Thermochemistry of sarcosine and sarcosine anhydride: Theoretical and experimental studies

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Abstract

The standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, of sarcosine, −(388.0 ± 1.0) kJ · mol−1, and sarcosine anhydride, −(334.5 ± 1.6) kJ · mol−1, were calculated by combining, for each compound, the standard molar enthalpy of formation, in the crystalline phase, and the standard molar enthalpy of sublimation, derived from measurements of the standard massic energies of combustion by static bomb combustion calorimetry, and from measurements of vapour pressures by the Knudsen mass-loss effusion method, respectively. The standard (po = 0.1 MPa) molar enthalpies, entropies and Gibbs functions of sublimation, at T = 298.15 K, were also calculated. A theoretical study at the G3 and G4 levels has been carried out, and the calculated enthalpies of formation have been compared with the experimental ones.

Highlights

► Study on the Energetics of the sarcosine and sarcosine anhydride. ► Experimental and computational thermochemistry of sarcosine and its anhydride. ► Ab initio calculations for two amino acid derivatives by G3(MP2)//B3LYP method.

Introduction

The amino acids are essential species present on the biochemical processes and the understanding their role on biological activity depends on the knowledge of thermochemical data related with the formation and dissociation of chemical bonds. The need for reliable data for this class of compounds leads to the study of selected molecules, in order to improve the situation. In previous papers, the thermochemical properties of l-cysteine, l-cystine (sulfur containing amino acids) [1], N-benzylalanines [2], α-alanine (DL) and β-alanine [3] were reported. More recently, we reported the study of the cyclic anhydrides of glycine and alanine [4]. The present work continues our thermochemical characterization of amino acids, the study of two other important molecules, the sarcosine (N-methylglycine) and the sarcosine anhydride (1,4-dimethyl-2,5-piperazinedione) (see figure 1).

Sarcosine is a natural amino acid playing a significant role in metabolic processes of living cells as a source of serine, creatine, purines or glutathione, and as an important intermediate in the metabolism of choline found in muscles and other body tissues. Several studies [5], [6], [7], [8] indicate the use of sarcosine in adjunctive treatment of patients with schizophrenia and depression. It has been also recently identified as a potential urine biomarker for prostate cancer diagnosis and prognosis [9], since its levels increase greatly during prostate cancer progression to metastasis. This subject has been analyzed by other researchers with different results [10], [11], although very recent papers reported that the doubt on its utility as a biomarker is related to the methods used for its quantification [12], [13].

Sarcosine anhydride, as a 2,5-diketopiperazine, is a cyclic polyfunctional peptide with ideal characteristics to be used as a starting material for synthesis of new heterocyclic compounds with potential application on the development of new bioactive compounds for pharmaceutical and agrochemical industries [14]. Some studies have shown that diketopiperazines present activities as anti-fungal, anti-bacterial, anti-tumour and anti-viral, among others [15].

The standard (p° = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, at the temperature T = 298.15 K, of sarcosine and sarcosine anhydride, were derived from the values of energies of combustion, measured by static bomb combustion calorimetry. The standard molar enthalpies of sublimation of these compounds were obtained from the temperature dependence of the vapour pressure using the Knudsen mass-loss effusion technique. Additionally, computational determinations of the gas-phase enthalpies of formation at the G3 and G4 levels were also performed.

Section snippets

Compounds and purity control

The two compounds studied were obtained commercially and purified by sublimation under reduced pressure. Further details of the origin and purification of the samples are presented in table 1. The average ratio of the mass of carbon dioxide recovered after combustion to that calculated from the mass of samples used in each experiment, together with the uncertainties (twice the standard deviation of the mean) was (0.99995 ± 0.00010) for sarcosine and (0.99991 ± 0.00022) sarcosine anhydride.

The

Computational details

Standard ab initio molecular orbital calculations [29] were performed with the Gaussian 09 series of programs [30]. The energies of the compounds studied were calculated using two different theoretical model chemistry Gaussian-n methods, at the G3 [31] and G4 [32] levels. Details on these methods have been given in our previous paper on glycine and alanine anhydrides [4].

Due to the lack of symmetry and the internal rotational degrees of freedom by free rotation of the NHMe, COOH, and OH groups

Experimental results

TABLE 2, TABLE 3 contain the combustion results obtained for sarcosine and sarcosine anhydride, in which Δm(H2O) represents the deviation of the mass of water added to the calorimeter from 3119.6 g, the mass assigned to εcalor, ΔTad is the calorimeter temperature change corrected for the heat exchange and the work of stirring, Δ is the correction to the standard state and the remaining terms are as previously defined [27], [33]. As samples were ignited at T = (298.150 ± 0.001) K, the internal

Molecular structures

The crystal structure of sarcosine was determined by X-ray diffraction by Mostad and Natarajan, in 1989 [40]. Amino acids exist as zwitterions in the crystalline state [41] as well as in aqueous solution, stabilized by electrostatic, polarization and hydrogen-bonding interactions with their environment. To our knowledge, there is not any experimental determination of the crystal structure of sarcosine anhydride.

In the gas phase, where the intermolecular interactions have no effect, amino acids

Condensed phase and phase transition

Sabbah and Laffitte [44] have measured the enthalpy of sublimation of sarcosine between 380 and 413 K and they obtained the value of (146 ± 1) kJ · mol−1, which corrected to T = 298.15 K yields (147 ± 1) kJ · mol−1. They have also determined the standard molar enthalpy of formation, in the solid phase of sarcosine as, −(513.24 ± 0.26) kJ · mol−1 [45]. Cox and Pilcher [46] have reanalysed the value of the standard molar enthalpy of formation obtained by Breitenbach, Derkosch, et al. [47] and they recommended the

Acknowledgments

Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal and to FEDER for financial support given to Centro de Investigação em Química da Universidade do Porto and to Programa Ciência 2008 (PEst-C/QUI/UI0081/2011). A.F.L.O.M.S thanks FCT and The European Social Fund (ESF) under the Community Support Framework (CSF) for the award of a post-doctoral fellowship (SFRH/BPD/41601/2007). The support of the Spanish Ministerio de Economía y Competitividad under Project

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