Modeling Interactions between an Amino Acid and a Metal Dication: Cysteine-Calcium(II) Reactions in the Gas Phase

Area of Science:

• Physical Chemistry
• Mass Spectrometry
• Computational Chemistry

Background:

• Cysteine (Cys) is a crucial amino acid with diverse biological roles.
• Understanding metal-ion interactions with biomolecules like cysteine is vital for biochemistry and medicine.
• Gas-phase studies provide fundamental insights into ion-molecule interactions, free from solvent effects.

Purpose of the Study:

• To investigate the gas-phase interactions between calcium dication (Ca²⁺) and cysteine (Cys).
• To elucidate the dissociation pathways and structural preferences of [Ca(Cys)]²⁺ complexes.
• To determine the energetic and electronic factors governing Ca²⁺ binding to cysteine.

Main Methods:

• Electrospray ionization mass spectrometry (ESI-MS) was employed to generate and analyze Ca²⁺/Cys complexes.
• Unimolecular collision-activated decomposition (CAD) experiments were performed on the doubly charged ions.
• Density functional theory (DFT) calculations at the B3LYP/6-311⁺⁺G(3df,2p)//B3LYP/6-311+G(d,p) level were used to model potential-energy surfaces and structures.

Main Results:

• Collision-activated decomposition of [Ca(Cys)]²⁺ primarily yields loss of ammonia, H₂S, or Coulomb explosion products.
• Coulomb explosion of [CaC₃H₄O₂S]²⁺ leads to [C₃H₃OS]⁺ and CaOH⁺, with subsequent decomposition of the cation.
• DFT calculations reveal that salt-bridge structures, despite being higher in energy, dictate the observed reaction pathways, with a strong electrostatic interaction (560 kJ/mol) between Ca²⁺ and cysteine.

Conclusions:

• The gas-phase fragmentation of Ca²⁺/Cys complexes is governed by Coulombic forces and structural preferences.
• Salt-bridge configurations play a critical role in directing the dissociation pathways of the doubly charged complex.
• The study highlights the importance of considering non-charge-solvated structures in understanding metal-ion interactions with amino acids.