[peptideismylife]

Hi guys,

I have a chain in which at the end I have a methilene bond to azido and another molecule with a methylene bond to amino. In one spectra I show a multiplet and in the second one I show a triplet. I guess that the triplet correspond to methylene bond to azido and the multiplet correspond to the methylene bond to amino (due to coupling to the amino protons). Is that assumption right?

and what about the chemical shift? the methylene protons next to the azido group could appear more deshielded compare to the methylene protons next to the amino?

[uglepik]

In my experience the protons sitting next to a primary amine do not couple with the protons of the amine and therefore would only give you a triplet (or a broad singlet) in this case, not a multiplet. I suppose this could depend on the solvent you're using.

I suppose the protons next to the amine would have a higher chemical shift than the ones next to the azide. Protons next to amines are usually around 2.5-3.0. Azides I don't know, but due to the resonance stabilization of the azide I would expect it to pull less and thus have a lower shift.

As you seem to have experience with peptides I have a question for you too :-)

Do you usually see the protons sitting on the nitrogens of the amide groups? I am running in CDCl3 (sometimes with CD3COOD added) and I just can't find them. What's your experience?

[peptideismylife]

try with DMSO! I found them always with DMSO...

[peptideismylife]

But I was talking about the methylene protons next to the amine or next to the azido. I would say if you do the NMR in DMSO u get a multiplet because the protons of methylene could coupled with the protons of amine...is that right?

[Dan]

In terms of chemical shift, if you are comparing similar environments you'd expect the protons next to the azide to be further upfield (lower shift).

Without knowing a bit more about the rest of the structure it's difficult to answer the question. It would be useful to know:

For RCH₂NH₂ and RCH₂N₃

What is R? At least show all the protons that could be coupling to your CH₂s if you are not in a position to divulge the entire structures.

Try to work out the pattern of the "multiplet", perhaps it's a dd, ddd, dt, etc. or even two overlapping shifts (check HMQC or HSQC spectrum).

Can you see the NH peak? Does it couple to the multiplet or the triplet in the COSY spectrum?

[peptideismylife]

Thanks Dan!

Right now I am waiting for the COSY...waiting for more information...

Regarding to the structure, it's an aminoacid:

Fmoc-Lys-OH and Fmoc-Lys(N3)OH

I am doing with DMSO....

in which chemical shift appears amino protons?

[Honclbrif]

If you've got an IR, its a great way to find azides. They have a pretty unique absorbance.

[Dan]

Regarding to the structure, it's an aminoacid:

Fmoc-Lys-OH and Fmoc-Lys(N3)OH

Ah OK, are we talking about two separate pure samples or a single mixture of two amino acids?

In the case of the former scenario, definitely IR.

NH signals can move around or be so broadened you can't see them. It's difficult to say where they'll be - but HMQC or HSQC will tell you (as they will not correlate to a C signal).

Also, the NMR data for these compounds must have been published before - you can compare your results to the literature.