Therefore, this result strongly suggests a high preference for the proR over a proS hydrogen transfer in apo BSS, if it is possible at all. Furthermore, the thiyl radical intermediate (I apo ) exhibits a higher energy than E apo by 7.2 kJ/mol in the case of the proR transfer and by approximately 78 kJ/mol in the case of the proS transfer (although such a high energy of proS I apo seems to be associated with increased size of QM part-see Table ). It should be noted, however, that QM:MM methodology, with mostly frozen coordinates outside of the active site, is not the best tool for precise estimation of energy differences between different conformations of the protein.