Influence of Water on Protein Transitions: Morphology and Secondary Structure

Fibrous protein secondary structural transitions are affected by bound water. A family of specially designed recombinant spider silk-like block copolymers with a gradient of block length, hydrophobilicity, and degree of crystallinity was biosynthesized and characterized to demonstrate the effect of water on the protein structural transitions. These proteins were inspired by the genetic sequences found in the dragline silk of Nephila clavipes, comprising an alanine-rich hydrophobic block, A, a glycine-rich hydrophilic block, B, and a C-terminus or a His-tag, H. Because the A-block is hydrophobic and the B-block is hydrophilic, the spider silk-like block copolymers behave as amphiphilic molecules and self-assemble into various structures in water solution. We employ time-resolved Fourier transform infrared (FTIR) spectroscopy to assign the origin of specific secondary structural transitions during heating. A transition from random coils to β-turns dominates during a lower temperature glass transition (of plasticized protein) mediated by the removal of bound water. Once the protein is in the dry solid state, further heating causes the now-dry protein to undergo the glass transition to the liquid state through conversion of α-helices into β-turns. The structural transitions during protein glass transitions are intrinsic to the amorphous region of protein and are hardly affected by protein hydrophobicity, block length, or crystallinity. The self-assembly morphology of the spider silk-like block copolymers, investigated by scanning electron microscopy, indicates that the large-scale morphology is stable during heating through both the lower and upper temperature glass transitions.