For the amino acid Lysine, the nitrogen with the H attached to it is not basic (since you can draw out the resonance for it). But the other nitrogen that has a double bond is not basic. Why is this?? I can draw out the resonance for the double bonded nitrogen as well.. so shouldnt it be considered nonbasic as well. can someone please explain this to me? Thanks

To understand why the nitrogen atom with the double bond in lysine is not basic, let's first review the concept of basicity and resonance.

In organic chemistry, basicity refers to the ability of a molecule or functional group to accept a proton (H+). A basic nitrogen atom, often referred to as a basic site, usually has a lone pair of electrons readily available to accept a proton. In the case of lysine, there are two nitrogen atoms: one with a hydrogen atom attached (NH2) and another with a double bond to a carbon atom (C=N).

Now, let's analyze why the NH2 nitrogen of lysine is basic. This nitrogen has a lone pair of electrons that can accept a proton, making it a suitable site for basic reactions.

Next, let's consider the nitrogen with the double bond (C=N) in lysine. While it is possible to draw resonance structures that involve the double bond, these resonance structures don't accurately represent the true nature of the molecule. Resonance structures are simply different ways to depict the movement of electrons within a molecule, and they contribute to the overall stability of the molecule.

In the case of lysine, the double-bonded nitrogen does have resonance forms that can be drawn. However, it's important to note that resonance structures do not imply that the atoms are constantly shifting or moving between different forms. Rather, they represent a distribution of electron density within the molecule.

The double bond in lysine is localized and does not have a considerable contribution from the nitrogen lone pair. The presence of the double bond decreases the availability of the lone pair on nitrogen for basic reactions. Consequently, the nitrogen atom with the double bond is not as basic as the NH2 nitrogen.

To summarize, the amino acid lysine has two nitrogen atoms: one with a hydrogen atom (NH2) and another with a double bond to a carbon atom (C=N). The NH2 nitrogen is basic because it has a readily available lone pair of electrons to accept a proton. On the other hand, the nitrogen with the double bond is not as basic since the double bond limits the availability of its lone pair for proton acceptance. The resonance structures associated with the double bond in lysine represent electron distribution, rather than indicating the constant switching between forms.