Elucidating Potential Energy Surfaces for Singlet O Reactions with Protonated, Deprotonated, and Di-Deprotonated Cystine Using a Combination of Approximately Spin-Projected Density Functional Theory and Guided-Ion-Beam Mass Spectrometry.

The reactivity of cystine toward electronically excited singlet O (aΔ) has been long debated, despite the fact that most organic disulfides are susceptible to oxidation by singlet O. We report a combined experimental and computational study on reactions of singlet O with gas-phase cystine at different ionization and hydration states, aimed to determine reaction outcomes, mechanisms, and potential energy surfaces (PESs). Ion-molecule collisions of protonated and di-deprotonated cystine ions with singlet O, in both the absence and the presence of a water ligand, were measured over a center-of-mass collision energy (E) range from 0.

J-coupling constants for a trialanine peptide as a function of dihedral angles calculated by density functional theory over the full Ramachandran space.

We present 13 (3)J, seven (2)J and four (1)J coupling constants (24 in all) calculated using B3LYP/D95** as a function of the φ and ψ Ramachandran dihedral angles of the acetyl(Ala)(3)NH(2) capped trialanine peptide over the entire Ramachandran space. With the exception of three of these J couplings, all show significant dependence upon both dihedral angles. For each J coupling considered, a two dimensional grid with respect to φ and ψ angles can be used to interpolate the values for any pair of φ and ψ values.