Calculates the partial charge of the amino acid.

    Parameters
    ----------
    aa : str
        Amino acid single-letter code.
    pH : float
        pH of interest.

def partial_charge(aa, pH):
    """Calculates the partial charge of the amino acid.

    Parameters
    ----------
    aa : str
        Amino acid single-letter code.
    pH : float
        pH of interest.
    """
    difference = pH - residue_pka[aa]
    if residue_charge[aa] > 0:
        difference *= -1
    ratio = (10 ** difference) / (1 + 10 ** difference)
    return ratio