Difficulties in the growth and characterization of non-linear optical materials: A case study of salts of amino acids

Fleck, Michel; Petrosyan, A.M.

doi:10.1016/j.jcrysgro.2010.04.054

Cited by 116 publications

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“…The dubious nature of the crystal can be evidenced from the reported infrared (IR) spectrum in the title paper which is identical to that of L-alanine [7,8]. As it is well known that glycine (or L-alanine) reacts with aqueous HCl to form glycine hydrochloride [9] diglycine hydrochloride [10] and L-alanine hydrochloride [11], we reinvestigated the reported crystal growth of a so-called glycyl-L-alanine hydrochloride in order to identify its exact nature.…”

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confidence: 71%

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Comments on “Crystal growth, spectral, optical, and thermal characterization of glycyl-l-alanine hydrochloride (GLAH) single crystal”

Srinivasan

Moovendaran

Natarajan

2014

J Therm Anal Calorim

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The authors of the title paper (J Therm Anal Calorim 2012,110:873-878) report to have grown a dipeptide hydrochloride crystal namely glycyl-L-alanine hydrochloride by the slow evaporation of an aqueous solution containing stoichiometric amounts of L-alanine and glycine and an excess of hydrochloric acid. In this letter, we prove that no such dipeptide hydrochloride can be crystallized simply by mixing two amino acids in aqueous hydrochloric acid.Keywords Dipeptide Á Crystal growth Á Glycyl-L-alanine hydrochloride Á Glycine hydrochloride CommentDuring a literature survey of nonlinear optical (NLO) materials from the amino acid family, we came across the title paper by Malliga et al. [1] reporting NLO properties of glycyl-L-alanine hydrochloride. As it appeared to be the first case of a NLO material from the peptide family, the title paper attracted our attention and was taken up for scrutiny. We found the synthetic methodology for the growth of a dipeptide (hydrochloride) crystal involving the reaction of glycine and L-alanine with excess aqueous hydrochloric acid at ambient temperature very unusual as it differed from the reported procedure of reaction of the dipeptide glycyl-L-alanine with 2 N HCl followed by evaporation to dryness in vacuum [2]. It is well documented in all standard chemistry and biochemistry text books [3,4] that the synthesis of a dipeptide is accomplished using a multi step reaction protocol which includes (i) incorporation of protecting groups for the amino (or carboxyl) functionalities of the concerned amino acids (ii) reaction of the protected amino acids to form peptide bond, and (iii) deprotection reaction to obtain the dipeptide. The importance of protecting group for peptide synthesis is highlighted in the text book by Morrison and Boyd [3] taking as an example the dipeptide glycyl-L-alanine, the synthesis of which was accomplished by Tranter [5] employing the N-carbobenzoxy method. It is quite intriguing to note that the authors of the title paper used aqueous HCl as a reagent for formation of peptide at room temperature (which is a dehydration reaction), in contrast to the well-known procedure of employing concentrated HCl albeit at elevated temperatures for the hydrolysis of proteins (polypeptides) to the individual amino acids as the hydrochloride salts [4].Although the reported procedure in the title paper using excess HCl as a reagent appears to be a convenient single pot reaction for peptide bond formation without use of any protecting groups and conversion of the dipeptide into its hydrochloride, the claim of the growth of this crystal is untenable, as the unit cell data reported in the title paper do not correspond to either glycyl-L-alanine hydrochloride [2] or glycyl-L-alanine hydrochloride monohydrate [6] ( Table 1). The dubious nature of the crystal can be evidenced from the reported infrared (IR) spectrum in the title paper which is identical to that of L-alanine [7,8]. As it is Electronic supplementary material The online version of this article (

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“…The dubious nature of the crystal can be evidenced from the reported infrared (IR) spectrum in the title paper which is identical to that of L-alanine [7,8]. As it is Electronic supplementary material The online version of this article (…”

Section: Commentmentioning

confidence: 85%

Comments on “Crystal growth, spectral, optical, and thermal characterization of glycyl-l-alanine hydrochloride (GLAH) single crystal”

Srinivasan

Moovendaran

Natarajan

2014

J Therm Anal Calorim

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“…Since salicylic acid is a centrosymmetric solid, the only possible explanation is that no such measurement was probably made. Several such claims of growing novel NLO crystals based on unit cell data or observation of strong SHG signals in centrosymmetric solids have been proved to be erroneous by the groups of Fleck, Petrosyan, Srinivasan and Natarajan [20][21][22][23][24][25][26][27] and L-valinium salicylate is another dubious NLO crystal. Our present results once again show the risk of formulating new compounds based only on unit cell data, a single IR spectrum or an NMR spectrum instead of structure refinement based on single crystal data.…”

