L-valine + zinc sulfate heptahydrate  L-valine and

Reinvestigation of growth of L-valine zinc sulfate crystal

Bikshandarkoil R. Srinivasan, Rita N. Jyai

Department of Chemistry, Goa University, Goa 403206, INDIA

Email: srini@unigoa.ac.in Telephone: 0091-(0)832-6519316; Fax: 0091-(0)832-2451184

Graphical Abstract

H

O

L-valine + zinc sulfate heptahydrate  L-valine and

NOT

L-valine zinc sulfate

Graphical Abstract (for review)

Published in: Spectrochimica Acta Part A-Molecular and Biomolecular Spectroscopy: 120; 2014; 621-624.

1

Reinvestigation of growth of L-valine zinc sulfate crystal

Bikshandarkoil R. Srinivasan, Rita N. Jyai,

Department of Chemistry, Goa University, Goa 403206, INDIA

Email: srini@unigoa.ac.in Telephone: 0091-(0)832-6519316; Fax: 0091-(0)832-2451184

Highlights

 Growth of L-valine zinc sulfate crystal is reinvestigated.

 Earlier reported L-valine zinc sulfate crystal is actually L-valine.

 ZnSO4

·

 ZnSO4

·

*Highlights (for review)

Published in: Spectrochimica Acta Part A-Molecular and Biomolecular Spectroscopy: 120; 2014; 621-624.

1

Reinvestigation of growth of L-valine zinc sulfate crystal

Bikshandarkoil R. Srinivasan, Rita N. Jyai

Department of Chemistry, Goa University, Goa 403206, INDIA

Email: srini@unigoa.ac.in Telephone: 0091-(0)832-6519316; Fax: 0091-(0)832-2451184

Abstract

Reinvestigation of the growth of L-valine zinc sulfate crystal is reported. Slow evaporation of an aqueous solution containing L-valine and zinc sulfate heptahydrate results in the fractional crystallization of L-valine and not the organic inorganic hybrid nonlinear optical L-valine zinc sulfate crystal, as reported by A. Puhal Raj, C. Ramachandra Raja, Studies on the synthesis, spectral, optical and thermal properties of L-Valine Zinc Sulphate: An organic inorganic hybrid nonlinear optical crystal, Spectrochim. Acta A97 (2012) 83-87.

Keywords: L-valine; zinc sulfate heptahydrate; fractional crystallization; L-valine zinc sulphate; crystal growth; improper characterization;

1. Introduction

An extensive biochemistry of zinc has been developed and it is now well recognized that zinc is a key element of life [1-3]. Coordination compounds of zinc, especially the zinc-amino acid chelates, find application in medicine and foodstuff [4, 5]. In addition to the biological

relevance, zinc complexes of the naturally occurring chiral amino acids are of interest in view of their possible nonlinear optical (NLO) behavior. Many examples of structurally characterized amino-acid compounds of zinc [6-12] including a five coordinated Zn(II) compound namely [Zn(val)2(H2O)] (val =L-valinate) obtained by reaction of zinc sulfate with L-valine and NaOH in 1:2:2 mole ratio are well documented. Although a thermochemical study of a zinc compound of composition [Zn(valH)SO4·H2O] (valH = L-valine) has been reported [13], no structural details were given.

Manuscript

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Published in: Spectrochimica Acta Part A-Molecular and Biomolecular Spectroscopy: 120; 2014; 621-624.

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A recent paper describes the slow evaporation solution growth of a „so called‟ nonlinear optical (NLO) organic inorganic hybrid crystal L-valine zinc sulphate (1) having formula C5H11NO2ZnSO4·7H2O and abbreviated by a strange code namely LVZS [14]. Although it is not clear why a compound with seven molecules of water in its formula was named as L- valine zinc sulphate, this compound is referred to as compound 1 in this paper, to avoid use of the non-standard abbreviation LVZS. Another paper also describes the growth of a crystal with the same name L-valine zinc sulphate but without any chemical formula or the code LVZS [15]. In both these papers the details pertaining to crystal growth in terms of amounts of reagents taken and yield of product crystal are not available. Although the authors of both papers [14, 15] claim to have performed a single crystal structure determination no structural details of 1 (for example the coordination sphere of zinc) were reported. The authors have not substantiated the claim of their single crystal work with a cif file. Based on a comparison of the unit cell data of the starting materials, L-valine [16], ZnSO 4·7H2O [17] with the unit cell data of 1, the authors of [14] made a remarkable conclusion, „A notable increase in cell volume strongly recommends the presence of both molecules (L-valine and zinc sulphate) within the unit cell‟. Such a finding cannot be considered as an acceptable result of a single crystal structure determination. Instead of providing the structural features of the zinc site based on their single crystal exercise, the authors state „SHG test confirmed the bonding between L-valine and zinc sulfate increases its NLO efficiency‟. In spectral discussion, the authors reported „Slight variations are observed in frequencies of C-C bond (L-valine) is due to mixing of zinc sulfate heptahydrate‟. In view of the possible importance of zinc(II) amino acid compounds in biological applications, a reinvestigation of the reported crystal growth of L-valine zinc sulphate [14] has been undertaken in this study, in order to establish the correct identity of L-valine zinc sulfate.

