Intersulfur Distance Is a Key Factor in Tuning Disulfide Radical Anion Vertical UV−Visible Absorption

Abstract

International audienceMaximum absorption wavelengths λmax for σ* ← σ vertical transition of peptidic disulfide radical anions of increasing complexity were investigated by means of time-dependent density functional theory. Values among a representative set of 17 two-cysteinyl peptides range between 385 and 624 nm (TD-BH&HLYP/DZP++//MP2/DZP++:HF/6-31G* level of theory). This considerable spread contrasts with the usually admitted value of ca. 400−450 nm typically ascribed to two-center three-electron bonds. It is traced back to the large range of equilibrium intersulfur distances d, with values comprised between 2.73 and 3.19 Å. More quantitatively, blue- and red-shifts follow a near-linear regime (slope of 46 nm per 0.1 Å). They can be mapped onto a relaxed scan of l,l-cystine taken as a prototypical system λmaxref = 436 nm, 2.79 Å. This result could assist future radioprotectants rational design, with disulfide-linking arcs of controlled geometry. Meanwhile, the presence of a secondary structure motif such as an α-helix does not affect the UV−vis transition

Performances of recently-proposed functionals for describing disulfide radical anions and similar systems

“…Such an analysis involves three relevant descriptors: adiabatic electron affinity (AEA), inter-sulfur distance d SS , and longest wavelength of maximal absorption k max . However, we established a nearly linear dependence between the two latter properties [31,6], and only AEA and d SS are considered in this Letter.…”

“…A first purely-geometric test is imposed: both the neutral and anionic dðSSÞ should be reproduced with a AE0:1 Å tolerance. It is critical to have a faithful estimate of this geometrical parameter, which deviates importantly from DMeDS equilibrium value in cyclic aliphatic [35,36] or proteinic [37] systems, subsequently inducing a marked UV/vis shift of the transition energy [6]. Obviously, this threshold is only discriminative for RA, as all DFT functionals adequately predicts the intersulfur distance for the neutral system.…”

“…In the latter context, covalent disulfide bonds are probably the most important entities able to capture a low-energy electron into a low-lying r H orbital, forming transient radical anions. Resort to sophisticated experimental techniques paved the way to increasingly detailed pictures of such two-center three-electron (2c-3e) systems: existence [2], stability [3], formation [4], redox potentials [5], UV/vis signature [6] have been tackled. For instance, the dramatic lengthening of the intersulfur distance (ca.…”

“…Emphasis was recently given to the formation [28,29] and UV/ vis signature of disulfide radical anions [7,6], by contrast with their dissociation, for which a correction has been proposed [30]. Such an analysis involves three relevant descriptors: adiabatic electron affinity (AEA), inter-sulfur distance d SS , and longest wavelength of maximal absorption k max .…”

Influence of Water on the Oxidation of Dimethyl Sulfide by the ·OH Radical

“…The ionic 2c-3e bond in general has been the subject of many fundamental theoretical studies. − However, these studies mainly focused on ionic 2c-3e bonded systems, with fewer studies on the neutral SO bonds formed in the adducts. , In particular, in ref the (2c-3e) SO bonds of different radicals was carried out using various computational methods (BH&HLYP, MP2, and CCSD­(T)), the possibility of the direct attachment of · OH to S atom in neutral radicals was analyzed with the ELF method − and the 2c-3e character of the SO bond in DMS- • OH was established.…”