Possible Physical Mechanisms in the Galaxy to Cause Homochiral Biomaterials for Life

Abstract

The origin of homochirality in life remains a mystery that some believe is essential for life, and which may result from chiral symmetry breaking interactions with galactic organic material.

1. Introduction

For more than a century, there has been evidence for the chiral nature of life forms on Earth. Pasteur was among the first to point this out (1848–1880), and the universal nature of chiral symmetry breaking in DNA and RNA is now very well established for all life forms. Figure 1 and Figure 2 show how the homochirality is manifest at the molecular level. We note that amino acids are L and DNA is largely D handed.

With the discovery of parity violation within charged current reactions in 1956, and of the weak neutral currents (WNCs) in 1973, two universal symmetry-breaking processes (WNC and β-decay) were uncovered that could have determined the handedness of DNA and RNA. The main problem is the extremely small symmetry-breaking effects (ΔE/k_B T ~ 10^-17). However there are plausible non-linear mechanisms that could have amplified this small, symmetry-breaking phase transition up to the full symmetry-breaking level observed in life forms [1,2,3,4,5,6,7,8,9,10,11].

For many years, there have been several issues associated with the homochiral structure of biomolecules, as first observed by Pasteur in 1848:

(a)

Is a homochiral structure necessary for life as we know it?

(b)

Did homochirality precede the formation of life (homochiral prebiotic medium hypothesis)?

(c)

Is there any reasonable physical mechanism that could have produced the large chiral symmetry breaking in the prebiotic medium or in the observed homochiral structure?

(d)

Is the homochiral structure an accident that occurred in biological systems, which was later amplified?

(e)

Can the homochirality be used as a signature for existing, or previous, living systems in the solar system or other parts of our galaxy?

(f)

Are there any experiments that can be carried out now to clarify the origin of homochirality?

(g)

Work on the RNA world and the homochirality of biomolecules is promising. See, for example, Reference [39,40,41].

Recently, there has been increasing interest in the chiral nature or handedness of biomolecules. In fact, there are some who claim that the complex biomolecular structure of life must have arisen from a “chiral pure” medium [12,13]. This may be a precondition for the emergence of self-replicating biomolecular systems.

Table 1. Trends in life origins.

1	DNA: Self replication would not work with heterochiral systems (50% L and 50% D).
2	Errors in DNA Replication: Without a pure chiral structure, the error rate in replication would be unacceptable for long-lived systems (higher animal forms, trees, etc.).
3	In a prebiotic medium, homochirality must have been either (a) or (b)
	(a) Established in a very short time on Earth (≤ 100 Million years)
	(b) Existed in Interstellar Medium (ISM) organic materials near the solar system

lists some of these ideas, which have been put forward largely by W. Bonner and V. Goldanskii [12,13]. We also point out a recent review in Reference [14]; see also Reference [15] for more recent details. Life on Earth is likely to have originated between 3.8 and 3.5 billion years ago. This estimate of time, along with the previous concept of the prebiotic medium, leaves a small window of 300 million years or less for life to have emerged from that prebiotic medium. Indeed, some speculate that the time could be less than 10 million years. Recently, some evidence has been obtained for an excess of L amino acids in the Murchison meteorite [37]. This and other observations may point to an extraterrestrial origin of homochirality [37].

We now turn to the experiments that study possible asymmetry processes. Over a period of 20 years, many experiments have been carried out (see Reference [12] for a nearly complete list). However, it appears that nearly every positive effect that was observed has turned out to be incorrect. In

Table 2. Experiments where a chiral effect has been observed (and not refuted).

	Experiment	Beam/Source	Target	Detection	Comments	Reference
1	CPL on L/D/ photo absorption	UV/keV radiation	DC tartaric acid, DL alanine, DL glutamic	Observe destructive difference in one chirality	To be expected from optical activity	Norden [19]
2	e- target → Č light	p32 source and Ca137 (no chiral electrons)	R- or S-PBA	Observe different Č light intensity due to chiral electrons	Effect too large to be due to Č radiation from spin effects	Garay et al. [20]
3	Co60 → γ + (L,D)	Co60 γs	D or L alanine	Observe different amounts of L,D after irradiation	Other experiments did not produce this effect	Akabosh et al. [21]
4	Introduction of L/D/ chiral particles	Sr90, Y90 β-decay	Y90 D or L alanine	Detects effects by electron spin resonance technique	The electron spin resonance may be sensitive to spin dependence	Conte et al. [22]
5	Low-energy polarized e- beam	GaAs polarized source 5 eV	Camphor L,D	Observe different electron polarization and beam attenuation in L/D	At such low energy, asymmetry may be too large	Campbell et al. [23]

, we list some experimental results that are not yet refuted or are in direct conflict with previous null effects. The observation that circularly polarized light destroys L and D isomers selectively (entry 1 in Table 2) is now well established.

