Electron distribution in cationic fragments generated mass spectrometrically from peptides

Ab initio Hartree-Fock (HF/6-31G, HF/6-31G* and HF/6-31G**) methods have be en used to study the mechanism of the formation of cationic fragments from peptides. The results show that the simplest B-3 ion of nominal structure H CONHCH2. CONHCH2CO+, can be fragmented into the A(3) ion in the first step by the loss of CO, but not directly into the B-2 ion, in addition, the A(3) ion can be further fragmented into the B-2 ion. The calculated activation barrier of the first reaction (from B-3 to A(3)) is 26.2 kcal/mol at the HF /6-31G** level of theory. The second reaction (from B-3 to B-2) has much hi gher energy barrier (77.15 kcal/mol at HF/6-31G** level of theory). The thi rd reaction (from A(3) to B-2) and the fourth reaction (from B-2 to A(2)) h ave barriers of 28.82 and 25.59 kcal/mol, respectively. In addition to the energetics of the fragmentation, the electronic structure and bonding of th e main stationary points have been analyzed by Bader's theory of atoms in m olecules (AIM). (C) 1999 Elsevier Science B.V. All rights reserved.