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2. Materials and Methods

Chemicals used in this study were purchased from commercial sources and were used as received without any further purification. Double distilled water was used as solvent. Infrared (IR) spectra of the samples diluted in KBr were recorded in the region 4000 – 400 cm ^-1 using a Shimadzu (IR Prestige-21) FT-IR Spectrometer, at a resolution of 4 cm ^-1. ¹H NMR spectra were recorded (in D2O) using a Bruker 400 MHz (Avance) FT-NMR spectrometer. X-ray powder pattern were recorded on a Rigaku Miniflex II powder diffractometer using Cu-K α

radiation with a Ni-filter. Optical rotation of the crystals dissolved in water, were measured in a Rudolph research analytical (Autopol IV) polarimeter.

2.1 Reinvestigation of crystal growth of L-valine zinc sulfate 1

A mixture of zinc sulfate heptahydrate (1.438 g, 5 mmol) and L-valine (0.586 g, 5 mmol) was taken in water ~25 ml and the reaction mixture was stirred well to obtain a clear colorless solution. The clear reaction mixture (pH ~4.5) was left undisturbed at room temperature.

Slow evaporation of the solvent resulted in the separation of transparent crystals, which were isolated by filtration and dried in air to yield 0.45 g of crystalline product labeled as 1. In addition, we have also investigated crystal growth reaction by taking L-valine (0.586 g, 5 mmol) and zinc sulfate heptahydrate (0.144 g, 0.5 mmol) in a 10:1 mole ratio and the crystalline product from this experiment was isolated as before.

3. Results and discussion

3.1 Synthetic aspects of crystal growth and spectral characterization

The recently reported growth of L-valine zinc sulfate crystal [14] is reinvestigated in order to unambiguously characterize the crystalline product. For the system L-valine / Zn(II) / water, the authors report that from an aqueous solution containing L-valine and zinc sulfate heptahydrate in a 10:1 mole ratio, 10 mm long crystals of 1 having formula

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C5H11NO2ZnSO4·7H2O were grown but did not mention the reason for use of a tenfold excess of L-valine for crystal growth of this „so called‟ nonlinear optical (NLO) organic inorganic hybrid 1. In this work, we have investigated crystal growth reaction using i) equimolar quantities of L-valine and ZnSO4·7H2O in view of the formula of 1 (Scheme 1) and ii) crystal growth reaction using a tenfold excess of L-valine. The products obtained in both crystal growth reactions were the same as evidenced by their identical IR spectra. A comparison of IR spectrum of 1, with that of pure ZnSO 4·7H2O, pure L-valine and an artificial mixture of ZnSO4·7H2O and L-valine (Fig. S1 – S4) reveals that the crystals obtained are those of pure L-valine as can be seen by the coincidence of the IR spectra (Fig. 1). The IR spectrum of 1 is in good agreement with the spectrum reported for L-valine by the Natarajan group [16]. Since L-valine is a known compound and its spectrum is reported in the literature [16] and IR spectroscopy is used as a characterization tool to infer new product formation, no discussion of the IR spectrum and band assignment is presented here.

Crystals of 1 exhibit a high melting point (in open capillary) of 298^oC providing added evidence for formation of L-valine crystals. In order to confirm formation of L-valine unambiguously, the crystals obtained in the crystal growth study were further examined for the presence of zinc and sulfate by using standard qualitative spot tests [18]. The negative tests for both zinc and sulfate provide evidence that the crystals of 1 obtained in the reaction do not contain any zinc sulfate. The phase pure nature of the L-valine crystals was further confirmed by a coincidence of the X-ray powder pattern of pure L-valine and that of 1 (Fig.

S5 – Fig S6). The optical spectrum of 1 showing its transparent nature in the UV and visible region perfectly matches with that of L-valine (Fig. S7). The ¹H NMR spectrum of the crystals grown in this work reveals the high purity of the crystals and the observed chemical shifts are as expected for L-valine (Fig. S8).

