def molecularMass_from_I0(I0, c, density=1.347):
    """ calculate the molecular mass in Da

        based upon:
        D Orthaber, A Bergmann and O Glatter
        J. Appl. Cryst. (2000). 33, 218-225


        Paramters
        ---------
        I0 : float
            extrapolated value (in cm-1)
        c : float
            concentration (in g/ml)
        density : float
            protein density (in g/ml)
    """
    delta_rho = 26700000000.0
    delta_rho_M = delta_rho / density
    Navo = 6e+23
    M = I0 * Navo / (c * delta_rho_M ** 2)
    return M
