Published: September 26, 2011 r2011 American Chemical Society 3807 dx.doi.org/10.1021/je200292z | J. Chem. Eng. Data 2011, 56, 3807–3812 ARTICLE pubs.acs.org/jced Thermophysical Study of Several α- and β-Amino Acid Derivatives by Differential Scanning Calorimetry (DSC) María Victoria Roux,* ,† Rafael Notario, † Marta Segura, ‡ Ram on Guzm  an-Mejía, § Eusebio Juaristi, § and James S. Chickos || † Instituto de Química Física “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spain ‡ PerkinElmer Espa~ na S.L., Ronda de Poniente 19, 28760 Tres Cantos, Madrid § Departamento de Química, Centro de Investigaci on y de Estudios Avanzados del IPN, Apartado Postal 14-740, 07000 M exico D.F., M exico ) Department of Chemistry and Biochemistry, University of Missouri-St. Louis, One University Boulevard, St. Louis, Missouri 63121-4499, United States ABSTRACT: The present study reports a differential scanning calorimetry (DSC) study of the amino acids sarcosine [CAS Registry No. 107-97-1], α-alanine (DL) [CAS Registry No. 302-72-7], β-alanine [CAS Registry No. 107-95-9], N-benzyl-α-alanine (DL) [CAS Registry No. 40297-69-6], and N-benzyl-β-alanine [CAS Registry No. 5426-62-0] in the temperature interval from T = 268 K to their respective melting/decomposition temperatures. Temperatures and heat capacities as a function of temperature of all solids and the enthalpies and entropies of fusion of two of the amino acids, sarcosine and N-benzyl-β-alanine, are reported. ’ INTRODUCTION Amino acids are among the most important building blocks of life and are also believed to play key roles in interstellar chemistry as well as in the origin of life on Earth. 1,2 They are the building blocks of peptides and the backbone of proteins, and for this reason, they have been extensively studied. In addition to their fundamental biological importance, amino acids present rather interesting physical properties; for example, in solution or in the crystalline state they exist as zwitterions 3 that are stabilized by intermolecular electrostatic polarization, as well as hydrogen-bonding interactions with their environment. 4,5 By contrast, in the gaseous state, or when isolated in a noble gas matrix at low temperature, the neutral forms are more stable. 4,5 The knowledge of heat capacity as a function of temperature is an essential property for the calculation of thermodynamic properties such as ΔH, ΔS, and ΔG at different temperatures, and this property plays an important role in identifying and understanding transitions occurring in the solid state of crystals and in liquid crystals. Heat capacities at T = 298.15 K have proven quite useful in adjusting vaporization, sublimation, and fusion enthalpies with temperature. Equations for doing this have recently been reported by Chickos and co-workers. 68 There are several compilations of critically evaluated calorimetrically measured heat capacities, 913 but new and reliable data on the heat capacity of important families of compounds are still needed, 14,15 particularly for crystalline solids. There has been an effort to develop reliable and accurate group contribution schemes to improve the estimation and compensate for the scarcity of this data. The simplest schemes are based on first-order additivity and only consider the constituent groups of the molecule. 16,17 Other methods use a second-order additivity scheme that takes into account nearest-neighbor interactions in the definition of the structural units of molecules. 1820 These schemes normally neglect all next-to-nearest neighbor interactions because of the limited accuracy of the available experimental heat capacity data. Although current improvements in instrumentation and proto- col allow the experimental determination of heat capacities by differential scanning calorimetry (DSC) within an error of 0.15 %, 21 the actual experimental uncertainty of the measured heat capacity is often larger due to the presence of impurities in the samples under investigation. Special care must be taken to remove or reduce the water content, especially for liquid samples, since the heat capacity of water departs considerably from that of the majority of organic compounds. Estimations of heat capacity of solids are more problematic than their liquid counterparts. This is due in part to the lack of data but also due to the anisotropic nature of the solid state. Phase transitions in solids can affect their heat capacities near these transitions. Solids that form liquid crystals, for example, appear to have larger heat capacities in certain temperature regions, and total phase change entropies seem to be attenuated in comparison to systems that melt directly to isotropic liquids. 22 Group values for estimating the heat capacity of crystalline solids have been reported, but the estimations in many cases have been hampered by the lack of sufficient reference data. During the past few years, we have been involved in the experi- mental determination of enthalpies of fusion and heat capacities, as well as the study of polymorphism of pure crystalline organic compounds. 2331 Nevertheless, despite the importance of amino acids, reliable experimental thermochemical studies are scarce. Very recently we have carried out 32,33 a thermochemical study of α-alanine (DL), β-alanine, N-benzyl-α-alanine (DL), and N-benzyl-β-alanine, Received: May 11, 2011 Accepted: September 8, 2011