We investigated the gas-phase nucleobases [CGH](+1), [Cu(CG)](+1), [Cu(CG-H)](+1), and nucleoside [Cu(dCdG)](+1) complexes using collision-induced dissociation (CID) MS/ MS spectra. Two forms, the Watson- Crick complex form (cation-base-base) and an interstrand complex form (base-cation-base), were included in the analysis of the [cation-base pair] complex structures. The main cytosine loss dissociation channel of the [Cu(CG)](+1) complex was analyzed as the loss of an interstrand [Cu(CG)](+1) form instead of the Watson-Crick form. The -43 amu (CONH) and -68 amu (cytosine-CONH) loss fragments of the [Cu(CG-H)](+1) complex can be explained by the dissociation process of the interstrand [Cu(CG-H)](+1) form. (C) 2011 Elsevier B.V. All rights reserved.