RESUMO

Copper is a ubiquitous metal in biology that, among other functions, is implicated in enzymatic redox catalysis and in protein electron transfer (ET). When it comes to ET, copper sites are found in two main forms, mononuclear type 1 (T1) and binuclear CuA sites, which share a common cupredoxin fold. Other relevant copper sites are the so-called type 2 (T2), which are more resilient to undergo direct electrochemistry and are usually involved in catalysis. Here we report the electrochemical and spectroscopic characterization of a novel T2-like copper site engineered following the loop swapping strategy. The ligand loop sequence of the newly discovered T1 copper site from Nitrosopumilus maritimus was introduced into the CuA scaffold from Thermus thermophilus yielding a chimeric protein that shows spectroscopic features different from both parental proteins, and resemble those of red T2 copper sites, albeit with a shorter Cu-S(Cys) bond length. The novel T2 site undergoes efficient direct electrochemistry, which allows performing temperature-dependent cyclic voltammetry studies. The obtained results reveal that this chimera constitutes the first example of a copper protein with entropically controlled reduction potential, thereby contrasting the enthalpic supremacy observed for all other copper sites reported so far. The underlying bases for this entropic control are critically discussed.

Assuntos

RESUMO

Knowledge of the movement of water molecules inside material is important for the study of interactions between water and chemical components of agricultural products. The thermodynamic properties calculation reports this need, being the source of information to design drying equipment, calculate the energy required in this process and evaluate the physical phenomena that occur on the food surface. Given the importance of the subject, the objective of this study was to determine the thermodynamic properties of the water desorption process in lettuce seeds. Lettuce seeds had an initial moisture content of 9.3% (d.b.). The equilibrium moisture content of seeds was determined by static gravimetric method at different temperatures (10, 20, 30, 40 and 50 ± 1 °C) and relative humidity values (between 11 and 96%). Experiments were conducted in triplicate. It was observed that, with increasing equilibrium moisture content, there was a reduction in the energy required for evaporation of water in lettuce seeds, with values of integral isosteric heat of desorption in the water content range 2.1 to 21.2% (d.b.) which varied from ‘4164.89 to 2477.43 kJ kg-¹. With increasing equilibrium moisture content there was a decrease in differential entropy, resulting in lower demand for mobility of water molecules. The Gibbs free energy also decreased with increasing equilibrium moisture content and was positive for all temperatures studied, showing non-spontaneity of water desorption in the seeds. The enthalpy-entropy compensation theory was has been satisfactorily applied to the sorption phenomenon and the water desorption process in lettuce seeds was controlled by enthalpy.(AU)

O conhecimento da movimentação das moléculas de água dentro do material é importante para o correto estudo das interações entre a água e os componentes químicos dos produtos agrícolas. O cálculo das propriedades termodinâmicas relata essa necessidade, sendo fonte de informação para projetar equipamentos de secagem, calcular a energia requerida nesse processo e avaliar os fenômenos físicos que ocorrem na superfície dos alimentos. Diante da importância do assunto, o objetivo do presente trabalho foi determinar as propriedades termodinâmicas do processo de dessorção de água em sementes de alface. As sementes de alface possuíam teor de água inicial de 9,3% (b.s.). O teor de água de equilíbrio das sementes foi determinado pelo método estático-gravimétrico para diferentes valores de temperatura (10, 20, 30, 40 e 50 ± 1 °C) e atividade de água (entre 11 e 96%), em três repetições. De acordo com os resultados encontrados, observou-se que com o aumento do teor de água de equilíbrio,ocorre redução da energia necessária para a evaporação da água nas sementes de alface, sendo que os valores do calor isostérico integral de dessorção, na faixa de teor de água de 2,1 a 21,2% (b.s.), variaram de 4164,89 a 2477,43 kJ kg-1. Com a elevação do teor de água de equilíbrio, há um decréscimo daentropia diferencial, implicando em menor demanda de mobilidade das moléculas de água. A energia livre de Gibbs também diminuiu com o incremento do teor de água de equilíbrio, sendo positiva para todas as temperaturas estudadas, demonstrando a não espontaneidade do processo de dessorção de água nas sementes. A teoria da compensação entalpia-entropia foi satisfatoriamente aplicada ao fenômeno de sorção, sendo o processo de dessorção de água das sementes de alface controlado pela entalpia.(AU)

