Lednev, IK and Karnoup, AS and Sparrow, MC and Asher, SA
(1999)
α-Helix peptide folding and unfolding activation barriers: A nanosecond UV resonance raman study.
Journal of the American Chemical Society, 121 (35).
8074 - 8086.
ISSN 0002-7863
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Abstract
We used UV resonance Raman spectroscopy to characterize the equilibrium conformation and the kinetics of thermal denaturation of a 21 amino acid, mainly alanine, α-helical peptide (AP). The 204-nm UV resonance Raman spectra show selective enhancements of the amide vibrations, whose intensities and frequencies strongly depend on the peptide secondary structure. These AP Raman spectra were accurately modeled by a linear combination of the temperature-dependent Raman spectra of the pure random coil and the pure α-helix conformations; this demonstrates that the AP helix-coil equilibrium is well-described by a two-state model. We constructed a new transient UV resonance Raman spectrometer and developed the necessary methodologies to measure the nanosecond relaxation of AP following a 3-ns T- jump. We obtained the T-jump by using a 1.9-μm IR pulse that heats the solvent water. We probed the AP relaxation using delayed 204-nm excitation pulses which excite the Raman spectra of the amide backbone vibrations. We observe little AP structural changes within the first 40 ns, after which the α-helix starts unfolding. We determined the temperature dependence of the folding and unfolding rates and found that the unfolding rate constants show Arrheniustype behavior with an apparent κ8 kcal/mol activation barrier and a reciprocal rate constant of 240 ± 60 ns at 37 °C. However, the folding rate constants show a negative activation barrier, indicating a failure of transitionstate theory in the simple two-state modeling of AP thermal unfolding, which assumes a temperature-independent potential energy profile along the reaction coordinate. Our measurements of the initial steps in the α-helical structure evolution support recent protein folding landscape and funnel theories; our temperature-dependent rate constants sense the energy landscape complexity at the earliest stages of folding and unfolding.
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