The hydrogen bond is also referred to as hydrogen bridge, and, as its name implies, it involves a hydrogen atom. Indeed, this is a particular case of a dipole-dipole interaction (an interaction established between polar molecules) that includes a hydrogen atom, and requires specific conditions to be established.
There are two requirements that have to occur in order to be established a hydrogen bond. Therefore, not all polar molecules having hydrogen atoms have the ability to establish this type of interaction... The first requirement that must be acomplished is the existence of a very electronegative atom in one of the involved molecules. When I say "very electronegative" I 'm referring to one of the 3 most electronegative atoms - oxygen, nitrogen or fluorine. This atom will function as an "acceptor" of hydrogen, due to the fact that it is very electronegative, and thus it will have a very high electron density on it, presenting a partial negative charge. The second condition that needs to occur is the existence of a hydrogen atom covalently bonded to a very electronegative atom. In this case, the latter acts as a "donor" of hydrogen, and the hydrogen will present partial positive charge because it is attached to a very electronegative atom.
So, what happens is an electrostatic attraction between opposite partial charges, settling the hydrogen bridge. In biochemistry, the hydrogen bonds, like the remaining non-covalent forces, are very important. The best known example concerns the interaction between complementary nitrogenous bases in DNA.
And now... despite having already written this in another post , I can not resist telling it again: ;)
Do you know how an electron commits suicide?
It jumps from the hydrogen bridge!