Origin of Terrestrial Bioorganic Homochirality and Symmetry Breaking in the Universe
Round 1
Reviewer 1 Report
The authors are reviewing the different possibilities for non-racemic mixtures of chiral compounds to be formed in space, before seeding the Earth for further chemical evolution towards life.
This review is indeed interesting and sound, and thus is worth being published in "Symmetry". I would however suggest few changes for a better text clarity:
1- The authors should use with much more care the different terminologies. For example, page 2 line 64, it is unclear what the authors means by "asymmetric reactions produced chiral (compounds)". Literally, this sentence would only mean than chiral compounds were produced from achiral compounds, but nothing is said concerning the racemicity of the obtained mixture. Instead, what was probably meant here is "produced a non-racemic mixture of chiral (compounds)"
It is important to keep in mind that "chiral" and "non-chiral" are the properties of a single molecule (e.g. valine is chiral, glycine is achiral), while "racemic", "non-racemic" and "homochiral" are the properties of a mixture of chiral compounds (e.g. a 50%D:50%L mixture of valine is racemic, a 25%D:75%L mixture is non-racemic, a sample of pure L-Valine, with no D-Valine molecule, is homochiral, while "racemic glycine" would be a nonsense). Please correct throughout the text with the correct terminologies. Typically the "chiral seeds" term used throughout the text is at the best imprecise, as the purpose of these chemical reactions in space is not the synthesis of chiral compounds, but the synthesis of a non-racemic mixture of them (which is much more complex to obtain).
2- Page 5: the addition of few precision in the part presenting the effect of muons on chemical reactivity would be interesting, as these effects are not often developped: What is the typical order of magnitude of the lifetime of "Mu radicals" and "muonic atoms"? What kind of chemical reactions can occur (and were observed) in this timelapse?
3- Page 5-6, 3.1.1 paragraph: It may be interesting to remind quickly how CPL can induce non-racemicity: the photolysis of a racemic mixture of chiral amino acids induces the differential destruction of some of L and D AA (asymmetric photons interact differently with each AA enantiomer), resulting in non-racemic intacts AAs.
4- Page 7, 3.2.2: Are all the process induced by spin-polarized leptons similar to the one induced by CPL (i.e. differential destruction of a racemic mixture of chiral compounds)?
Other minor comments/corrections that may be addressed:
1- page 1, line 24: "The traditional hypothesis"?: I am not sure that "terrestrial scenarios" are more "traditionally" studied than "cosmic scenarios"... I have the feeling that both processes are acknowledged by the "homochirality" community.
2- page 1, line 32 : "asymmetric conditions in the primitive interstellar media resulting in terrestrial bioorganic homochirality"... it would rather be "can result", as there is no guarantee that non-racemic spatial seeding will automatically result in the emergence of homochirality...
3- page 4, line 157 "the spin angular momentum vectors of electron/positron are perfectly polarized as antiparallel/parallel to the vector direction of the kinetic momentum" it may be interesting to emphasize here (or earlier in the text) that this is precisely this property of having the spin angular momentum (anti)parallel with the kinetic momentum that confers to the particle its "chiral" property. This point may not be obvious for some of the readers.
Author Response
Thank you for your peer review.
I am responding to your comments.
Revised file with correction in red is attached.
1- The authors should use with much more care the different terminologies. For example, page 2 line 64, it is unclear what the authors means by "asymmetric reactions produced chiral (compounds)". Literally, this sentence would only mean than chiral compounds were produced from achiral compounds, but nothing is said concerning the racemicity of the obtained mixture. Instead, what was probably meant here is "produced a non-racemic mixture of chiral (compounds)"
Response 1. I corrected "chiral seeds" to "asymmetric seeds" and made some minor corrections.
2- Page 5: the addition of few precision in the part presenting the effect of muons on chemical reactivity would be interesting, as these effects are not often developped: What is the typical order of magnitude of the lifetime of "Mu radicals" and "muonic atoms"? What kind of chemical reactions can occur (and were observed) in this timelapse?
Response 2. I cannot find the lifetimes of "Mu radicals" and "muonic atoms" from any articles.
