Thermodynamics of interaction of water vapour with 20 different poly-l-amino acids Original Research Article

The uptake of water vapour by 20 different polyaminoacids have been evaluated by an isopiestic vapour pressure technique in absence of solute at three different temperatures (22°C, 30°C and 37°C). The water vapour adsorption isotherm for different polyaminoacids in the range of water activity varying between zero and unity apparently agreed with that expected from a type III BET isotherm. From the linear BET plots, obeyed in the lower range of water activity, the BET constants nm and Qm for different polyamines have been evaluated. The amount of water vapour adsorbed (n1)was calculated in moles/kg of polyaminoacids as well as in moles/mole of amino acid residue. Its value at unit water activity (Δn1o) has been evaluated by an extrapolation method and the results support that the multilayer adsorption of water vapour at the interface of polyaminoacids takes place. Values of Δn1o are strictly comparable in terms of moles per kg rather than mole per mole unit. In case of β lactoglobulin (βlg), lysozyme and BSA, theoretically obtained Δn1o values were observed to be considerably larger than experimental values of Δn1o. This indicated that amino acid residues in the polypeptide release a large amount of water due to the formation of a globular structure. Using the Bull equation in the integrated form, standard free energies, ΔGwo for water–polyamino acid binding interaction at two different temperatures have been evaluated. Based on the Clausius–Clapeyron equation in an integrated form, the integral enthalpy for water–polyamino acid interaction has also been evaluated.