Abstract
Specific hydration induced coil-globule transition in gelatin-B(GB) chain, a polypeptide, in ethanol, ethylene glycol and glycerol solutions (organo-solutions) was studied by Raman spectroscopy, viscosity and dynamic light scattering techniques to map its differential hydration behavior in mono, di and tri-ol solutions (alcohols are non-solvents for this polypeptide). The chain stiffness, determined from the hydrodynamic radius to radius of gyration ratio Rg/Rh, was estimated to be around 0.67 in water which was found to be dependent on the nature of alcohol and its concentration. This clearly attributed same random coil conformation to GB in all thesolutions at low alcohol concentration, but chain collapse to a near-globular shape (Rg/Rh=0.77) was observed when alcohol concentration was increased which was found to occur in glycerol at ≈ 60% (v/v), in ethylene glycol at ≈ 35% and in ethanol at ≈ 40% (v/v) concentrations consistence with the intrinsic viscosity data. The Huggins (KH) and Kraemer (KK) solute-solvent interaction parameter determined from the concentration dependence plot of relative viscosity revealed poorer interaction between GB and organo-solvents with increase in concentration of the organic phase. Differential chain hydration was evaluated from characteristic Raman active modes of water molecule. Three signatures Raman peaks were observed at 3200, 3310, and 3460 cm-1 specific to structured, partially structured and amorphous water respectively. The peak at 3310 cm-1 was observed to decrease with alcohol concentration while the peak at 3200 cm-1 was observed to increase indicating the increase in the water structure in all the solutions and depletion of water density near the first hydration sheath of the protein molecule which caused GB chain to collapse.
Highlights
Hydrogen bonding liquids and their mixtures occupy a special place among complex systems due to the existence of directed H-bonds
When the O-atom has all the 4 bonds engaged with 4H-atoms, we describe it as fully structured, when 3 bonds are engaged with 3 different H-atoms, it is partially structured and when only 2 bonds are engaged with 2 H-atoms, we refer to it as free water molecule
The ratio of H-atoms available for hydrogen bonding to O-atoms in the system was calculated for various concentrations of glycerol and the same is shown in figure 1
Summary
Hydrogen bonding liquids and their mixtures occupy a special place among complex systems due to the existence of directed H-bonds. An enormous amount of literature exists which relates to the investigation of H-bonding systems, on glycerolwater mixture systems [1,2,3,4,5,6,7], there is still a lack of clear understanding even of the level of dynamics in “simple” water or alcohols. Alcohols and their mixtures with water are widely used as excellent models to study cooperative dynamics. A systematic way to study the same can be attempted through appropriate tuning of solvent hydrophobicity and its hydrogen bonding capability by choosing alcohol solutions as solvent and a known biopolymer as probe molecule. We have chosen ethanol, ethylene glycol and glycerol solutions to observe how they affect the hydration of gelatin B (GB) biopolymer
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