Circular Dichroism of Amino Acids in the Vacuum‐Ultraviolet Region

Abstract

Dedicated to Professor Henri B. Kagan on the occasion of his 80th birthday Biopolymers such as nucleic acids and proteins are composed of chiral monomers that show identical stereochemical configuration. Naturally occurring proteins are made up of l-amino acids.[1] Hypotheses for the origin of symmetry breaking in biomolecules include the absolute asymmetric photochemistry model by which circularly polarized (CP) light induces an enantiomeric excess (ee) in chiral organic molecules. [2][3][4] This model is supported by both the observation of CP light in the star-forming region of Orion [3,5] and the occurrence of l-enantiomer-enriched amino acids in carbonaceous meteorites. [6][7][8] However, the differential absorption of CP light by amino acid enantiomers, which determines the speed and intensity of enantioselective photolysis, is unknown over a large spectral range. Here we show that significant circular dichroic transitions in amino acids can be observed by extending circular dichroism (CD) spectroscopy to the vacuum-ultraviolet (UV) spectral range. a-H amino acids show the same CD magnitude and sign over a large wavelength range. In a given spectral window [9] CP light is therefore capable of inducing enantiomeric excesses of the same handedness into the proteinogenic amino acids we have studied. Absolute asymmetric photochemistry might thus well have triggered the appearance of l-amino acid based life on Earth. Our results demonstrate that enantiomers of "meteoritic" a-methyl amino acids show dichroic absorption with equal magnitude, yet opposite sign to a-H amino acids. Therefore CP light cannot induce l enantiomeric excesses into a-methyl and a-H amino acids as found in meteorites.To explain the cause of symmetry breaking in biomolecules a well-known theory [2-4, 10, 11] proposes that CP interstellar UV radiation-similar to that identified in the starforming region of Orion in the infrared [3,5] -induced enantiomeric excesses into interstellar and circumstellar organic compounds by asymmetric photochemical reactions prior to their deposition on the early Earth. [12] In support of this theory chiral amino acid structures were identified in interstellar ice analogues [13] and a large number of l-enantiomer-enriched amino acids have been identified in the interior of the Murchison [6] and Murray [7] carbonaceous meteorites.[8] To verify the absolute asymmetric photochemistry model the differential CP-light absorption of proteinogenic and meteoritic amino acid enantiomers requires systematic examination.Until now, the popular and extensively used technique of CD spectroscopy has been used to record electronic CD for chiral molecules in aqueous solution above 190 nm.[14] Water absorbs photons of l < 190 nm, making the vacuum-UV region inaccessible for CD spectroscopy in aqueous solution. By using a synchrotron radiation source for CP light and preparing isotropic amorphous solid-state samples immobilized on MgF 2 windows, we have extended electronic CD measurements to the vacuum-UV spectral range.We observed...

An infrared study of solid glycine in environments of astrophysical relevance

“…Upon heating the neutral form transforms completely to the zwitterionic structure, which is in good agreement with previous observations of room temperature vapour deposited amino acids. 15,26 The evolution is easily followed by the changes undergone by two IR bands, assigned to the neutral species. This process had been studied before for a 9 K substrate.…”

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Polarimetric technique to study (pre)biological organics in cosmic dust and planetary aerosols

“…Multiple amino acids have been found to show optical characteristics akin to chlorophyll a in both circular birefringence and dichroic spectra around a wavelength of 200 nanometers (Iizuka and Yang, 1964;Hawkins and Lawrance, 1973;Meierhenrich et al, 2010). We thus suggest that observations of circular polarization spectra around absorption bands of amino acids may allow for their remote detection in comets, and potentially other objects that may contain (pre)biological organics.…”

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“…Another suggested mechanism lies with the processing of a population of amino acids by circularly polarized light [20][21][22][23][24][25]; this could select one chirality over the other. However, this solution does not easily explain why the physical conditions that would select one form in one place would not select the other in a different location.…”