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In this reinvestigation we have performed crystal growth reactions using different ratios of L- valine and zinc salt and in all experiments, the yield of the crystals obtained was always slightly less than the amount of the amino acid employed for crystal growth and in terms of yield this amounted to ~75% of L-valine. In view of the recovery of a major portion of one of the reactants no attempts were made to fully recover the amino acid content. We also note that the product obtained in all cases is always pure L-valine and not a mixture of crystals of ZnSO4·7H2O and L-valine as evidenced by a comparison of the IR spectrum of the product with that of an artificial mixture of ZnSO 4·7H2O and L-valine (Fig. S3, Fig. S4). The grown crystals in this study exhibit a positive optical rotation which could unambiguously confirm

the crystals to be same as those of the starting material L-valine indicating that the use of ZnSO4·7H2O crystals does not result in any isomerization / racemization of the optically active L-valine used for crystal growth. The presence of ZnSO4·7H2O in the medium does not inhibit the crystal growth of L-valine.

3.2 Chemistry of the crystal growth reaction from an aqueous solution containing zinc sulfate and L-valine

The authors of the title paper attempted to grow crystals of a „so called‟ L-valine zinc sulfate by use of L-valine and zinc sulfate heptahydrate in 10:1 mole ratio. The grown crystals were formulated based on an incorrect assumption that use of L-valine and ZnSO 4·7H2O in a 10:1 mole ratio will result in a so called L-valine zinc sulfate crystal, disregarding the chemistry of zinc(II) with L-valine in water, which dictates product formation. It is well-known that Zn(II) reacts with L-valine and forms a bis(valinate) complex [Zn(val) 2(H2O)] [12]. In contrast, in an acidic medium (pH ~ 4.5), zinc sulfate does not exhibit any chemical reaction with L-valine and no new product is formed (Scheme 1). This chemical inertness of Zn(II) towards L-valine in an aqueous acidic solution can explain the formation of only L-valine in all crystal growth reactions irrespective of the ratio of zinc sulfate:L-valine. The less soluble

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L-valine (88.5 g / litre) [19] separates out and the more soluble (1 g in 0.6 ml) ZnSO 4·7H2O [19] remains in solution. The fractional crystallization of L-valine which is ~19 times less

soluble than ZnSO4·7H2O is in accordance with its chemistry and our result is in good agreement with crystallization of L-valine from acidic solutions (pH 3.75) [12].

Several instances of isolation and reporting of starting material or known compounds under the name novel NLO crystal are well documented in the literature in the form of critical comments [20-23]. Hence we do not find the report on L-valine crystal with the name L- valine zinc sulfate [14-15] very surprising. What we find unfortunate is that the authors of

[14] could get a different (a large volume) unit cell for L-valine crystal making one wonder if

they really made such a measurement. In this context, mention must be made of the work of authors of [15] who published the second paper on L-valine zinc sulfate 1. This report does not provide any molecular formula for 1, and the characterization is not supported by chemical analysis or any spectral data. Although no ratio of reagents taken for the crystal growth and the details of instrumentation for single crystal study is missing, these authors could conclude 1 as a monoclinic crystal based on unit cell data without even knowing the

monoclinic angle β. Although the authors of [15] seem to be unfamiliar with the earlier published work on L-valine zinc sulfate namely [14] (as this work is not cited), it is remarkable to note that the values for a, b, c given in [15] are in perfect agreement with those reported in [14]. The dubious nature of single crystal study of both these reports on L-valine zinc sulfate [14, 15] can be evidenced by the absence of cif files.

4. Conclusions

In summary, we have shown that slow evaporation of an aqueous solution containing L- valine and zinc sulfate heptahydrate at room temperature, results in the fractional crystallization of L-valine and not the so called nonlinear optical L-valine zinc sulfate crystal.

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The presence of ZnSO4·7H2O in the crystal growth medium does not inhibit the growth of L- valine crystals.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at *****

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[4] The roles of amino acid chelates in animal nutrition, H.D. Ashemead (Editor) Noyes Publications, New Jersey 1993.

[5] J.W. Swinkels, E.T. Kornegay, W. Zhou, M.D. Lindemann, K.E. Webb Jr, M.W.

Verstegen, Effectiveness of a zinc amino acid chelate and zinc sulfate in restoring serum and soft tissue zinc concentrations when fed to zinc-depleted pigs, J. Anim. Sci. 74 (1996) 2420-2430.

