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- Author or Editor: Elena Boldyreva x
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Abstract
Heat capacity C p(T) of the crystalline dl-cysteine was measured on heating the system from 6 to 309 K by adiabatic calorimetry; thermodynamic functions were calculated based on these data smoothed in the temperature range 6–273.15 K. The values of heat capacity, entropy, and enthalpy at 273.15 K were equal to 142.4, 153.3, and 213.80 J K−1 mol−1, respectively. At about 300 K, a heat capacity peak was observed, which was interpreted as an evidence of a first-order phase transition. The enthalpy and the entropy of the transition are equal, respectively, to 2300 ± 50 and 7.6 ± 0.1 J K−1 mol−1.
Abstract
Heat capacity C p(T) of the orthorhombic polymorph of L-cysteine was measured in the temperature range 6–300 K by adiabatic calorimetry; thermodynamic functions were calculated based on these measurements. At 298.15 K the values of heat capacity, C p; entropy, S m 0(T)-S m 0(0); difference in the enthalpy, H m 0(T)-H m 0(0), are equal, respectively, to 144.6±0.3 J K−1 mol−1, 169.0±0.4 J K−1 mol−1 and 24960±50 J mol−1. An anomaly of heat capacity near 70 K was registered as a small, 3–5% height, diffuse ‘jump’ accompanied by the substantial increase in the thermal relaxation time. The shape of the anomaly is sensitive to thermal pre-history of the sample.
Low-temperature heat capacity of diglycylglycine
Some summaries and forecasts for the heat capacity of amino acids and peptides
Abstract
Heat capacity of tripeptide diglycylglycine was measured in a temperature range from 6.5 to 304 K. The results were compared with those for glycine and glycylglycine. Peptide bonding was found not to change C P(T) virtually above 70 K, where heat capacity does not obey the Debye model. Comparison with literature data allows one to expect a significant difference in the heat capacity for enantiomorph and racemic species of valine and leucine, like it was found recently for D-and DL-serine.
Abstract
Monoclinic (I) and orthorhombic (II) polymorphs of paracetamol were studied by DSC and adiabatic calorimetry in the temperature range 5 - 450 K. At all the stages of the study, the samples (single crystals and powders) were characterized using X-ray diffraction. A single crystal → polycrystal II→ I transformation was observed on heating polymorph II, after which polymorph I melted at 442 K. The previously reported fact that the two polymorphs melt at different temperatures could not be confirmed. The temperature of the II→I transformation varied from crystal to crystal. On cooling the crystals of paracetamol II from ambient temperature to 5 K, a II→ I transformation was also observed, if the 'cooling-heating' cycles were repeated several times. Inclusions of solvent (water) into the starting crystals were shown to be important for this transformation. The values of the low-temperature heat-capacity of the I and II polymorphs of paracetamol were compared, and the thermodynamic functions calculated for the two polymorphs.
Abstract
Heat capacity of D- and DL-serine was measured using adiabatic calorimetry in a temperature range of 5.5 to 300 K, and then thermodynamic functions were calculated. The difference in heat capacity (C PD-C PDL) between two species indicates a small anomaly in D-serine near 15 K and a systematic excess over DL for temperatures > 30 K. This is much larger, than a difference in thermodynamic functions measured so far for the polymorphs of organic molecular crystals. The excess is fitted well to Einstein contribution with characteristic temperature of 185 K which is equivalent to vibrational mode at 129 cm−1.
Abstract
Five polymorphs of chlorpropamide (α, β, δ, γ, and ε) were investigated near the melting point by using DSC. Structure of samples was tested by X-ray powder diffraction. Four first polymorphs were found to transform into ε-polymorph, which melts at T m=128°C, Δm H=24 kJ mol−1. Enthalpy of the polymorph transitions ranges from +3 kJ mol−1 for α→ε to −0.8 kJ mol−1 for β→ε. Structure of three first polymorphs was published elsewhere, and the structure of δ-polymorph is published for the first time. XRPD patterns for all polymorphs are reported, together with the atomic coordinates for the δ-polymorph.
Abstract
Heat capacity of crystalline L- and DL-phenylglycines was measured in the temperature range from 6 to 305 K. For L-phenylglycine, no anomalies in the C p (T) dependence were observed. For DL-phenylglycine, however, an anomaly in the temperature range 50–75 K with a maximum at about 60 K was registered. The enthalpy and the entropy changes corresponding to this anomaly were estimated as 20 J mol−1 and 0.33 J K−1 mol−1, respectively. In the temperature range 205–225 K, an unusually large dispersion of the experimental points and a small change in the slope of the C p (T) curve were noticed. Thermodynamic functions for L- and DL-phenylglycines in the temperature range 0–305 K were calculated. At 298.15 K, the values of heat capacity, entropy, and enthalpy are equal to 179.1, 195.3 J K−1 mol−1, and 28590 J mol−1 for L-phenylglycine and 177.7, 196.3 J K−1 mol−1 and 28570 J mol−1 for DL-phenylglycine. For both L- and DL-phenylglycine, the C p (T) at very low temperatures does not follow the Debye law C – T 3. The heat capacity C p (T) is slightly higher for L-phenylglycine, than for the racemic DL-crystal, with the exception of the phase transition region. The difference is smaller than was observed previously for the L-/DL-cysteines, and considerably smaller, than that for L-/DL- serines.
