Optical Activity as a Biosignature in theSearch for Extraterrestrial Life

Andrew K. Boal

AST74021 April 2006

Outline of this talkScience

How will we detect life on other planets?Some proposed biosignaturesBiomolecular structure, chirality, andcircular dichroism spectroscopy

Exploring the possibilitiesSteps to take to explorefrozen biosignatures

ConjectureWhat happens if there'ssomething in the ice?

Photo: Kjell Ove Storvik/AMASE

Image source: NASA

Image source: NASA

Life- How will we know it?Perhaps the most central of Astrobiological questions is: Howwill we know life when we see it?

It is almost impossible to simultaneously frame this question withoutterrestrial bias and think of ways to explore the solar system for lifeConsideration of biosignatures relevant to Astrobiology must be madewith the fewest terrestrially-biased assumptions to decrease theambiguity in finding a positive result

??Image from:http://commtechlab.msu.edu

Image source: http://library.thinkquest.org

Biosignatures- Immunology and BiomarkersBiochemical tests which are used to study terrestrialbiological molecules have been proposed to study otherplanets

Immunological Studies Fluorescence probes

While a positive result from biochemical tests on another planet wouldbe a unambiguous detection of life, a negative result would beambiguous e.g. Mariner experiments

Use of antibodies to detectproteins

Use of fluorescent probemolecules to detect biomolecules

antibody

antigen

Bindingevent

Signalemitted

hν

Bindingevent

DNA

Fluorescentprobe

Signalemitted

hν

Biosignatures- Isotope fractionationIsotope fractionation for several elements is known to occurand may be taken as a possible biosignature

Sulfur is fractionated by sulfatereducing bacteria

Iron may be fractionated by ironreducing/oxidizing bacteria

Resaerch being worked on by NAI

Fe(aq) Fe(aq)

Fe(aq)

Fe(aq)Ironoxides

Carbon is alsofractionated byorganisms that useCO2 or carbonatesas a carbon source

34SO42-

32SO42-

Leads to an enrichment of 32S ininorganic sulfates

34SH2

32SH2

We don’t really understand how isotopicfractionation occurs so its hard to know if it isa ubiquitous sign of lifeSome abiological processes can also lead toisotopic fractionation

Biosignatures- Complex organic moleculesComplex organic molecules are often thought of as beingsigns for life:

Polycyclic Aromatic Hydrocarbons(PAHs) are found in oil depositsand are the result of decayingbiological matter

Amino acids (AAs) are afundamental component ofterrestrial life and are the buildingblocks of proteins

Since both have been found in meteorites and ISM, complex organicmolecules are considered ambiguous biosignatures

NH

H

OH

O

NH

H

OH

O

NH

H

OH

O

NH

H

OH

O

HO

NH

H

OH

O

NH2

NH

H

OH

O

S

Biosignatures- ChiralityChirality may be an ideal biosignature since it makes only oneassumption

Though chiral organic molecules occur both as a result of biological andabiological processes, only in the case of biological origins is one form ofa chiral molecule selected almost completely over the other

Image from: http://web99.arc.nasa.gov/~astrochm/chirality.jpg

What is Chirality?The classic definition of a chiral object is one which is notsuperimposable with it’s mirror image

Hands are an example of chiralobjects

In chemistry, a pair ofmolecules whose mirror imagesare nonsuperimposable areknown as enatiomersWhen there is more of oneenatiomer than the other (likeamino acids in proteins), anentiomeric excess (ee) occurs

When “chirality” is heralded as a biosignature of life on otherplanets, what we are really looking for is a case where there is anentiomeric excess of chiral molecules

Chirality in organic moleculesOrganic molecules can be chiral due to the spatialarrangement of bonds around a carbon center:

In tetrahedral carbon (a carbon that has four single bonds to it), whenthere are four chemically distinct atoms that the central carbon is boundto, then that carbon is a stereogenic center and the molecule is chiral

Stereogenic centers exsist in aminoacids and sugars used in DNA

CO2HH2N

H

O

OH

HO N

NH

O

O

Biomolecules are made from exclusiveenantiomers of these components

Structural chirality in biological moleculesOther aspects of biomolecular structures are also of a chiralnature

Most duplex DNA exists in the“B”-form where the helix has aright handed twist

In proteins, nearly all of the α-helices are right handed

Image source:http://www.bmsc.washington.edu

Studying chirality with spectroscopy

Circular dichroism (CD) spectroscopy is used to study chiralmolecules

“R” and “L” designations ofmolecules indicates that themolecule absorbs more rightor left circularly polarized lightat a given wavelength

Image from: Brown, Z.; Starkey, R. J. Chem.Educ. 2005 82 1100.

CD spectroscopy utilizescircularly polarized (chiral)light since you need a chiraltest to study a chiral system

Measuring a CD spectrumA CD spectrum is measured by looking at the difference inthe absorption of left and right circularly polarized light by asample as a function of wavelength

Left and right handed lightImage source: http://repairfaq.ece.drexel.edu/

sample

Spectrum calculated as:CD = Al - Ar = θ/32982 ateach wavelength

Image source: http://www.umassmed.edu/

UV-Vis CDMost biological work is done in the UV-Vis (190-800 nm)spectral region

UV-Vis looks at electronictransitions

Image source: http://www.cryst.bbk.ac.uk

Image source: http://www.cryst.bbk.ac.uk

In proteins, UV-Vis CD canprovide information aboutsecondary structure

Example of a UV-Vis CDspectrum for the proteinmyoglobin

Vibrational circular dichroism (VCD)A recently developed technique is VCD which can be used toexamine the infra-red spectral region

