Models of protein chains containing a seleno-cysteine (Sec) residue have been investigated by gas phase laser spectroscopy in order to document the effect of the H-bonding properties of the SeH group in the folding of the Sec side chain, by comparison with recent data on Ser- and Cys-containing sequences. Experimental data, complemented by quantum chemistry calculations and natural bonding orbital (NBO) analyses, are interpreted in terms of the formation of a so-called 5γ intra-residue motif, which bridges the acceptor chalcogen atom of the side chain to the NH bond of the same residue. This local structure, in which the O/S/Se atom is close to the plane of the N-terminal side amide, is constrained by local backbone-side chain hyperconjugation effects involving the S and Se atoms. Theoretical investigations of the Cys/Sec side chain show that (i) this 5γ motif is an intrinsic feature of these residues, (ii) the corresponding H-bond is strongly non-linear and intrinsically weak, (iii) but enhanced by γ- and β-turn secondary structures, which promote a more favorable 5γ H-bonding approach and distance. The resulting H-bonds are slightly stronger in selenocysteine than in cysteine, but nearly inexistent in serine, whose side chain in contrast behaves as a H-bonding donor. The modest spectral shifts of the Cys/Sec NH stretches measured experimentally reflect the moderate strength of the 5γ H-bonding, in agreement with the correlation obtained with a NBO-based H-bond strength indicator. The evolution along the Ser, Cys and Sec series emphasizes the compromise between the several factors that control the H-bonding in a hyperconjugation-constrained geometry, among them the chalcogen van der Waals and covalent radii. It also illustrates the 5γ H-bond enhancements with the Sec and Cys residues favoured by the constraints imposed by the γ- and β-turn structures of the peptide chain.
