A recent discussion amongst colleagues brought this question up and I hope some of you can help me here. We have been taught for a long time that Arginine as an amino acid is positively charged. A recent discussion ( mostly heated) involved role of an arginine amino in an active site of an enzyme. Most of the people agree that arginine is positively charged but disagreed on its role in electrostatic and hydrogen bond interactions. Because of the conjugation between the double bond and the nitrogen lone pairs in the side chain, the positive charge is delocalized.This means that the electrons are not associated to one atom but are free to move around...Now here lies the confusion.. Is it a golden rule that delocalized electrons cannot participate in eletrostatic interactions? How does this rule affect arginine? If the positive charge cannot play its role as a "charge" then how does the molecular interaction work?
Hi there,
The case for the guanidinium of arginine is similar to the case of carboxylate in acidic aa (possibility of charge delocalization). They are both able to participate to salt bridge interaction which is actually made of both electrostatic interaction and hydrogen bonding contributions. Indeed global charge of arginine is delocalized between the 3 nitrogens of the guanidinium group but electrostatic interaction may actually contribute to stabilize one of its resonance forms.
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