• Which atoms in the protein backbone can form hydrogen bonds? N-H and O
• Which backbone atom donates the hydrogen atom (hydrogen bond donor)? N
• Which backbone atom accepts the hydrogen atom (hydrogen bond acceptor?) O

Intermezzo about Hydrogen Bonds (only for the fanatics):

Geometry of Hydrogen Bond D-H ?.

Distance

H-A ~ 2.5 Ångström
D-A ~ 3.5 Ångström
D..A is smaller than sum of VdW radii
Hbonds are getting stronger If D and A become more charged; strong Hb => salt bridge The ideal hydrogen bond would have an angle of 180^o  between the lone-pair off the acceptor atom, the polar hydrogen and the donor atom

Electronegativity is the ability of an atom to attract electrons to itself. Pull at the shared electrons in the N-H or O-H bond. Electronegativity of N 3.0 O 3.4 S 2.6 H 2.2 C 2.5 bond=polar if difference in EN >  0.6 (exception is S-H)

Partial negative charge (δ-) near oxygen due to unshared pairs of electrons. Partial positive charge (δ+) near the hydrogen atoms. Opposite charges attract. So if relative orientation of atoms is OK => overlap of orbitals =>Hbond

In Trp N-H (side chain) is weak donor. Cysteine (meaning the non-disulphide form of Cys) very rarely accepts a hydrogen bond, but it does donate them. The pKa of the SH group in a single cysteine molecule is typically 8.5, implying that deprotonation will be very rare. Methionine: SD can in theory accept two hydrogen bonds. Chemists do not regard sulphur as an element that is electronegative enough to accept hydrogen bonds. Neither, apparently, do proteins.

Hbond ca. 5% strength compared tot covalent bond

HB 1-5 kcal/mol

intermolecular or intramolecular Hbonds are about the same strength, there is little energetic advantage on exchange of one type of HBonds for the other