Analysis is trying to keep your assignments IUPAC compliant - but that's a little complicated if you have both methyl Hs and Cs assigned. The IUPAC rule says that the "a" designation should be driven (for the resonance with the lowest ppm shift) by the H shift but by the C shift if there's no H assigned. So it's likely that Cga/b designation will switch depending on whether the resonance is connected to an attached methyl H or not. In older versions of analysis, the assignments could end up confused, but that is rarer now.
When I try assigning C resonances (new resonances, not connected to Hs) in isolation in an up to date analysis 2.4 the behaviour is as expected - i.e. if I assign the lower ppm as b and the higher as a, analysis will switch the designations. The fact that you are seeing Cga switch immediately to Cgb suggests that there is already another Cg resonance, or that your Cg resonance is connected to an H resonance that is driving the switch.
To minimise confusion it's good to assign the lower ppm shift first in each case (to "a") and to take care when making the first peak assignment that explicitly connects a particular methyl H to a particular methyl C resonance.
Carsten's suggestion is good one for sorting the problem out once you have confused prochiral methyl assignments. Just trying to reassign one or other of the already assigned resonances is a recipe for frustration!
Dr. Brian O. Smith --------------------------- Brian Smith at glasgow ac uk
Institute of Molecular, Cell and Systems Biology & School of Life Sciences,
College of Medical, Veterinary & Life Sciences,
Joseph Black Building, University of Glasgow, Glasgow G12 8QQ, UK.
Tel: 0141 330 5167/6459/3089
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The University of Glasgow, charity number SC004401
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From: CcpNmr software mailing list [[log in to unmask]] on behalf of Carsten Krichel [[log in to unmask]]
Sent: 27 March 2015 10:14
To: [log in to unmask]
Subject: Re: Analysis
Hi Guillaume,
I once had the same problem for Val/Leu and circumvented it by deleting
the gamma resonances, creating new ones, linking them to the gamma /
delta, and then adding the resonances once again in the resonances ->
spin system dialog to the respective amino acid. This 'solution' is a
bit ugly, but it worked ;)
Carsten
On 27/03/15 10:12, Guillaume Nars wrote:
> Hello,
>
> I have a problem to assign methyl on hCCcoNH for Valine and Leucine.
> When I assign the first methyl to Cga, analysis change directly the name to Cgb.
> So I assign the second methyl to Cga, but analysis change again the name to Cgb.
> As result, all my Valine and Leucine have two Cgb and no Cga and are reported as resonance assignment error on the Quality report.
>
> Do you have any suggestion ? Am I missing something ?
>
> Thanks a lot
>
> Guillaume
>
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