You and others with interests in 3D RNA drawings may want to look
at the Macintosh RNA 3D toolkit that Jim Brown here put together
for his work. These are a selection of RNA building blocks
drawn with the Mac application Swivel 3D, which is a general Mac
3D drawing program.
You ask for "tetraloops, stacks, helices, bends, pseudoknots" and
I seem to recall that this collection includes those.
Here is the brief description of these drawing tools:
3D RNA toolkit is a "toolkit" of RNA pencil-type structures in
Swivel 3D format. These files can be used to construct 3D models
of RNAs either from known structures (e.g. tRNA) or to test potential
from James W. Brown, jwbrown at bio.indiana.edu
You can pick these up by anonymous ftp to ftp.bio.indiana.edu,
currently in folder Incoming/ as rna-3d-tools.hqx ( a 1.9 megabyte file).
Don Gilbert gilbert at bio.indiana.edu
biocomputing office, biology dept., indiana univ., bloomington, in 47405