Biophys J, November 2000, p. 2714-2721, Vol. 79, No. 5

Kinetics of Local Helix Formation in Poly-L-Glutamic Acid Studied by Time-Resolved Photoacoustics: Neutralization Reactions of Carboxylates in Aqueous Solutions and Their Relevance to the Problem of Protein Folding

Stefania Abbruzzetti,^* Cristiano Viappiani,^* Jeanne R. Small, Louis J. Libertini, and Enoch W. Small

 ^*Dipartimento di Fisica, Università di Parma and Istituto Nazionale per la Fisica della Materia, Parma, Italy;  Department of Chemistry and Biochemistry, Eastern Washington University, Cheney, Washington; and  Quantum Northwest, Inc., Spokane, Washington, USA

Photoactivatable caged protons have been used to trigger proton transfer reactions in aqueous solutions of acetate, glutamate, and poly-L-glutamic acid, and the volumetric and enthalpic changes have been detected and characterized by means of time-resolved photoacoustics. Neutralization of carboxylates in aqueous solutions invariably results in an expansion of the solution due to the disappearance of two charges and is accompanied by little enthalpic change. The reactions occur with thermally activated, apparent bimolecular rates on the order of 10¹⁰ M¹s¹. In the case of aqueous solutions of poly-L-glutamic acid at pH around the pK_a of the coil-to-helix transition, diffusional binding of a proton by carboxylates is followed by a sequential reaction with rate 1.06 (± 0.05) × 10⁷s¹. This step is not thermally activated in the temperature range we have investigated and is likely related to local formation of hydrogen bonds near the protonation site. This structural event may constitute a rate-limiting step in helix propagation.

Biophys J, November 2000, p. 2714-2721, Vol. 79, No. 5
© 2000 by the Biophysical Society   0006-3495/00/11/2714/08  $2.00

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