[Q] Sign convention for torsion angles

[Maria Ortner]

Hi!
I have a question concerning the definiton of the sign of torsion angles
in peptides. I know of the IUPAC-IUB definition, which is e.g. 
decribed at URL:http://www.chem.qmw.ac.uk/iupac2/misc/ppep1.html#160.

I heard of a so-called 'mathematical' definiton 
for the sign of torsion angles, which is said to be opposite to 
the 'chemical' one. Unfortunately now I cannot find any literature 
which explains the mathematical definition, nor something where 
comparison is done. 

So I ask if anybody can give me hints to the following 
questions:
How is the sign of a torsion angle defined mathematically?
Is the term 'chemical convention' identic to the IUPAC-IUB nomenclature?
Has there been an older chemical convention, which is opposite from
the one described in IUPAC-IUB?

Thanks for any help,
Maria
-- 
--------------------------------------------------
Maria Ortner                            
Center of Applied Molecular Engineering              
University of Salzburg, Austria         
Maria.Ortner at came.sbg.ac.at