Modelling consecutive chemical reaction (ode45, lsqcurvefit)

Hello community

I want to model some experimental data with the ode's of the unimolecular consecutive chemical reaction to get the values of the kinetic konstants (k). The odes are:

I have experimental data to fit for I. To do so, I adopted the code from Star Strider in this post to my needs:

ydata = [1;5;5;2.5;3.96;2.37]; % Experimental data for I
xdata = [0;49.8000000000000;100.200000000000;150;199.800000000000;250.200000000000]; % time

B0 = [0.2 0.8];
lb = [0 0]
[B,Rsdnrm,Rsd,ExFlg,OptmInfo,Lmda,Jmat] = lsqcurvefit(@MonodKinetics2,B0,xdata,ydata,lb);

% Plot
a= MonodKinetics2(B, xdata)

plot(xdata, ydata,'bo')
hold on
plot(xdata,a,'ro')
hold off

function S = MonodKinetics2(B, t);
% k1 = B(1), k2 =(B2)
% A = x(1), I = x(2), P = x(3)

 x0 = [1 1 0];
[T,Sv] = ode45(@DifEq, t, x0);
      function dS = DifEq(t, x);
          xdot = zeros(2,1);
          xdot(1) = -B(1) .* x(1); 
          xdot(2) = B(1) .* x(1) - B(2) .* x(2); 
          xdot(3) = B(2) .* x(2);
          dS = xdot;
      end
S = Sv(:,1);
end

It is obvious that something is wrong, because the calculated values for I are always 1 and for the k's nearly 0. Can somebody help me?

[Torsten]

If you have data for I, you'll have to set S = Sv(:,2) instead of S = Sv(:,1).

[Marco Knobloch]

Oh that's it!! It is more obvious than I thought. Thank you Torsten