[ajax0604]

I understand why arginine is a basic amino acid but I am not sure where the proton would attach. I have looked at various resources and some show -NH₂⁺C(NH)NH₂ whereas others show --NHC(NH)NH₃⁺. Are both acceptable? Also, could a second proton attach on to the structure. If not, is there a reason? Thank you.

[Babcock_Hall]

The second structure is definitely wrong.  As a general rule it usually helps to draw out the structure and look for resonance forms.  However, there is another issue of which you should be aware.  Many tables of the amino acids show the side chains as they appear at or near pH 7.  For example, aspartic acid has an acidic side chain, -CH₂-C(O)OH, but at pH 7, the predominant form of the side chain is -CH₂-C(O)O^1-.  With arginine we have the opposite problem.  The basic form of the side chain has no charge, but its conjugate acid has a positive charge.  Why don't you draw the neutral form?

[ajax0604]

In our chemistry course, we have to be able to draw the structure of an amino acid in acidic and basic conditions. Could arginine accept another proton or is it limited to just one?

[Babcock_Hall]

At sufficiently low pH I imagine it could accept another proton, but doubt that it is physiologically relevant.  I may have misunderstood your original question.  Were your two initial drawings intended to represent the conjugate acid form of arginine?  One other general piece of advice with respect to drawing the amino acids.  For me it has always been helpful to draw out the lone pair on nitrogen, when there is one.

[Dan]

I have looked at various resources and some show -NH₂⁺C(NH)NH₂ whereas others show --NHC(NH)NH₃⁺. Are both acceptable?

Actually both are wrong, or at least do not show the major tautomer of the guanidinium cation.

Generally, guanidines protonate at the imine N to give a highly resonance stabilised cation.

See for example: https://en.wikipedia.org/?title=Guanidine#Guanidinium_cation

[ajax0604]

Sorry Babcock-Hall I didn't make myself clear. In our chemistry course, we have to take an amino acid in its zwitterion form and draw the resulting structure if it were placed in acidic or basic conditions. I was trying to draw the structure of arginine in acidic conditions - as you said, its conjugate acid form. Since there are three nitrogens in the Z group on arginine, I wasn't sure where to put the proton. Dan, you mentioned the guanidinium cation. Would the structure second from the right be correct in the image below?

[mjc123]

Your second and fourth structures look wrong. I think the second should have HN=, not H₂N=; and the fourth should have H₂N-, not H₂N⁺-

[Babcock_Hall]

mjc123, I disagree about the fourth structure because I prefer option c, which is not yet shown.  At pH ≈ 8, we are below pK₂, the pK_a of the alpha-ammonium group. 

@ OP, I seem to notice that your structures sometimes lose a proton but not a charge, as in going from the third structure to the fourth, or from the fourth structure to the first.  I would look into formal charge and how it relates to the common Lewis structures for nitrogen.  That is why I recommend writing out the lone pairs.  I also think it would help to arrange the structures to conform to the titration curve.  In other words, structure 3 should move to become the leftmost structure.

[mjc123]

I take your point; I simply meant that R-NH₂ should not have a + charge. Either -NH₃⁺ or -NH₂.