Temperature-dependent helix-coil transition of an alanine based peptide

The helix-coil transition of a synthetic alpha -helical peptide (the D-Arg peptide), Ac-YGG(KAAAA)(3)CO-D-Arg-CONH2, was studied by static far-UV circular dichroism (CD) and time-resolved infrared spectroscopy coupled with the laser-induced temperature-jump technique for rapid relaxation initiation. Equilibrium thermal unfolding measurements of the D-Arg peptide monitored by CD spectroscopy reveal an apparent two-state helix-coil transition, with a thermal melting temperature around 10 degreesC. Time-resolved infrared (IR) measurements following a laser-induced temperature jump, however, reveal biphasic (or multiphasic) relaxation kinetics. The fast phase rises within the 20 ns response time of the detection system. The slow phase has a decay lifetime of similar to 140 ns at 300 K and exhibits monotonic temperature dependence with an apparent activation energy around 15.5 kcal/mol.