Backbone torsional angles are a characteristic and useful parameter for the
description and characterisation of protein structures determined by x-ray
crystallography or NMR spectroscopy. For the comparison of an ensemble of
three-dimensional structures the calculation of the statistical parameters
mean and standard deviation would be very useful. However, they are not def
ined unambiguously for periodic quantities such as the dihedral angles. In
this paper a plausible and unique definition of these parameters is introdu
ced and a straightforward method for their calculation is given. (C) 1999 A
cademic Press.