Influence of Water on Protein Transitions: ThermalAnalysis.

We have developed a methodology usingadvanced thermal analysisto characterize the role of water in a specially synthesized familyof recombinant spider silk-like block copolymers. These proteins wereinspired by the genetic sequences found in the dragline silk of Nephila clavipes, comprising an alanine-rich hydrophobicblock, A; a glycine-rich hydrophilic block, B; and a C-terminus ora His-tag, H. This family of proteins serves as a model system inwhich the hydrophobicity is controlled by A and B block lengths, allowingsystematic comparison of water effects within the family. Temperature-modulateddifferential scanning calorimetry and thermogravimetric analyses wereemployed to capture the glass to rubber transition, Tg, in water-cast protein films. Modeling of the solidand liquid state heat capacity baselines allows us to determine thecritical role played by bound water which plasticizes and stabilizesthe protein through interchain bonding. In samples containing boundwater, two sequential glass transitions, Tg(1) and Tg(2), were observed during heating.The lower temperature glass transition, Tg(1), is related to conformational change induced by bound water removal,the hydrophobicity of the protein sequences, and the crystallinityof the protein. The higher temperature glass transition, Tg(2), is characteristic of the dry protein. The bindingenergy of water to protein compares favorably to ligand–waterbinding affinities. The energy absorbed by evaporating water dependsupon the volume fraction of the hydrophilic B-block. [ABSTRACT FROM AUTHOR]