Section: Commentmentioning

confidence: 99%

Comment on the paper “Growth and characterization of organic nonlinear optical crystal: l-Valinium salicylate (LVS)”

Natarajan¹,

Moovendaran²,

Srinivasan³

2015

Optik

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“…The determination of unit cell parameters can be considered as standard crystallographic procedure for identification of previously known crystals, but not for new crystals. For formulating new crystals, unit cell data are not enough and can lead to erroneous conclusions [5]. The authors provided also the infrared spectrum and thermogravimetric curves, which according to them confirm the composition and structure of the obtained crystal.…”

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On the existence of “l-arginine acetamide”

Petrosyan

Srinivasan

2015

J Therm Anal Calorim

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We argue that the ''L-arginine acetamide'' crystal reported by Anitha et al. (J Therm Anal Calorim 119:785-789, 2015) is actually the well-known monoclinic form of anhydrous L-arginine hydrochloride crystal.The authors of a recent paper [1] claim to have grown a socalled L-arginine acetamide crystal by the slow evaporation of an aqueous solution containing L-arginine (L-Arg) and acetamide in equimolar ratio. The name ''L-arginine acetamide'' means that the obtained crystal is a 1:1 adduct viz. L-ArgÁCH 3 CONH 2 . However, the authors consider that they obtained a compound of formula H 2 NC(NH)NH (CH 2 ) 3 CH(NH 2 )CONH(CO)CH 3 (Fig. 1), whose actual systematic name is N-acetyl-2-amino-5-guanidinopentanamide.According to the authors of [1], the ''L-arginine acetamide'' crystal is monoclinic (space group P2 1 ) with a = 20.29 Å ; b = 5.18 Å ; c = 9.54 Å ; b = 91.68°. These cell parameters differ from the reported data for L-arginine dihydrate and anhydrous L-arginine [2] as well as trigonal [3] and orthorhombic [4] forms of acetamide, indicating that the title crystal is different from either of the starting materials. The authors write: ''The crystal structure is elucidated using standard crystallographic procedure.'' The determination of unit cell parameters can be considered as standard crystallographic procedure for identification of previously known crystals, but not for new crystals. For formulating new crystals, unit cell data are not enough and can lead to erroneous conclusions [5]. The authors provided also the infrared spectrum and thermogravimetric curves, which according to them confirm the composition and structure of the obtained crystal. We cannot agree with the authors in this point. Then, the authors added an erratum [6], where an alternate reaction scheme and a different formula (H 2 NC(CH 3 )NC(NH)NH(CH 2 ) 3 CH(NH 2 )COOH) were proposed for the title crystal (Fig. 2). The authors retained the earlier name ''L-arginine acetamide'' even though the systematic name for the proposed formula is (E)-2-amino-5-(3-(1-aminoethylidene)guanidino)pentanoic acid. The reaction scheme explaining the formation of the title crystal due to the coupling of the carbonyl group ([C=O) of acetamide with the -NH 2 group of the guanidine moiety of L-arginine forming an imine (Schiff base) is incorrect because Schiff base formation is a characteristic reaction of aldehydes or ketones and not that of an amide [7].However, this reaction scheme contains the essential information, that is, the presence of HCl, which helped us to clear up what actually was obtained by the authors. The use of HCl can lead to formation of L-arginine hydrochlorides: L-ArgÁHClÁH 2 O (monoclinic), L-ArgÁHCl (monoclinic) [8,9], L-ArgÁHCl (triclinic) [10], L-ArgÁ 2HClÁH 2 O (orthorhombic) [11,12]. Triclinic L-ArgÁHCl and orthorhombic L-ArgÁ2HClÁH 2 O can be excluded, because obtained crystal is monoclinic. The crystal L-ArgÁHClÁH 2 O also can be excluded because obtained crystal decomposes above 200°C. Comparison of unit cell

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Difficulties in the growth and characterization of non-linear optical materials: A case study of salts of amino acids

Cited by 116 publications

References 68 publications

Comments on “Crystal growth, spectral, optical, and thermal characterization of glycyl-l-alanine hydrochloride (GLAH) single crystal”

Comments on “Crystal growth, spectral, optical, and thermal characterization of glycyl-l-alanine hydrochloride (GLAH) single crystal”

Comment on the paper “Growth and characterization of organic nonlinear optical crystal: l-Valinium salicylate (LVS)”

On the existence of “l-arginine acetamide”

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