Building a PDB file from amino acid sequence of non-folded structure
So to be clear, it sounds like you want the 'coil' or unfolded structure of a protein based on the sequence?

There are plenty of programs out there to do homology modelling, which is taking a sequence of unknown structure and modelling it onto one with a known structure. On the other hand, there are many libraries for analyzing existing structures. What you want is somehow in between these two.

I'll add some links here as I find them:


There is a Lua library that might be useful : https://github.com/rob-miller/rFold
Modeller has functions for this: https://salilab.org/modeller especially this page
The Structural Bioinformatics Library : http://sbl.inria.fr/doc/index.html
The Rosetta library (as used in fold.it): https://www.rosettacommons.org/


I'm sure there are more, but it really depends on what programming language you want to use, or if you are implementing stuff yourself versus running some standalone software.