HYDROGEN-TRANSFER IN THE PRESENCE OF AMINO-ACID RADICALS

Quantum chemical model studies of hydrogen transfer between amino acid s in the presence of radicals have been performed using the density fu nctional theory method B3LYP. These studies were made to investigate a lternative mechanisms to the conventional electron transfer-proton tra nsfer mechanisms. The model reactions studied are such that the net re sult of the reaction is a transfer of one neutral hydrogen atom. Simpl e models are used for the amino acids. Three different mechanisms for hydrogen transfer-were found. In the first of these, a transition stat e with a protonated intermediate residue is found: in the second, the proton and electron take different paths and in the third, a neutral h ydrogen atom call be identified along the reaction pathway. A key feat ure of these mechanisms is that charge separation is always kept small in contrast to the previous electron transfer-proton transfer mechani sms. It is therefore proposed that the processes normally considered a s electron transfer-in the biochemical literature could in fact be bet ter explained as hydrogen atom transfer, at least in cases where a sui table hydrogen bonded chain pathway is present in the protein. The pre sence of such chains in principle allows the protein to define the pat h of net hydrogen transfer. Another important conclusion is that stand ard quantum chemical methods can be used to treat these mechanisms for hydrogen transfer, allowing for an accurate representation of the geo metric changes during the reactions.