[6] Y. Yukawa, N. Yasukawa, Y. Inomata, T. Takeuchi, The structure of dichlorobis- (L-proline)zinc(II), Bull. Chem. Soc. Jpn. 58 (1985) 1591–1592.

[7] M. S. Nandhini, R. V. Krishnakumar, S. Natarajan, Dichlorobis(DL-valine)zinc(II), Acta Crystallogr. E57 (2001) m498-m500.

[8] M. Subha Nandhini, R. V. Krishnakumar, S. Natarajan, Dichlorobis(DL-alanine)zinc(II), Acta Crystallogr. E58 (2002) m127-m129.

[9] M. Lutz, R. Bakker, Dichlorobis(DL-proline)zinc(II), Acta Crystallogr. C59 (2003) m18- m20.

[10] M. Fleck, L. Bohaty, Three novel non-centrosymmetric compounds of glycine: glycine lithium sulphate, glycine nickel dichloride dihydrate, and glycine zinc sulphate trihydr- ate, Acta Crystallogr. C60 (2004) m291–m295.

[11] A. Alagha, D.A. Brown, M. Elawad, H. Müller-Bunz, H Nimir, A.I. Yanovsky, K. B.

Nolan, The preparation and crystal structure of acetatobis(L-arginine)zinc(II) acetate trihydrate, the first reported X-ray structure of a zinc(II)–arginine complex, Inorg. Chim Acta 377 (2011) 185-187.

8 [12] C-S. Hwang, N. Lee, Y-A. Kim, Y. B. Park, Synthesis of the Water Dispersible

L-Valine Capped ZnS:Mn Nanocrystal and the Crystal Structure of the Precursor Complex: [Zn(Val)2(H2O)], Bull. Korean Chem. Soc., 27 (2006) 1809–1814 [13] Y-Y. Di, Z-C. Tana, S-L. Gao, L-X Sun, Thermochemistry of Zn(AA)SO4·H2O(s) (AA = l-α-valine and methionine), Thermochim. Acta, 436 (2005) 150–152.

[14] A. Puhal Raj, C. Ramachandra Raja, Studies on the synthesis, spectral, optical and thermal properties of L-Valine Zinc Sulphate: An organic inorganic hybrid nonlinear optical crystal, Spectrochim. Acta 97A (2012) 83-87.

[15] S.Suresh, K. Anand, Dielectric and Photoconductivity Studies on L-Valine Zinc sulphate NLO single crystal, Int. J. Chem. Tech. Res. 5 (2013) 284-287.

[16] E. Ramachandran, S. Natarajan, Synthesis of L-valine crystals, Cryst. Res.

Technol. 44 (2009) 641–646.

[17] J. L. Anderson, R. C. Peterson, I.P. Swainson, Combined neutron powder and X-ray single-crystal diffraction refinement of the atomic structure and hydrogen bonding of goslarite (ZnSO4·7H2O), Mineralogical Magazine, 69 (2005) 259-271.

[18] G. Svehla, Vogel‟s Qualitative Inorganic Analysis, Pearson, Seventh Edition (2011) 151-155.

[19] Merck Index, Ninth Edition, Published by Merck & Co. Rahway, New Jersey 1976.

[20] M. Fleck, A.M. Petrosyan, Difficulties in the growth and characterization of non-linear optical materials: A case study of salts of amino acids, J. Cryst. Growth 312 (2010) 2284–2290.

[21] A.M. Petrosyan, V.V. Ghazaryan, M. Fleck, On the existence of “L-threonine formate”, “L-alanine lithium chloride” and “bis L-alanine lithium chloride” crystals, Spectrochim. Acta 105A (2013) 623-625.

[22] B.R. Srinivasan, On the existence of L-proline cadmium bromide, Spectrochim Acta 116A (2013) 639-641.

[23] B.R. Srinivasan, P. Raghavaiah, V.S. Nadkarni, Reinvestigation of growth of urea thiosemicarbazone monohydrate crystal, Spectrochim Acta 112A 84-89 (2013).

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Caption for figures:

Fig. 1. IR spectral comparison reveals L-valine (top) and L-valine zinc sulfate (1) (bottom) are one and the same.

Fig. 1. IR spectral comparison reveals L-valine (top) and L-valine zinc sulfate (1) (bottom) are one and the same.

H

O

L-valine + zinc sulfate heptahydrate  L-valine ^C

H

NO

ZnSO

·7H

O

and NOT

L-valine zinc sulfate

C

H

NO

ZnSO

·7H

O

Scheme 1