Quantitative insights into tightly and loosely bound water in hydration shells of amino acids.

The hydration of amino acids closely correlates the hydration of peptides and proteins and is critical to their biological functions. However, complete and quantitative understanding about the hydration of amino acids is lacking. Here, tightly and loosely bound water of 20 zwitterionic amino acids are quantitatively distinguished and determined by Raman spectroscopy with multivariate curve resolution (Raman-MCR) and differential scanning calorimetry (DSC). The total hydration water obtained from Raman-MCR and the tightly bound water determined by DSC have certain relevance, but they are not exactly corresponding. Especially, Pro, Arg and Lys exhibit larger number of tightly bound water molecules (4.026.59), showing significant influences on the onset transition temperature and the melting enthalpy values of water molecules, which provides direct evidence for their unique functions associated with biological water. Asn, Ser, Thr, Met, His and Glu have smaller number of tightly bound water molecules (0.301.31), whilst the other remaining 11 amino acids only contain loosely bound water. Four exceptional amino acids Ile, Leu, Phe and Val show lower tightly bound water number but the high loosely bound water number. As to hydration shell structure, most amino acids except Pro and Trp enhance tetrahedral water structure and H-bonds relative to pure water and at least 1.9 % of the hydration water molecules associated with the amino acids show non-hydrogen-bonded OH defects. This work combines two effective experimental techniques to reveal the hydration water structure and quantitatively analyze the two kinds of bound water of 20 amino acids.