The K2(9-ethylguanine)122+ quadruplex is more stable to unimolecular dissociation than the K(9-ethylguanine)8+ quadruplex in the gas phase: a BIRD, energy resolved SORI-CID, IRMPD spectroscopic, and computational study

Abstract

A combination of experimental trapped-ion mass spectrometric studies and computational chemistry has been used in the present work to assess the intrinsic properties of the potassiated 9-ethylguanine (9eG) self-assembled quadruplex, K₂(9eG)₁₂²⁺, in the gas phase. Infrared multiple photon dissociation (IRMPD) spectroscopy in the N–H/C–H stretching region (2700–3800 cm⁻¹) revealed that this G-quadruplex is a sandwich-type structure with two G-tetrads sandwiching each of the two K⁺, very similar to the structure determined previously for the K(9eG)₈⁺ complexes. The stability of K₂(9eG)₁₂²⁺ toward unimolecular dissociation and its binding energy were examined using energy-resolved sustained off-resonance collision induced dissociation (SORI-CID) and blackbody infrared radiative dissociation (BIRD) kinetics experiments. SORI-CID experiments showed that the self-assembled K₂(9eG)₁₂²⁺ complex undergoes charge separation forming K(9eG)₈⁺ and K(9eG)₄⁺ compared to K(9eG)₈⁺ which loses neutral 9eG. More interestingly, K₂(9eG)₁₂²⁺ is more stable toward unimolecular dissociation activated by SORI-CID than the K(9eG)₈⁺ complex. Temperature dependent BIRD kinetics for K₂(9eG)₁₂²⁺ were consistent with energy-resolved SORI-CID results showing K₂(9eG)₁₂²⁺ to have an activation energy of 225 ± 15 kJ mol⁻¹, approximately 50 kJ mol⁻¹ greater than that determined for K(9eG)₈⁺. The extra stability of K₂(9eG)₁₂²⁺ is apparently not thermodynamic stability, but most likely due to an energy barrier for dissociation.