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The interactions of Selenocysteine (Sec) with Ca have been investigated through the use of B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) calculations. The global minimum of the [SecCa] potential energy surface is a charge-solvated (CS) species in which the metal dication is bound to the SeH, to the amino groups and to the carbonyl oxygen. Within a gap of 5.0 kJ mol in terms of free energies there are four more complexes three of which are salt-bridged (SB) structures. The interactions between Ca and the amino acid are essentially electrostatic, in contrast with what happens when Ca is replaced by Cu, where the charge transfer from Sec to the metal cation is so large that in the [Sec-Cu] global minimum the amino acid moiety is oxidized. The significant electron density redistribution undergone by the amino acid when interacting with both doubly-charged metal ions is reflected in a significant enhancement of its intrinsic acidity, although this effect is much larger for Cu than for Ca. One of the direct consequences of the oxidation undergone by the amino acid when attached to Cu is that the complexes formed are thermodynamically unstable with respect to a proton loss since the process Sec + Cu → [(Sec-H)Cu] + H is strongly exothermic. Conversely, a similar process when Cu is replaced by Ca is endothermic and therefore the [Sec-Ca] complexes should be stable with respect to a proton loss.

Complexation of Ca2+ with selenocysteine and effects on its intrinsic acidity