How to improve curve fit for data with multiphasic exponential equation?

I'm working on a project and I need to fit some data to the equation y = bmax_1 * np.exp(-koff_1 * x) + bmax_2 * np.exp(-koff_2*x) with bmax_1, koff_1, bmax_2, koff_2 being the parameters. I have tried using curve_fit, but the result is quite poor and give an R-squared value of 0.16. I'm wondering if there is something I can do to improve the fit?


    #Define model function
    def func(x, bmax_1, koff_1, bmax_2, koff_2):
        return bmax_1*np.exp(-koff_1*x) + bmax_2 * np.exp(-koff_2 *x)
    
    # function for genetic algorithm to minimize (sum of squared error)
    def sumOfSquaredError(parameterTuple):
        warnings.filterwarnings("ignore") # do not print warnings by genetic algorithm
        val = func(xData, *parameterTuple)
        return np.sum((yData - val) ** 2.0)
    
    def generate_Initial_Parameters():
        parameterBounds = []
        parameterBounds.append([0.0, 200.0]) # search bounds for bamx_1
        parameterBounds.append([0.0, 10.0]) # search bounds for koff_1
        parameterBounds.append([0.0, 200.0]) # search bounds for bmax_2
        parameterBounds.append([0.0, 10.0]) # search bounds for koff_2
        # "seed" the numpy random number generator for repeatable results
        result = differential_evolution(sumOfSquaredError, parameterBounds, seed=4)
        return result.x

    column = input('Column to be analysed: ')
    xData = df.loc[:, 'T']
    yData = df.loc[:, column]
    geneticParameters = generate_Initial_Parameters()
    
    fittedParameters, pcov = curve_fit(func, xData, yData, geneticParameters, bounds = )
    print('Fitted parameters:', fittedParameters)

The fitted parameters are [1.24066146e+02 1.48240328e-02 1.34805335e+01 8.26108828e-01]

Result of the fit:
Result for fitting



Read more here: https://stackoverflow.com/questions/64945709/how-to-improve-curve-fit-for-data-with-multiphasic-exponential-equation

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