The other experiment to which we will refer is that of entry 2 (also in Table 2), the study of Cherenkov light in L or D material with chiral e^- (β-decay) [20]. The authors claim an effect of perhaps 10^-2 magnitude. This is, in our belief, far too large an effect, but a future study of Cherenkov radiation from L or D materials with polarized e^- beams could be promising.

In Figure 3, we show the current limits or observations of asymmetry. The e⁺ measurement of Gridley et al. [18] is a very nice experiment. See also references [16] and [17]. Our conclusion is that no present experiment has reached the level of sensitivity needed to observe an effect. Therefore, it is premature to count out the weak force as a determining factor in the origin of homochirality in life.

2. Organic Molecules in Space and the Possible Role of Nearby Supernovae and Neutron Stars

One of the main themes of recent attempts to understand life on Earth is the likelihood that most of the early organic material on Earth was brought in by comets and asteroids. Reference [15] gives a nice introduction, from different points of view, to this concept. There are some interesting “large numbers” to consider in this regard:

(1)

The estimated amount of dust matter in the galaxy is ~10^-4 mG, or ≤ 10⁷ solar masses, largely in the form of dust grains. A fraction of that material is in the form of organic materials [26].

(2)

It has not been possible to measure the amount of interstellar dust that has accumulated on the Earth (some of this dust would have brought organic material) [27,28].

(3)

In a molecular cloud with a density of 10⁴ M/cm³ and a radius of 1 parsec, there could be a complex of organic matter equal to 100 solar masses.

(4)

The Earth revolves around the galaxy every ~250 million years, and it likely encounters several dense layers of molecular clouds in this trajectory.

(5)

It is likely that large quantities of organic material were deposited in the Earth in the first billion years.

The above information is obtained by observing the infrared scattering of dust in the galaxy and by modeling various UV-driven processes here on Earth [26]. Ultraviolet photo processing plays an important role in the organic chemistry of the dust particles [27]; see also Reference [28]. Figure 4 shows the nature of a dust grain with prebiotic molecules inside [29].

We discuss here two scenarios where chiral interactions in the ISM could have led to a preponderance of homochiral molecules. These two concepts are outlined in

Table 3. Possible sources of CPL (UV/keV) radiation in dense molecular clouds.

	Primordial Soup – Molecular Clouds (ISM)
1	Synchronotron radiation from neutron stars ([12,26])
	CPL helicity depends on position (i.e., above or below star)
	In principle, this mechanism works. However, on the 250-Myr orbit around the galaxy, this effect is expected to average out.
2	Radiation from weak interaction processes [30]-injection into molecular cloud
	Processes: supernova II $\overline{{\nu }_e}$ interaction, Al26 from nearby supernovas, etc.;
	Because of grain structure, dE/dx will be very different from that of solids, gases, or liquids;
	Always gives the same chiral symmetry breaking;
	An act as a chiral impulse along with WNC.

, with Figure 5 giving a fairly complete description of the hypothesis. The second hypothesis is illustrated in Figure 6, where the possibility of the relative survival of the e^± polarization deep within the cloud is also illustrated. Of course, during this time, the effect of the WNC can be driving the system towards a homochiral state.

Let us consider the rate of these three effects:

(1)

For $\overline{{\nu }_e}$ absorption and a supernova 1 parsec away (or inside a 1-parsec dense cloud), the number of interactions will be ~10^-3 /kg of material for 100 M_O of organic material (which would be 10¹² g of organic matter that is active), therefore the positron from the $\overline{{\nu }_e}$ interactions would lose energy at a rate of 10^-19 MeV/cm, and thus travel over a parsec. ($\overline{{\nu }_e}$ is an antielectron neutrino).

(2)

For the coherent vx + N → vx + N, and for the carbon in the hydrocarbons, we would have ~10² more or ~10¹⁴ grams of active material. Note that νx stands for all types of neutrinos. This effect could be very important in light of the small energy difference that separates L and D molecules, and the possibility of large coherent effects.

(3)

For the Al²⁶ over the half-life, there would be ~10⁵⁰ decays producing ~10⁵⁰ positrons that lose energy at the rate of 10^-19 MeV/cm; for MeV positrons, the range would be on the order of a parsec (ignoring possible magnetic-field effects).