Possible reactions are already mentioned in the text in 2.1.2 - Muon Section.
3- Page 5-6, 3.1.1 paragraph: It may be interesting to remind quickly how CPL can induce non-racemicity: the photolysis of a racemic mixture of chiral amino acids induces the differential destruction of some of L and D AA (asymmetric photons interact differently with each AA enantiomer), resulting in non-racemic intacts AAs.
Response 3. I added one sentence 3.1.1 paragraph line 272-273 in the corrected file.
4- Page 7, 3.2.2: Are all the process induced by spin-polarized leptons similar to the one induced by CPL (i.e. differential destruction of a racemic mixture of chiral compounds)?
Response 4. I added sentences 2.1.2 paragraph line 160-163 in the corrected file.
Other minor comments/corrections that may be addressed:
1- page 1, line 24: "The traditional hypothesis"?: I am not sure that "terrestrial scenarios" are more "traditionally" studied than "cosmic scenarios"... I have the feeling that both processes are acknowledged by the "homochirality" community.
Response 1. I corrected "The traditional hypothesis" to "One of the hypotheses" and made some minor corrections.
2- page 1, line 32 : "asymmetric conditions in the primitive interstellar media resulting in terrestrial bioorganic homochirality"... it would rather be "can result", as there is no guarantee that non-racemic spatial seeding will automatically result in the emergence of homochirality...
Response 2. I made minor correction in line 31 in the corrected file
3- page 4, line 157 "the spin angular momentum vectors of electron/positron are perfectly polarized as antiparallel/parallel to the vector direction of the kinetic momentum" it may be interesting to emphasize here (or earlier in the text) that this is precisely this property of having the spin angular momentum (anti)parallel with the kinetic momentum that confers to the particle its "chiral" property. This point may not be obvious for some of the readers.
Response 3. I added sentences 2.1.2 paragraph line 160-162 in the corrected file.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript by Takahashi and Kobayashi reviews different hypotheses rationalizing chiral symmetry breaking and origin of Earth homochirality. The manuscript is clearly written and I recommend acceptance after few minor revisons.
1) As the authors write, CD intensity (or better absorption dissymmetry factor ΔA/A) can give indication about the efficiency of CPL-induced symmetry breaking. As the authors know for most organic molecules, such dissymmetry factor is very low (around 10-3) and in fact, in this way, only tiny enantiomeric excess can be obtained. So this alone is probably not enough to explain a symmetry breaking at the scale we observe it on Earth, unless amplification mechanisms follow, such as Soai reaction (featuring both autocatalysis and strong positive non-linear effects), which in turn are very rarely observed. Can the authors comment on that?
2) Some reference numbers in the text do not correspond to the cited article in the reference list. Please check.
3) Some typos need to be corrected, for example: in the abstract “parity violence” (should read “parity violation”), page 6 “Resent” (should read “recent”), “monochromatic of CPL” (should read “monochromatic CPL”).
p { margin-bottom: 0.25cm; line-height: 115%; }Author Response
Thank you for your peer review.
I am responding to your comments.
Revised file with correction in red is attached.
1) As the authors write, CD intensity (or better absorption dissymmetry factor ΔA/A) can give indication about the efficiency of CPL-induced symmetry breaking. As the authors know for most organic molecules, such dissymmetry factor is very low (around 10-3) and in fact, in this way, only tiny enantiomeric excess can be obtained. So this alone is probably not enough to explain a symmetry breaking at the scale we observe it on Earth, unless amplification mechanisms follow, such as Soai reaction (featuring both autocatalysis and strong positive non-linear effects), which in turn are very rarely observed. Can the authors comment on that?
Response 1. I added a sentence in line 75-78 in the corrected file. A new reference has been added.
2) Some reference numbers in the text do not correspond to the cited article in the reference list. Please check.
Response 2. I checked the numbers and corrected them.
3) Some typos need to be corrected, for example: in the abstract “parity violence” (should read “parity violation”), page 6 “Resent” (should read “recent”), “monochromatic of CPL” (should read “monochromatic CPL”).
Response 3. I checked and corrected them.
Author Response File: Author Response.docx