Assuntos

RESUMO

Knowledge of the movement of water molecules inside material is important for the study of interactions between water and chemical components of agricultural products. The thermodynamic properties calculation reports this need, being the source of information to design drying equipment, calculate the energy required in this process and evaluate the physical phenomena that occur on the food surface. Given the importance of the subject, the objective of this study was to determine the thermodynamic properties of the water desorption process in lettuce seeds. Lettuce seeds had an initial moisture content of 9.3% (d.b.). The equilibrium moisture content of seeds was determined by static gravimetric method at different temperatures (10, 20, 30, 40 and 50 ± 1 °C) and relative humidity values (between 11 and 96%). Experiments were conducted in triplicate. It was observed that, with increasing equilibrium moisture content, there was a reduction in the energy required for evaporation of water in lettuce seeds, with values of integral isosteric heat of desorption in the water content range 2.1 to 21.2% (d.b.) which varied from ‘4164.89 to 2477.43’ kJ kg-¹. With increasing equilibrium moisture content there was a decrease in differential entropy, resulting in lower demand for mobility of water molecules. The Gibbs free energy also decreased with increasing equilibrium moisture content and was positive for all temperatures studied, showing non-spontaneity of water desorption in the seeds. The enthalpy-entropy compensation theory was has been satisfactorily applied to the sorption phenomenon and the water desorption process in lettuce seeds was controlled by enthalpy.

O conhecimento da movimentação das moléculas de água dentro do material é importante para o correto estudo das interações entre a água e os componentes químicos dos produtos agrícolas. O cálculo das propriedades termodinâmicas relata essa necessidade, sendo fonte de informação para projetar equipamentos de secagem, calcular a energia requerida nesse processo e avaliar os fenômenos físicos que ocorrem na superfície dos alimentos. Diante da importância do assunto, o objetivo do presente trabalho foi determinar as propriedades termodinâmicas do processo de dessorção de água em sementes de alface. As sementes de alface possuíam teor de água inicial de 9,3% (b.s.). O teor de água de equilíbrio das sementes foi determinado pelo método estático-gravimétrico para diferentes valores de temperatura (10, 20, 30, 40 e 50 ± 1 °C) e atividade de água (entre 11 e 96%), em três repetições. De acordo com os resultados encontrados, observou-se que com o aumento do teor de água de equilíbrio,ocorre redução da energia necessária para a evaporação da água nas sementes de alface, sendo que os valores do calor isostérico integral de dessorção, na faixa de teor de água de 2,1 a 21,2% (b.s.), variaram de 4164,89 a 2477,43 kJ kg-1. Com a elevação do teor de água de equilíbrio, há um decréscimo daentropia diferencial, implicando em menor demanda de mobilidade das moléculas de água. A energia livre de Gibbs também diminuiu com o incremento do teor de água de equilíbrio, sendo positiva para todas as temperaturas estudadas, demonstrando a não espontaneidade do processo de dessorção de água nas sementes. A teoria da compensação entalpia-entropia foi satisfatoriamente aplicada ao fenômeno de sorção, sendo o processo de dessorção de água das sementes de alface controlado pela entalpia.

Assuntos

RESUMO

Freeze-dried chia mucilage adsorption isotherms were determined at 25, 35 and 40°C and fitted with the Guggenheim-Anderson-de Boer model. The integral thermodynamic properties (enthalpy and entropy) were estimated with the Clausius-Clapeyron equation. Pore radius of the mucilage, calculated with the Kelvin equation, varied from 0.87 to 6.44 nm in the temperature range studied. The point of maximum stability (minimum integral entropy) ranged between 7.56 and 7.63kg H2O per 100 kg of dry solids (d.s.) (water activity of 0.34-0.53). Enthalpy-entropy compensation for the mucilage showed two isokinetic temperatures: (i) one occurring at low moisture contents (0-7.56 kg H2O per 100 kg d.s.), controlled by changes in water entropy; and (ii) another happening in the moisture interval of 7.56-24 kg H2O per 100 kg d.s. and was enthalpy driven. The glass transition temperature Tg of the mucilage fluctuated between 42.93 and 57.93°C.

Assuntos