The effect of the three-dimensional structure of polypeptides and proteins on their ability to undergo isotopic exchange under the action of spillover hydrogen (SH) in the high temperature solid state catalytic isotope exchange reaction (HSCIE) was theoretically and experimentally studied. The HSCIE reaction in the β-galactosidase protein from Thermoanaerobacter ethanolicus (83kDa) was studied. The influence of the β-galactosidase structure on isotopic exchange as peptide fragments with spillover tritium was studied. The most accessible peptide fragment, which does not contribute to α-helix and β-strand formations (KEMQKE215-220), had the largest relative reactivity. The one located in the contact area between the subunits (YLRDSE417-422) showed the smallest relative reactivity. The relative reactivities of these peptides differ more than 150 times. Data collected during a study devoted to the HSCIE reaction of the β-galactosidase protein indicate that the HSCIE reaction might be employed for acquiring information about their three-dimensional structure and protein-protein interactions. The results of ab initio calculations have shown that α-helix formation in polypeptides decreases the reactivity in HSCIE. Hydrogen exchange in the α-helical fragment Trp1-Leu8 of zervamycin IIB was also analyzed using theoretical methods. It was shown by ab initio quantum-chemical calculations that the high degree of substitution of CαH for tritium in Gln3 might be associated with the participation of electron donor O and N atoms in transition state stabilization in the HSCIE reaction. © 2003 Elsevier Inc. All rights reserved.