310 6T 1, close to the observed value of ee/B = 1.710 6T 1.
Figure 17: Enantiomeric excess obtained with the magnetic field of 7.5 T parallel to the irradia-tion direcirradia-tion, as a funcirradia-tion of the irradiation wavelength, with unpolarized light.
MChA has been suggested as an explanation for the homochirality of life. So far, the two possible causes for this homochirality considered as most likely, are photochemistry with circularly polarized light, and the electroweak interaction. The latter, although never observed in this context, is predicted to lead to extremely small ee (10 17). The photo-chemistry with circularly polarized light can yield ee close to unity. At this moment it is unknown how large a prebiotic ee is required in order to arrive at homochirality. We can therefore only wonder if the magnetic fields observed in nature, ranging from the Earth’s magnetic field of 10 4 T, up to the stray field of neutron stars (108 T on the surface), might through MChA lead to sufficiently large ee. Furthermore, issues of spectral, spatial and temporal averaging have to be addressed. Clearly the question of the origin of the homochirality of life is far from answered. Our results merely suggest that MChA merits consideration in this discussion.