Entropy/enthalpy compensation: hydrophobic effect, micelles and protein complexes

Abstract

Molecular interpretations are here presented of the hydrophobic effect, which is the cause of the low solubility of apolar substances in water. The solubilization process of substances such as the noble gases consists in the formation of a cavity in the solvent with expulsion of n w water molecules. The process is associated to an entropy/enthalpy (S/H) compensation linearly dependent upon the temperature. The observed enthalpy DH app , either determined calorimetrically or by van't Hoff equation, shows DC p,app a 0 and positive. We set DC p,app ¼ n w C p,w where C p,w is the isobaric heat capacity of water. The number n w (n w 4 0) of relaxed water molecules is proportional to the size of the solute molecule and hence of the cavity. The term n w C p,w T is actually an entropy term, which compensates for part of the reaction enthalpy (DH 0 o 0). The entropy change at 298 K linearly depends on n w , thus showing that cavity formation is associated to a negative entropy change (Ds cav ¼ -23.2 J K À1 mol À1 n w À1). Beyond a temperature T min typical of each compound, the reaction becomes endothermic. The highly negative entropy change DS min (at T min we have DH app ¼ 0) is related to the loss of kinetic energy by the solute molecule when trapped in the cage. Another example of S/H compensation occurs in the formation of micelles. The resultant cage volume after formation of the micelle is smaller than the sum of the cavities previously hosting the single separated apolar moieties. Therefore, some floating water molecules need to be reintroduced into the structure of the solvent (n w o 0) to fill the void. The contraction of the cavity is associated to a positive entropy change (Ds fill ¼ 22.4 J K À1 mol À1 |n w | À1 ). Protein folding and protein-substrate association behave in a way similar to micellisation (n w o 0). The present interpretation of the complexation reactions of proteins, and also of micellisation, leads to a new formulation of the so-called 'hydrophobic bond': the positive entropy change for cavity contraction is the main driving force of hydrophobic bonding. In the denaturation process as opposite to folding, the denaturation enthalpy DH den at different temperatures T den depends on positive numbers (n w 4 0) of water molecules. The presence of polar groups and/or charges in the solute molecule, on the other hand, exerts on the water molecules the same action as that produced by micellisation (n w o 0).

Hydrophobic Hydration Processes. I: Dual-Structure Partition Function for Biphasic Aqueous Systems

“…15 Next, we passed to the study of the denaturation processes 16,17 and then to the study of micelle formation processes. 18 All of the experimental data of every hydrophobic hydration process give origin to linear plots of the function Δ H dual = Δ H 0 + g ( T ). This behavior conforms to the ergodic algorithmic model (EAM), whereby Δ C p ,hydr is necessarily constant for analytical geometry constraint.…”

Thermodynamics of aqueous solutions of dodecyldimethylethylammonium bromide

“…Their changes upon micellization are primarily due to the change in structure of the water molecules involved in the hydrophobic effect [35] and to the changes in electrostriction of the polar head and of the counterion. A great change in these interactions occurs when the micelle formation begins.…”

Doxorubicin and ABT-199 Coencapsulated Nanocarriers for Targeted Delivery and Synergistic Treatment against Hepatocellular Carcinoma

“…The process is associated with compensation of entropy and enthalpy and related to the temperature. Temperature influences movement of the hydrophobic chain at temperatures above their glass transition temperature [47].…”