2.1. Direct Interaction of the Supernova II Neutrino

Consider the example where 0.001 M_O of Al²⁶ is produced and assume (M_O is the mass of the sun), for simplicity, that the energy of the e⁺ is 1 MeV and is contained in the gas cloud. Assume that the cloud has a density of 10⁴ atoms/cm³ and that 10^-3 of the atoms are organic. The stopping power for e⁺ would then be

$$dE/dx ~ MeV g^{-1} c{m}^{-3}$$

and for a density of ρ = 10⁴ atoms/cm³ ~ 10^-17 g/cm³, we find

$$dx ~ (MeV/\rho )dE ~ {10}^{19}cm (~ 3 par \sec )$$

and for an average energy exchange of 10 eV, we have

$${10}_e^5 collisions / A{l}^{26} decay$$

For 0.001 M_O of Al²⁶ and a 10^-3 organic fraction, we obtain a total of ~10⁵⁰ collisions of polarized positrons, with organic materials in the cloud, assuming all of the e⁺ stop in the cloud (we assume that only one of the collisions can result in spin exchange). There will also be the same order of polarized photons from the e⁺e^- → γγ annihilation. It is estimated in [17] and [24] that the asymmetry due to the weak interaction would be of order 10^-11 to 10^-6, depending on the positron energy {it scales like α²[α/(ν/c)]²}. Thus, it takes N ~10²² interactions for the asymmetric to become statistically important. In this example there are far more interactions.

More recently we have considered the main effects of a SN II explosion in a hydrocarbon cloud. We estimate that the larger anti-neutrino interaction rate would cause a chirality breaking effect and estimate this effect in

Table 4. Estimated rate of νe interactions in the dense presolar cloud.

$\mathbf{Estimated}\mathbf{rate}\mathbf{of}\overline{{\mathit{\nu }}_{\mathit{e}}}\mathbf{Interactions}\mathbf{in}\mathbf{the}\mathbf{Dense}\mathbf{Presolar}\mathbf{Cloud}$
$\left[N_{\overline{v}}~{10}^{57}fromSNII\right]\left[AssumeSNIpar\sec fromcloud\right]$
Assume: Cloud density 104 protons/cm3 [fraction of organic material] 10-3-- Range of e+ 1 parsec; Number of $\nu$ interactions ~ 1035 = Number of e+ for interaction with organics in cloud > 1030 We estimate that only 1022 interactions are necessary to produce asymmetry. From this, it is clear a much larger asymmetry is produced.

. See Reference [34].

It appears that supernova neutrinos could include a chiral symmetry breaking in the ISM that could be transferred to the biomolecules of life. I wish to thank Goldanskii for some very useful conversations while he was a Regents Lecturer at UCLA and I regret his passing; please see Reference [5].

3. Other Calculations of Chiral Symmetry Breaking

The process shown in Figure 6 could come about in similar ways. Here are two examples:

(a)

Effects of cosmological neutrinos on the discrimination between the two enantiomers of a chiral molecule [35]. This concept is similar in spirit to that of Cline and the authors reference that work [34]. The basic concept in this work is that cosmological neutrinos and antineutrinos interact with the electrons in organic materials in the galaxy. This results in a split in the energy of the different chiral systems. This energy split is enhanced by the contributions if all the electrons in the molecules, and other mechanisms, up to the point of a larger chiral symmetry breaking.

(b)

A relativistic neutron fireball from a supernova explosion is a possible source of chiral influence [36]. This concept is also similar to the one by Cline [34]. However, in this case, it is the decay of neutrons $n\to p+e^{-}+\overline{{\nu }_e}$ that leads to polarized electrons that destroy organic material differently for L and D enantiomers. This is shown in Figure 3 and explained in the text. This work studies the effect of neutrinos from the supernova fireball with a Lorentz factor of one hundred. The relativistic electron-proton plasma (from the neutron decays) is slowed down by collective effect. There is high chiral efficiency for such electrons. The electron interactions and the photons produced both help destroy one chiral state, leading to the dominance of the other. This idea is very similar to that of Cline [34].

In Reference [38], new arguments are made for the emergence of homochiral materials from the circular polarization found in star forming regions. This concept is similar to that of Bonner [7,12] but with a different origin for the polarized light.

4. Summary of Concepts of the Production of Homochiral Molecules in the Galaxy

The best of all these ideas is to find organic material that is homochiral in prebiotic systems. The Rosetta mission to an asteroid or comet could discover this effect [33]. There is already some evidence from the L excess of meteoritic amino acids [37] in the morchism and other meteorites.