[semor]

Hi !

I have a quite general question about the value used for the CSA while studying protein dynamics of 15N-1H vectors with model-free approach.

In the litterature, we mainly find two values for the CSA (-160 and -172 ppm).

There is, if I understand well, a link between the bond length and the CSA, but everyone seems to agree about using the same value of 1.02 A which should give rise to a mean S2 of 0.85 for secondary structure when combined to a CSA of -172 ppm. When using a CSA of -160 ppm, the mean S2 for secondary structure should slightly rise up from 0.85.

The manuals for several programs as relax (d'Auvergne) and Model-free (Palmer) suggest a value of -160 ppm for CSA (combined with a bond length of 1.02 A).

However, some recent papers (Biochemistry, 45, 11414-11424, 2006 ; JACS, 127, 3220-3229, 2005) suggest using a value of -172 ppm.

Also, what about the CSA variability from one vector to another (JACS, 128, 7855-7870, 2006) ?

What is the best (site specfific or not) value to use and, most importantly, why ?

Thanks !


Sébastien