As shown in this diagram, we have electromagnetic interaction between the oxygen and the H. And what this does is it basically positions this entire side chain of histidine, 157, in the correct orientation so that the next interaction can take place. Now, what exactly is the next interaction? Well, this nitrogen contains two electrons, a lone pair of electrons. On top of that, we can also say that the nitrogen has a partial positive charge, a partial negative charge, because nitrogen is more electronegative than the nearby carbons. And so we can say there is this partial negative charge that exists on this nitrogen. And so it will interact with the H atom because this H atom of this alcohol of the Serine contains a partial positive charge because this oxygen contains a partial negative charge because of its high electronegativity value.