code: def partial_charge(aa, pH):
      
      difference = pH - residue_pka[aa]
      if residue_charge[aa] > 0:
          difference *= -1
      ratio = (10 ** difference) / (1 + 10 ** difference)
      return ratio
docs: Calculates the partial charge of the amino acid.
  
      Parameters
      ----------
      aa : str
          Amino acid single-letter code.
      pH : float
          pH of interest.