| סיכום: | The thermodynamic stabilities of three monomeric variants of the bacteriophage Cro repressor that differ
only in the sequence of two amino acids at the apex of an engineered -hairpin have been determined. The
sequences of the turns are EVK-XX-EVK, where the two central residues are DG, GG, and GT, respectively.
Standard-state unfolding free energies, determined from circular dichroism measurements as a function of
urea concentration, range from 2.4 to 2.7 kcal/mole, while those determined from guanidine hydrochloride
range from 2.8 to 3.3 kcal/mole for the three proteins. Thermal denaturation yields van’t Hoff unfolding
enthalpies of 36 to 40 kcal /mole at midpoint temperatures in the range of 53 to 58°C. Extrapolation of the
thermal denaturation free energies with heat capacities of 400 to 600 cal/mole deg gives good agreement
with the parameters determined in denaturant titrations. As predicted from statistical surveys of amino acid
replacements in -hairpins, energetic barriers to transformation from a type I turn (DG) to a type II turn
(GT) can be quite small.
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