Chemical reaction scheme with loop

Question

I'm trying to produce the scheme in the figure on latex, so far my attempts are abortive. I've added here what I have so far using the chemfig package:

\schemestart
 A \arrow{->[$k_{on}$]} 
 B \arrow{->[$k_{c}$]}
 C
  \arrow(C---){->[$k_{d}$]}[30,1.75] D
 \arrow(@C---){->[$k_{d}^{'}$]}[-30,1.75] E
 \arrow{<=>[*{0} $k_{a}^{}$][*{0} $k_{a}^{'}$]}[90,1.85]
\schemestop

Answer 1

You can define a new arrow type as explained in section 11 of part IV of the chemfig manual:

\documentclass{article}
\usepackage{chemfig}

\makeatletter
\definearrow{3}{-|>}{%
  \CF@arrow@shift@nodes{#3}%
  \expandafter\draw\expandafter[\CF@arrow@current@style,-CF]
    (\CF@arrow@start@node) -| (\CF@arrow@end@node)
    node[midway] (arrow@middle){} ;%
  \CF@arrow@display@label
    {#1}{0.5}{+}{\CF@arrow@start@node}
    {#2}{0.5}{-}{arrow@middle}%
}
\makeatother

\begin{document}

% \schemedebug{true}
\schemestart
  A \arrow(A--){->[$k_{on}$]} 
  B \arrow{->[$k_{c}$]}
  C \arrow(C--D){->[$k_{d}$]}[30,1.75] D
  \arrow(@C--E){->[$k_{d}^{'}$]}[-30,1.75] E
  \arrow(@E--@D){<=>[*{0} $k_{a}^{}$][*{0} $k_{a}^{'}$]}
  \arrow(@E--@A){-|>}
  \arrow(@D--@A){-|>}
\schemestop

\end{document}

For reasons I don't understand the arrows don't quite end at node A but a little bit beside it. And if one adds an anchor (\arrow(@[email protected]){-|>}) the node gets shifted instead of the arrow tip. I suspect that there's a bug when an arrow connects two nodes with the (@node1--@node2) syntax but I'm not sure… probably a question for Christian, the author of chemfig.