Kinetic Isotope Effect in the Unfolding of a Protein Secondary Structure: Calculations for Beta-Sheet Polyglycine Dimers as a Model.

In the present work, we performed calculations of the kinetic isotope effect (KIE) on H/D, ¹⁴N/¹⁵N, ¹⁶O/¹⁸O, and ¹²C/¹³C isotopic substitution in the dissociation of beta-sheet polyglycine dimers of different lengths into two monomer chains. This dissociation reaction, proceeding via breaking of the interchain hydrogen bonds (H-bonds), is considered to be a model of unfolding of the secondary structure of proteins. The calculated strengthening of the interchain hydrogen bonds N-H⋯O=C due to heavy isotope substitution decreases in the row H/D >> ¹⁴N/¹⁵N > ¹⁶O/¹⁸O > ¹²C/¹³C. The KIE for H/D substitution, defined as the ratio of the rate constants k(H)k(D), was calculated with the use of a "completely loose" transition state model. The results of the calculations show that a very high H/D isotope effect can be achieved for proteins even with moderately long chains connected by dozens of interchain H-bonds. The results obtained also indicate that the heavy isotope substitution in the internal (interchain) and external H-bonds, located on the periphery of a dimer, can provide comparable effects on secondary structure stabilization.