Thermodynamic investigation of l-carnitine

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Highlights

  • Temperature dependence of heat capacity of l-carnitine has been measured by precision adiabatic vacuum calorimetry.

  • The thermodynamic functions of the l-carnitine have been determined for the range from T → 0 to 350 K.

  • The character of heterodynamics of structure was detected.

  • Enthalpy of combustion of the L-carnitine was measured using high-precision combustion calorimeter.

Abstract

In the present work temperature dependence of heat capacity of l-carnitine has been measured for the first time over the range from 6 K to 346 K by precision adiabatic vacuum calorimetry. Based on the experimental data, the thermodynamic functions of the l-carnitine, namely, the heat capacity , enthalpy H°(T) − H°(0), entropy S°(T) − S°(0) and Gibbs function G°(T) − H°(0) have been determined for the range from T → 0 to 350 K. The value of the fractal dimension D in the function of multifractal generalization of Debye’s theory of the heat capacity of solids was estimated and the character of heterodynamics of structure was detected. Enthalpy of combustion (−4142.1 ± 2.3) kJ mol−1 of the l-carnitine was measured for the first time using high-precision combustion calorimeter. The standard molar enthalpy of formation in the crystalline state (−756.2 ± 2.5) kJ mol−1 of l-carnitine at 298.15 K was derived from the combustion experiments. Using combination of the adiabatic and combustion calorimetry results the thermodynamic functions of formation of the l-carnitine at T = 298.15 K and p = 0.1 MPa have been calculated. The low-temperature X-ray diffraction was used for the determination of coefficients of thermal expansion.

Introduction

l-carnitine or vitamin B11 (CAS: 541-15-1) is a non-protein amino acid (β-hydroxy-γ-trimethyl-amino-butyric acid), that is synthesized from the essential amino acids lysine and methionine [1].

One of the functions of l-carnitine is the interconvertion, in the mechanisms of regulation, of cetogeneze and thermogeneze [2]. Due to its intrinsic interaction with the bioenergetic processes, its role is crucial in diseases associated with metabolic compromise, especially mitochondrial related disorders [3]. Furthermore, the carnitine system plays an essential physiological role in fatty acid b-oxidation and in the maintenance of acyl-coenzyme A (acyl-CoA)2 pools.

Small amounts of l-carnitine can be synthesized by adults, the majority of l-carnitine needed in humans is taken through food consumption. In addition, l-carnitine is an essential nutrient for infants [4].

This work is a continuation of systematic studies of vitamins B. Earlier in the articles [5], [6], [7], [8], [9], we have investigated the thermodynamic properties of vitamins Bn (n = 2, 3, 8, 9, 12). The goals of this work include calorimetric determination of the standard thermodynamic functions of the l-carnitine with the purpose of describing biochemical and industrial processes with its participation.

Section snippets

Sample

l-carnitine was purchased from Sigma Aldrich. For phase identification, an X-ray diffraction pattern of the vitamin B11 sample was recorded on a Shimadzu X-ray diffractometer XRD-6000 (CuKα radiation, geometry θ-2θ) in the 2θ range from 5° to 60° with scan increment of 0.02°. The X-ray data and Sigma Aldrich certificate led us to conclude that the l-carnitine sample studied (the content of impurities 0.1 wt%) was an individual crystalline compound (monoclinic modification, space group P21 [10]).

Heat capacity

The Cpo measurements were carried out between 6 K and 346 K (see Table 2). The mass of the sample loaded in the calorimetric ampoules of the BKT-3.0 device was 0.4022 g. 146 experimental Cpo values were obtained in two series of experiments (Fig. 1). The heat capacity of the sample varied from 20% to 50% of the total heat capacity of calorimetric ampoule + substance over the range between 6 and 346 K. The experimental points of Cpo in the temperature interval 6–346 K were fitted by means of the

Conclusions

The general aim of these investigations was to report the results of the thermodynamic study of the l-carnitine. The heat capacity of this vitamin B11 is measured in the temperature range from (6 to 346) K, the thermodynamic functions are calculated and the fractal dimension D is evaluated. Thermochemical parameters of formation are determined by combining the data obtained by using combustion calorimetry and heat capacity measurements.

Acknowledgements

The work was performed with the financial support of the Russian Foundation of Basic Research (Project Number 16-03-00288). This work was financially supported by the Ministry of Education and Science of The Russian Federation (Organization of Scientific Research, No. 4.6138.2017/6.7). Also, this work has been supported by Russian Government Program of Competitive Growth of Kazan Federal University.

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    The heat capacity measurements and data evaluation.

    2

    Results from combustion calorimetry and QCM measurements.

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