Force Spectroscopy of an Elastic Peptide: Effect of D(2)O and Temperature on Persistence Length

This study explores the mechanical unfolding of elastic protein analogues as a function of temperature, in both H(2)O and D(2)O, using atomic force microscopy (AFM) force spectroscopy in a specially constructed AFM liquid cell. This represents the first time that the effect of D(2)O on protein flexibility has been investigated at the single molecule level by this technique. Model elastic peptides, R6, were encoded from synthetic genes expressed in Escherichia coli. The peptides possess short N- and C-terminal domains with central repetitive domains containing 13 repeats of the motif PGQGQQ-plus-GYYPTSLQQ. These sequences mimic those in native high molecular weight subunit glutenin proteins which confer elasticity to bread dough. Fitting single molecule stretching events to the worm-like chain model, allows determination of the molecular persistence length under various experimental conditions. The effect of increasing the temperature is to increase the persistence length of the peptide in both H(2)O and D(2)O, consistent with the expected "thermal softening" effect. However, the effect is significantly enhanced in D(2)O, in which the persistence length at 45 degrees C is similar to 25% greater than the value measured in H(2)O at the same temperature. Stronger intrapeptide H-bonding due to isotopic substitution of hydrogen for deuterium is the most likely cause of the enhanced backbone rigidity.