VCD (top)andabsorbance(bottom) IRspectrum ofthe amideregion of aprotein

IR spectroscopy looks atbond vibrations, stretching,etc

This not only gives a widevariety of things to look atbut also provides moreinformation about themoleculeVCD is a relatively newtechnique and commercialmachines have only beenavailable for a couple ofyears: much to be done

Image from: Shanmugam, G.; Polavarapu, P. L. J.Am. Chem. Soc.2004, 126, 10292-10295

How can CD be used to detect life?One major problem with using CD as the basis of detectingextraterrestrial life is that it assumes chemical similarity

Since we don’t know what an extraterrestrial biomolecule will be made of,we may or may not be able to use these methods, but…

In UV-Vis CD and the primarybond detected is:

VCD looks for bond stretches,and signature bands forbiological molecules include:

Amide carbonylbond, proteinbackbones, twotransitions from190-240 nm

Nucleobaseshave severaltransitions from197-298 nm

HN

N NH

N

O

H2N

O

NH

HN

O

Carbonyl:1600-1800 cm-1

Amide NH3300-3500 cm-1

Alcohol OH3590-3650 cm-1

Various CH2835-2962 cm-1

O

NH

HN

O

O

OHH

H

NH

O

HO

Terahertz circular dichroism(TCD)TCD is still a developing technology but could be useful indescribing whole molecule vibronic modes

When fully developed, TCD will be a powerful tool for Astrobiology,but until then…

Illustration of the atomic motions ofbacteriorhodopsin at 0.498 THz

Simulatedbacteriorhodopsin TCDspectrum

Xu, J. et. al. Astrobiology 2003, 3, 489-504

Induced circular dichroism (ICD)In some situations, a CD spectrum can be measured for anachiral molecule

When an achiral molecule becomes physically (not chemically) attachedto a chiral molecule, then it is in a chiral environment

In a chiral environment, an achiral molecule will become CD active

An ambidextrous latexglove is achiral until…

…it is placed on one handor the other and then is aleft or right handed glove

Image credit:imperial.ac.ukImage credit nwmedicalsolutions.com

Example of ICD effectOne example of this is the insertion of achiral chromophoresinto DNA films:

So, to use ICD for Astrobiology, we need to identify a simple, ubiquitous“probe” molecule that can be studied by either UV-Vis or IR and that isexpected to be intimately associated with biological molecules

NH

NNHN

SO3--O3S

-O3S

SO3-

Jiang, S.; Liu, M. J. Phys. Chem. B 2004, 108, 2880-2884Porphyrin chromophore in solutionAchiral environment = no CD signal

Porphyrin in a DNA film-Chiral environment = CD signal

Follow the water…in the cell?While we are on the topic of water and ice in the solar system

Water is, of course, a primary component of living systems:

About 5% of the water in a cell iscoupled to biological molecules

Cells are made primarily ofwater

Image from:http://www.angelfire.com/ill/biochemistry/WaterI3.gif

Image from: www.isis.rl.ac.uk

Even in the liquid state, this water istightly coupled to the biomolecule suchthat molecule-associated waterexchanges very slowly with bulk water

E. coli composition: water(70%), proteins (15%),RNA/DNA (7%),polysaccerides (2%), lipids(3%), inorganics (3%)

Image from:http://commtechlab.msu.edu

Clathrates beyond methaneClathrates are the cages that water forms around a nonpolarmolecule in waterClathrates form to minimizein interaction of a nonpolarmolecule with polar water

Image from:http://www.brookscole.com/

The same is true for biomolecules,where water is intimately associatedwith the molecule

You can see where this is going…

Image from: http://www.imb-jena.de

water

DNA

ICD in bioclathrates?Here is the conjecture: is the water that surrounds a biologicalmolecule ordered enough to exhibit ICD?

On an icy bodyin the solarsystem, theremay beevidence forlife in the iceitself

Photo: Kjell Ove Storvik/AMASE

Image source: NASA

This biosignature may befound as a result of frozenextraterrestrial cells orbiological molecules trappedin the ice layers around theplanet, especially in an activeenvironment like Europa

It may bepossible todetect life inthese placesby using VCDto study the ice

Image source:http://images.cambridgesoft.com/

Molecular modelingComputer simulations could be used to understand water/iceassembles around biomolecules

These studies could beused to predict whether ornot the clathrate structurewould be chiral

Image from: http://www.lsbu.ac.uk

Predictions could also bemade as to how strong thisordering would be andtherefore what kind ofsignal to expect

Could also look fortemperature variations

Laboratory studiesThe next step is to actually take a look at VCD of water inbiomolecule liquid/frozen solutions

Effects to look for: biological source (whole cell/individual molecules),concentration, temperature effects

Commercially available VCD spectrometer

Space missionTo get to the final target, flight mission would be needed

Mars

EuropaImage source: NASA

Image source: NASA

A flight mission isneeded to carry aspecially designedspectrometer

The mission would go afterplanetes/moons with surfaceice that has been clearlyobservedHere, reflectance VCD wouldbe used to study water icespectra

SummaryThere are several biosignatures we can look forProbably no one single technique will tell the whole storyLooking for high degrees of entiomeric excess in molecules makes theleast number of life assumptions and may provide the most unambiguouslife detection technique

Circular dichroism is the best way to look for eeMolecular CD can be studied at several wavelengths thus can be used tolook at several different types and aspects of molecules

“Bioclathrates” may hold the keyClathrates formed around biological molecules may be the moststraightforward thing to look forThis biosignature requires only two assumptions: that water is themedium of life and the ee is a signature of lifeMolecular modeling and simple VCD studies would provide a look intothis possibility