Binding of Arsenic by Common Functional Groups: An Experimental and Quantum-Mechanical Study

Round 1

Reviewer 1 Report

The aim of this paper is for the evaluation of the strength of interaction between As(III) and some different ligand for the first time. Authors provide a novel method to determining the contribution of each functional group to the stabilization process in a competitive environment where all donor groups are present.

The study is a fully scientific work and according to my opinion deserves to be published in the Applied Sciences Journal after minor revisions.

The only sugggestion I would like to express is the authors to make more clear the orientation of this work to the real applications. For example if the findings of this work improves processes such as water treatment or other in real applications.

Author Response

Please find our reply as an attached *pdf file.

Author Response File: Author Response.pdf

Reviewer 2 Report

The current manuscript aims to study the interactions of small anionic metabolites and amino acids with As(III) in aqueous solution.  I was pretty much looking forward to reviewing this manuscript but ran into problems understanding the experimental and computational design as well as the final conclusions.  It has been known for quite awhile that As(III) and not AS(V) is the toxic form of the metalloid, and that As tends to form highly covalent adducts with thiol and phosphorus ligands.  The present study concentrates (at the end) on Cysteine, the main protein ligand for low-valence transition metals (nickel, iron, copper and zinc) with which As competes with, in addition to Selenium.  That is not addressed here, nor it is the well know binding of As with methionine, the other sulfur containing amino acid present in proteins. So as it is the study is of limited interest to the toxicology or biological community.  This is a very Chemistry-oriented study, and I am not sure that it is suitable for publication in this Journal, regardless of the merits of the results.  None of the facts above was introduced in the introduction, so that needs more work.

There are a few corrections:

line 107 it should read sulfhydryl or sulphydryl.

ethanamine is commonly knows as ethyl amine, not ethan amine.

Line 190 should remove "electric" as it is not needed.  The same goes for the other instances.

The adduct formation in lines 199-200

The term "pure" should be eliminated in line 256.

The concentrations are indicated in mmol/L and I presume Cm and Cl are the metal (Arsenic is NOT a metal) and Ligand concentrations, respectively.

I am confused about the results shown in Table 1.  Stability constants are usually expressed as beta or log (beta).  But the progression of overall stability constants appears to be backwards because MLH2 has a higher value than MLH and in turn higher than MLH (see the result for tca for example) , which is the opposite of what is stated in the text.  The constants would make sense if they were expresse as pK(beta) = -log(beta), but the authors need to elaborate this. 

I am surprised the structure optimization experiments of cysteine did not include dimers or trimers of Cys to As(III).  Those complexes have more relevance for biological and toxicological reasons, and would be more stable since they involve two or three sulfur-As bonds.

The columns for n(SH), n(CO2H) and n(NH2) are not needed in Table 2 and can be confusing.  Some of the protonation states do not appear to make sense.  How are the numbers in Table 2 obtained?  For example, for Cysteine I would expect the equilibrium to be HSCH2CH(NH3+)CO2H == HSCH2CH(NH3+)CO2- == HSCH2CH(NH2)CO2- == -SCH2CH(NH2)CO2-.  The Table 2 shows a state where the thiol (pKa~9-10) is ionized while the ammonium group is protonated (pKa ~6), which I don't think is feasible.  In this part of the manuscript (lines 263-264) what calculations are being shown here?  I assume the ab initio calculations which are microscopic, so they cannot account for the acidity (pH) of the medium.  I am not clear here what is being conveyed.

Why is cysteine treated as a tridentate ligand where as lysine and asparate treated as bidentate?  and why is glycine treated as a monodentate ligand when it has 2 electron donating groups (NH2 and CO2-)?

All the equations need to be numbered.  There is no need to type them again later in the text since they can be referred to in the text by the assigned number.

The pH phase diagram does not appear to make sense for the Asp-As couple.  As the pH increases one should expect to see more carboxylic groups ionized, which should bind more As(III) (incorrectly labeled M) so the concentration of free As should decrease, which only happens at a might higher pH.  I would expect that as MLH2 undergoes deprotonation to MLH (with the formation of water) it would keep the As chelated.  The As-cys system shows something closer to what I would expect:  low As available (~20%) should form a stable compound with protonated Cysteine at low pH, which further decreases as the carboxylate and thiol groups become deprotonated.

Where does the semi-empirical equation (40) in line 319 comes from?  It seems to me that the contribution to logK of the charged groups ought to be larger than the neutral ones, but the thiol contribution to the microscopic equilibrium constant is much higher (2.39) than that of the thiolate (0.17).

Author Response

Please find our reply as attached *pdf file.

Author Response File: Author Response.pdf

Reviewer 3 Report

well put together study, clearly described and useful data on As-ligand interactions.

alot of citation of previous work by the group makes me hesitate that it has been broadly referenced. there are other studies by the group of As (III) with relevant molecules, so how novel is this study? particularly with thiol groups e.g. ref 14?? how different is this study from the previous work and have we simply repeated approaches with little new science?

are the experimental conditions representative of realistic exposure situations? these are lab simulations so what about other competing ligands compared to just the carboxylic, amino and thiol groups? the complexity of solution conditions in living systems or external environment are an issue for toxicity and mobility. some commentary on this would be useful

Author Response

Please find our reply as an attached *pdf file.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

See the attached Word document. Thanks

Comments for author File: Comments.pdf

Author Response

Please see the Letter to the Editors.

Reviewer 3 Report

happy with the response

Author Response

We greatly thank the Reviewer for her/his positive comment.