Fabiana Sussich

Reversible dehydration of trehalose and anhydrobiosis: from solution state to an exotic crystal?

Abstract Physico-chemical properties of the trehalose–water system are reviewed with special reference to the transformations that may shed light on the mechanism of trehalose bio-protection. Critical analysis of solution thermodynamics is made in order to scrutinize trehalose properties often called ‘anomalous’ and to check the consistency of literature results. Discussion on the conversion between the solid state polymorphic forms is given, with a special emphasis of the transformations involving the newly identified anhydrous crystalline form of α,α-trehalose, TREα. This exotic crystal is almost ‘isomorphous’ with the dihydrate crystal structure, and possesses the unique feature of reversibly absorbing water to produce the dihydrate, without changing the main structural features. The reversible process could play a functional role in the well-known ability of this sugar to protect biological structures from damage during desiccation. The final aim of the paper is to add some new insights into and to reconcile previous hypotheses for the peculiar ‘in vivo’ action of trehalose.

Transitions and phenomenology of α, α-trehalose polymorphs inter-conversion

The thermal transitions of α,α-trehalose polymorphs have been carefully studied by differential scanning calorimetry (DSC) at different scan rates. Attention has been paid to the correlation of the thermal data with the proper crystalline forms (when present), in order to attribute the observed transitions correctly. In particular, the subtle dehydration process of dihydrate trehalose has been already shown to produce several species, depending on the experimental conditions.Thermodynamic data (i.e., temperatures and enthalpies) for all the known transitions are reported and the results are critically discussed.

Enthalpy relaxation and glass transition behaviour of sucrose by static and dynamic DSC

Abstract The relaxation process of the sucrose in the glassy state and its glass transition are studied by differential scanning calorimetry in both the conventional and the dynamic mode. The data on the kinetics of glassy relaxation are experimentally evaluated by measuring the enthalpy relaxation peaks as a function of the ageing times at three different ageing temperatures, namely 45, 55 and 60°C. It is shown that the ageing process is a complex phenomenon with a rather large distribution of relaxation times. The value of the breath of the distribution of relaxation processes, β, is 0.32 and it is in the lower range of the distributions commonly observed. The dynamic components of the heat capacity ( C ′ p and C ″ p ) have been obtained from the experiments carried out at different frequency of temperature modulation ( ν =8.3−83 mHz). These components are unaffected by the ageing process and therefore are more reliable for the correct T g determination. The activation energies for the relaxation process and for the glass transition process are also evaluated.

Trehalose amorphization and recrystallization.

The stability of the amorphous trehalose prepared by using several procedures is presented and discussed. Amorphization is shown to occur by melting (T(m)=215 degrees C) or milling (room temperature) the crystalline anhydrous form TRE-beta. Fast dehydration of the di-hydrate crystalline polymorph, TRE-h, also produces an amorphous phase. Other dehydration procedures of TRE-h, such as microwave treatment, supercritical extraction or gentle heating at low scan rates, give variable fractions of the polymorph TRE-alpha, that undergo amorphization upon melting (at lower temperature, T(m)=130 degrees C). Additional procedures for amorphization, such as freeze-drying, spray-drying or evaporation of trehalose solutions, are discussed. All these procedures are classified depending on the capability of the undercooled liquid phase to undergo cold crystallization upon heating the glassy state at temperatures above the glass transition temperature (T(g)=120 degrees C). The recrystallizable amorphous phase is invariably obtained by the melt of the polymorph TRE-alpha, while other procedures always give an amorphous phase that is unable to crystallize above T(g). The existence of two different categories is analyzed in terms of the transformation paths and the hypothesis that the systems may exhibit different molecular mobilities.

The kinetics of periodate oxidation of carbohydrates 2. Polymeric substrates

A study of the kinetics of periodate oxidation on a series of dextran oligomers and polymers is carried out by isothermal microcalorimetry. In addition to these substrates, some dimeric carbohydrates and hyaluronan were studied. Rate constants were calculated from the calorimetric decay curves, which, properly corrected for calorimetric response, are proportional to the rate of periodate conversion. The dependence of the kinetic rates on the molecular weight of dextran samples and on the substrate concentration, is described in terms of the much higher rates of terminal reducing units. The presence of two sites with comparable reaction rates makes the analysis of the calorimetric curves difficult, even in the simple overall pseudo-first-order condition. The suitability of a phenomenological treatment of kinetic data is explored.

The kinetics of periodate oxidation of carbohydrates: a calorimetric approach

A calorimetric approach is described for analysing the kinetics of periodate oxidation on a series of monosaccharidic substrates. Rate constants at several temperatures were calculated from the calorimetric decay curves that are proportional to the rate of conversion. Arrhenius plots provided the activation parameters for the various carbohydrates and a linear correlation was found between the values of enthalpy and entropy of activation. The dependence of the values of kinetic rates on stereochemistry is interpreted in terms of conformational probability of the reactive state. The suitability of the calorimetric method to track the kinetic process of slow reactions is emphasised, in particular its ability to monitor, directly and continuously, the course of the reaction.

Thermodynamic behavior of mixed biopolymers in solution and in gel phase

The thermodynamic properties of mixtures of two biopolymers, namely maltodextrin and gelatin, have been studied from the gelation as well as from the solution properties points of view. Differential scanning calorimetry has been used to monitor the changes in enthalpy due to the melting of the gel and to evaluate the cooperativity parameter of the gelatin in dilute and semi-dilute concentration. Heats of dilution of the single biopolymers and heats of dilution of the mixed biopolymer solutions have been used to evaluate the Flory interaction parameters within the framework of a new experimental procedure. These data are useful in the description of the thermodynamics of mixed biopolymers and complement other data in progress on this or similar systems.