Thermodynamic and Conformational Study of Proline Stereoisomers

Abstract

Amino acids play fundamental roles both as building blocks of proteins and as intermediates in metabolism. Proline, one of the 20 natural amino acids, has a primordial function in enzymes, peptide hormones, and proteins. The energetic characterization of these molecules provides information concerning stability and reactivity and has great importance in understanding the activity and behavior of larger molecules containing these structures as fragments. In the present work, parallel experimental and computational studies have been performed. The experimental studies have been based on calorimetric and effusion techniques, from which the enthalpy of formation in the crystalline phase and the enthalpy of sublimation of the sterioisomers L-, D-, and the DL-mixture of proline have been derived. Additionally, vapor pressure measurements have also enabled the determination of the entropies and Gibbs energies of sublimation, at T = 298.15 K. From the former results, the experimental standard (p(o) = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, of L-proline, D-proline, and DL-proline have been calculated as -388.6 ± 2.3, -391.9 ± 2.0, and -391.5 ± 2.4 kJ·mol(-1), respectively. A computational study at the G3 and G4 levels has been carried out. Conformational analysis has been done and the enthalpy of formation of proline as well as other intrinsic properties such as acidity, basicity, adiabatic ionization enthalpy, electron and proton affinities, and bond dissociation enthalpies have been calculated. There is a very good agreement between calculated and experimental values, when they are available.

Volatility Study of Amino Acids by Knudsen Effusion with QCM Mass Loss Detection

“…only presented smoothed data and therefore it is difficult to estimate the real uncertainty of the data, but the agreement with this work is very good (around 5 %). The data for L‐proline and D‐proline by Santos et al . are in good agreement with this work, but possess about twice larger scatter.…”

“…This work: •: L‐proline; ▾: L‐isoleucine; ⧫: L‐leucine; ▾: L‐methionine;◄: L‐threonine. Literature: ○: L‐proline; ▾: L‐isoleucine; ⧫: L‐leucine; ▾: L‐methionine; ○: L‐proline; ⊕: L‐proline; ◊: L‐leucine . The lines are included as a guide for the eye.…”

“…Figure shows good agreement with our results required for mutual correlation of the data. Unfortunately, neither of the studies presented uncertainty of their data, which is here considered equal to 5 % for the purposes of weighting within the correlation procedure. Besides the vapor pressures of L‐proline, Santos et al .…”

“…Unfortunately, neither of the studies presented uncertainty of their data, which is here considered equal to 5 % for the purposes of weighting within the correlation procedure. Besides the vapor pressures of L‐proline, Santos et al . also presented measurements for D‐proline, and correlated them separately obtaining two values of sublimation enthalpy and entropy of enantiopure proline.…”

“…also presented measurements for D‐proline, and correlated them separately obtaining two values of sublimation enthalpy and entropy of enantiopure proline. Reasons for a separate treatment of thermodynamic data for L‐ and D‐enantiomer, which should be equal, are not explained . The data for D‐proline are not included in the correlation in this work to keep the weights of the three studies better balanced.…”