text: user704: I was wondering how I can calculate the charge of a protein peptide (e.g. "RKTTLVPNTQTASPR") computationally in R or another tool.
  
  user929: I've used PropKa in the past to get isoelectric points. Pretty simple to use:
  https://github.com/jensengroup/propka-3.1
  
  user77: A quick google search turns up protcalc, which is able to give a nice pH-dependent table of peptide charges (yours ranges from 3.1 at pH 4 to 1.5 at pH 10). It's on sourceforge, so hopefully the source code (and maybe a publication) is somewhere in there. Granted, this isn't R, but it's not clear from your post how necessary that really is.
  BTW, in general "best and most accurate" turns out to be highly subjective (at least the "best" part of that).
  
  user150: Another quick Google search reveals the Peptides R package.
  library(Peptides)
  s <- "RKTTLVPNTQTASPR"
  charge(s)
  
  [1] 2.997683
  
  Optional arguments to charge() are pH (default = 7) and pKscale (choice of nine, default = "Lehninger"). See ?charge for details.
  Another (non-R) option is the EMBOSS suite program iep. I highly recommend installing EMBOSS, it provides a suite of useful command-line tools with output that can be easily read and processed by other sofware, including R.
  
original_text: 704: <p>I was wondering how I can calculate the charge of a protein peptide (e.g. "RKTTLVPNTQTASPR") computationally in R or another tool.</p>
   929: <p>I've used PropKa in the past to get isoelectric points. Pretty simple to use:</p>
  
  <p><a href="https://github.com/jensengroup/propka-3.1" rel="nofollow noreferrer">https://github.com/jensengroup/propka-3.1</a></p>
   77: <p>A quick google search turns up <a href="http://protcalc.sourceforge.net/" rel="nofollow noreferrer">protcalc</a>, which is able to give a nice pH-dependent table of peptide charges (yours ranges from 3.1 at pH 4 to 1.5 at pH 10). It's on sourceforge, so hopefully the source code (and maybe a publication) is somewhere in there. Granted, this isn't R, but it's not clear from your post how necessary that really is.</p>
  
  <p>BTW, in general "best and most accurate" turns out to be highly subjective (at least the "best" part of that).</p>
   150: <p>Another quick Google search reveals the <a href="https://cran.r-project.org/web/packages/Peptides/index.html" rel="nofollow noreferrer">Peptides R package</a>.</p>
  
  <pre><code>library(Peptides)
  s &lt;- "RKTTLVPNTQTASPR"
  charge(s)
  
  [1] 2.997683
  </code></pre>
  
  <p>Optional arguments to <code>charge()</code> are <code>pH</code> (default = 7) and <code>pKscale</code> (choice of nine, default = "Lehninger"). See <code>?charge</code> for details.</p>
  
  <p>Another (non-R) option is the <a href="http://emboss.sourceforge.net/apps/release/6.6/emboss/apps/iep.html" rel="nofollow noreferrer">EMBOSS suite program iep</a>. I highly recommend installing EMBOSS, it provides a suite of useful command-line tools with output that can be easily read and processed by other sofware, including R.</p>
  