Calmodulin-RS20-Ca4 complex in the gas phase:: electrospray ionisation and Fourier transform ion cyclotron resonance

Dissociation of the complex between calmodulin, four calcium ions and the synthetic peptide RS20 derived from the phosphorylation site of smooth-muscle myosin light-chain kinase was studied by sustained off-resonance irradiation/collision-induced dissociation (SORI-CID) and electrospray ionisation (ESI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Gas-phase dissociation of isolated ions of calmodulin-RS20-Ca, in the 8+ charge state yielded fragments via two parallel pathways; calmodulin-Ca-4, complex ions in 5+ and 6+ charge states and RS20 ions in 2+ and 3+ charge states were detected at high resolution. The results are interpreted as indicating that RS20 binding to calmodulin in this complex is specific, involving particular non-covalent interactions that decompose readily during collision-induced dissociation. More specifically, it is proposed that the gas-phase dissociation of calmodulin-RS20-Ca-4 complex reflects the existence in vacuo of salt bridges.