How to read the "SEQRES" section from a PDB file, using R
I have not used Rpdb but I have used, and would recommend, bio3d.

To read the PDB file 1A4G:

library(bio3d)
pdb1a4g &lt;- read.pdb("1a4g")


To get the SEQRES information, there are two options. Get it from the pdb object directly:

seq1a4g &lt;- pdb1a4g$seqres


Or call pdbseq:

seq1a4g &lt;- pdbseq(pdb1a4g)


Using the second method, the 17th position (note this is not the same as the 17th position of the original protein sequence):

seq1a4g[17]
 92 
"Q"