More on figure 2.4

The electrostatic interaction between glutamate or aspartate and arginine

The reason that the interaction can be spread over both oxygen atoms of the carboxylate and/or two nitrogen centres of the arginine guanidinium group is explained by resonance, or delocalisation. The charge on the side chain carboxylate group of the glutamate is delocalised over both oxygen atoms,

For the carboxylate, the 'real' structure is between:

i.e. the charge is spread around the carboxylate group

Similarly the positive charge of the guanidinium group at the end of the arginine side chain is delocalised across the nitrogen centres:

i.e. the positive charge is spread around the guanidinium group.  

Put the delocalised carboxylate next to the delocalised guanidinium group, and you find that the 'salt bridge' can involve both oxygens and two nitrogen centres.  There are two possible such interactions:

 

Additionally, there are a number of possible interactions involving fewer interacting atoms, for instance with a single interaction between one of the oxygen atoms and one of the nitrogen centres.

All of these possible interaction can be considered in terms of both an electrostatic (ionic) interaction and hydrogen bonding.

 

As a side note, the actual charge distribution is quite complex, since there is an interplay between the resonance effect (which would apparently place half a negative charge on each oxygen of the carboxylate, and a third of a positive charge on each nitrogen of the guanidine) and the inductive effect.  This is the ability for an atom or group to polarise the electrons in a bond.  Both oxygen and nitrogen are highly electronegative (electronegativities of 3.44 and 3.04 on the Pauling scale respectively,  for aficionados of such things!), and so both exert a strong inductive effect.  As a result, the nitrogen and oxygen atoms are negatively charged, the 'central' sp² carbon atom is strongly positively charged, as are the hydrogen atoms of the guanidinium group.  The charge distributions are very approximately:

a)  Carboylate:  Each oxygen -0.85, the 'central' carbon +0.85.

b)  The guanidinium:  The terminal nitrogens -0.9, the terminal hydrogens +0.45; The 'central' carbon +1; the e-nitrogen -0.7 and its hydrogen +0.4.

(These values are roughly rounded averages of the values within the AMBER94 and MMFF molecular mechanics forcefields)