Positive charge of NAD+ molecule

[tickle_monste]

Why is the positive charge of the NAD+ molecule located on the nitrogen in the nicotinamide group? This nitrogen has 4 bonds attached to it, but nitrogen only has 3 holes in its valence shell, so wouldn't this make a negative charge on this nitrogen?

 

[chemisttree]

Why is the positive charge of the NAD+ molecule located on the nitrogen in the nicotinamide group? This nitrogen has 4 bonds attached to it, but nitrogen only has 3 holes in its valence shell, so wouldn't this make a negative charge on this nitrogen?

Draw the lewis dot structure for the nitrogen in this system and you will see a free lone pair of electrons before protonation by H⁺. Is it true that when you protonate a neutral molecule with H⁺ you are left with an anion?

 

[Ygggdrasil]

In general, for determining the formal charge on an atom, we consider all of the electrons in lone pairs and inner shells as belonging to that atom as well ashalf of the electrons in chemical bonds as belonging to that atom. Therefore, the nitrogen has 4 bonds attached to it (giving 4 electrons) plus two inner shell electrons, giving a total of 6 electrons to nitrogen's 7 protons.