# Why there is hydrogen bonding involved in salt bridge formation between glutamic acid and lysine?

I saw this figure on Wikipedia:

After seeing this image I got really confused about the difference between these two, or maybe the similarities. Of course, I understand that a hydrogen bond can be seen as +/- interaction (because of the dipole). However, I don't understand why this Lys + Glu interaction would also make a hydrogen bond. If it would, I would expect the hydrogen bond between the double bounded oxygen and the $$\ce{N}$$ atom from the $$\ce{NH3+}.$$

I hope someone can explain me what this figure means in respect to both of the interactions shown.

• Electrostatic interaction: the interaction between the negatively charged oxygen atom and the positively charged nitrogen atom.
– DHMO
Oct 2 '16 at 15:10
• Hydrogen bonding: the interaction between the oxygen and the hydrogen.
– DHMO
Oct 2 '16 at 15:10
• it does: "salt bridge is a combination of two noncovalent interactions: hydrogen bonding and electrostatic interactions " @DHMO Oct 2 '16 at 15:27

An electrostatic interaction is one between a centre of positive charge and a centre of negative charge. In your example, instead of a primary ammonium cation ($\ce{H3\overset{+}{N}R}$) you could also consider the fully methylated quarternary ammonium cation $\ce{Me3\overset{+}{N}R}$ which would still be able to build up an electrostatic interaction with the acid anion. The centre of positive charge is somewhere between the three ammonium hydrogens and the centre of negative charge somewhere between the two carboxylic oxygens.
A hydrogen bond, on the other hand, is a directional interaction of the type $\ce{X-H\bond{....}Y}$, where $\ce{X}$ and $\ce{Y}$ are electronegative elements and $\ce{Y}$ must feature a lone pair. Thus, hydrogen bonds always contain three atoms and only those three. One one of the three ammonium hydrogens can form a hydrogen bond with only one of the two oxygen atoms.