Interaction of some hydrophobic amino acids, peptides, and protein with aqueous 3-chloro-1,2-propanediol and 3-chloro-1-propanol: Biophysical studies

Abstract

The apparent molar volume V_2,ϕ, apparent molar isentropic compressibility K_S,2,ϕ, and heat of dilution (q) of aqueous glycine, alanine, α-amino butyric acid, valine, leucine, diglycine, triglycine, and hen egg white lysozyme have been determined in aqueous solutions of 3-chloropropano-1-ol and 3-chloropropan-1,2-diol solutions at T = 298.15 K. These data have been used to calculate the infinite dilution standard partial molar volume $V_{2\text{,}m}^0$, partial molar isentropic compressibility $K_{S\text{,}2\text{,}m}^0$, and enthalpy of dilution Δ_dilH° of the amino acids and peptides in aqueous 3-chloropropano-1-ol and 3-chloropropan-1,2-diol, and the standard partial molar quantities of transfer of the amino acids and peptides to the aqueous alcohol and diol solutions. The linear correlation of $V_{2\text{,}m}^0$ for a homologous series of amino acids has been utilized to calculate the contribution of the charged end groups $({\text{NH}}_3^{+}\text{,}{\text{COO}}^{-})$, CH₂ group and other alkyl chains of the amino acids to the values of $V_{2\text{,}m}^0$. The results on the standard partial molar volumes of transfer, compressibility and enthalpy of dilution from water to aqueous alcohol and diol solutions have been correlated and interpreted in terms of ion–polar, ion–hydrophobic, and hydrophobic–hydrophobic group interactions. The heat of dilution of these amino acids, peptides, and hen egg white lysozyme measured in aqueous solutions of 3-chloropropano-1-ol and 3-chloropropan-1,2-diol by using isothermal titration calorimetry along with the volumetric, compressibility, and calorimetric results on amino acid and peptides have been correlated to understand the nature of interactions operating in these systems.

Introduction

Understanding the role of the solvent in maintaining the native and an active conformation of a protein is very important as it gives information on the forces that contribute to its stability and mechanism of folding–unfolding transitions. Non-aqueous solvents have been widely used to generate the partially folded states in proteins in vitro. The 2,2,2-trifluoroethanol [1], [2], [3], [4], [5], [6], [7] and 1,1,1,3,3,3-hexafluoroisopropanol [8], [9], [10], [11], [12], [13], [14], [15] have been widely used in producing the partially folded states in the proteins. Generally, alcohols have been reported to affect the proteins in three distinct ways: disruption of the rigid native structure, induction of α-helix and dissolution of peptide aggregates that sometimes occur during peptide synthesis, investigation of prion diseases and Alzheimer’s amyloid peptides [16], [17]. The structure stabilizing property of fluoroalchols has generally been attributed to the hydrophobicity of the fluoroalkyl CF₃ group and the strong hydrogen bond donating/poor hydrogen bond accepting property of –OH group [18], [19]. In view of the importance of fluoroalcohols in the generation of partially folded states of proteins, it is important to understand the effect of chloro-substituted alcohols on the conformation of proteins.

Due to the importance of short-chain alcohols in understanding the partially folded states of proteins, effects of 4-chlorobutan-1-ol, n-butanol, 3-chloropropan-1-ol, 3-chloropropan-1,2-diol, propan-1,2-diol, n-propanol, 2,2,2-trifuoroethanol and ethanol, on the conformational stability and the heat denaturational behaviour of some globular protein has been studied [20]. The chloro-substituted alcohols have been observed to be more effective destabilizers for proteins compared with the corresponding normal alcohols in the order: 4-chlorobutan-l-ol > n-butanol > 3-chloropropan-l-ol > n-propanol > 3-chloropropane-1,2-diol [20]. Though there are reports in literature on the effect of fluoro- and chloro-substituted alcohols on the conformational stability of proteins, the information on the physicochemical properties of the constituents of proteins such as amino acids and peptides in aqueous substituted alcohols is scarce. In this work we have chosen to study the interaction of amino acids and peptides with aqueous 3-chloro-1-propanol and 3-chloro-1,2-propanediol. The energetics of the interaction of the alcohol and diol with hen egg white lysozyme has also been studied.

Understanding the physicochemical properties of the constituents of proteins in aqueous alcohol and diol solutions can throw light on the nature of interaction of these co-solvents with the amino acids and peptides. This can further lead to an understanding of the role of solvent mediated interactions operating in such systems. Therefore there is a need for experimental data on the interaction of amino acids and peptides in these mixed hydrophilic–hydrophobic solvent environment and correlation to the effect on the macromolecule as a whole.

Standard partial molar volume, isentropic compressibility and heats of dilution are sensitive and informative thermodynamic characteristics that reflect intermolecular and intramolecular interactions. In this work, we have measured standard partial molar volumes, isentropic compressibilities and heats of dilution of a homologous series of amino acids and peptides in aqueous 3-chloro-1-propanol and 3-chloro-1,2-propanediol solution and analyzed the results in terms of different intermolecular interactions. We have also performed studies on the isothermal titration calorimetric measurements on the interaction of 3-chloro-1-propanol and 3-chloro-1,2-propanediol with hen egg-white lysozyme with an objective to understand the mode of interaction of these co-solvents with the protein at the macromolecular and amino acid/peptide level. It is important to understand whether the interactions are binding type or through solvent mediated effects.