For many years it had been believed that steric compatibility of helix
interfaces could be the source of the observed preference for particular angles
between neighbouring helices as emerging from statistical analysis of protein
databanks. Several elegant models describing how side chains on helices can
interdigitate without steric clashes were able to account quite reasonably for
the observed distributions. However, it was later recognized (Bowie, 1997 and
Walther, 1998) that the ``bare'' measured angle distribution should be
corrected to avoid statistical bias. Disappointingly, the rescaled
distributions dramatically lost their similarity with theoretical predictions
casting many doubts on the validity of the geometrical assumptions and models.
In this report we elucidate a few points concerning the proper choice of the
random reference distribution. In particular we show the existence of crucial
corrections due to the correct implementation of the approach used to
discriminate whether two helices are in contact or not and to measure their
relative orientations. By using this new rescaling, the ``true'' packing angle
preferences are well described, even more than with the original ``bare''
distribution, by regular packing models.Comment: 23 pages, 5 figure
