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I have the following reaction scheme in chemfig:

enter image description here

I need for some of the reversible reactions: forward reactions arrow in black and the backward reactions arrow in red. Is it possible in chemfig?

MWE

\documentclass{article}
\usepackage{chemfig}

\begin{document}

\schemestart
P \arrow(P--Q){<=>[$\mathrm{k_{pq}}$]}[,,,red] Q
\arrow{<=>[$\mathrm{k_{qs}}$]}[30,,,red] S \arrow(S--T){<=>[][$\mathrm{k_{st}}$]}[-30,,,red] T
\arrow(@Q--R){<=>[][$\mathrm{k_{qr}}$]}[-30,,,red] R
\arrow(@R--@T){<=>[][$\mathrm{k_{rt}}$]}[,,,red]
\arrow(@T--U){<=>[][$\mathrm{k_{tu}}$]}[,,,red] U
\arrow(@U--V){->[$\mathrm{k_{uv}}$]} V
\schemestop

\end{document}
Bernard
  • 271,350
nxkryptor
  • 1,478

1 Answers1

3

As requested in the comments, here is a possible solution, that does not overwrite the definition of <=>, but rather defines its own new arrow <=r>.

\documentclass{article}
\usepackage{chemfig}

\catcode\_=11 \definearrow3{&lt;=r&gt;}{% \CF_arrowshiftnodes{#3}% \path[allow upside down](\CF_arrowstartnode)--(\CF_arrowendnode)% node[pos=0,sloped,yshift=1pt](\CF_arrowstartnode u0){}% node[pos=0,sloped,yshift=-1pt](\CF_arrowstartnode d0){}% node[pos=1,sloped,yshift=1pt](\CF_arrowstartnode u1){}% node[pos=1,sloped,yshift=-1pt](\CF_arrowstartnode d1){};% \begingroup \pgfarrowharpoontrue \expandafter\draw\expandafter[\CF_arrowcurrentstyle](\CF_arrowstartnode u0)--(\CF_arrowstartnode u1);% \expandafter\draw\expandafter[\CF_arrowcurrentstyle,red](\CF_arrowstartnode d1)--(\CF_arrowstartnode d0);% \endgroup \CF_arrowdisplaylabel{#1}{0.5}+\CF_arrowstartnode{#2}{0.5}-\CF_arrowendnode% } \catcode_=8


\begin{document}


\schemestart
P \arrow(P--Q){<=r>[$\mathrm{k_{pq}}$]}[,,,] Q
\arrow{<=>[$\mathrm{k_{qs}}$]}[30,,,] S \arrow(S--T){<=>[][$\mathrm{k_{st}}$]}[-30,,,] T
\arrow(@Q--R){<=>[][$\mathrm{k_{qr}}$]}[-30,,,] R
\arrow(@R--@T){<=>[][$\mathrm{k_{rt}}$]}[,,,]
\arrow(@T--U){<=>[][$\mathrm{k_{tu}}$]}[,,,] U
\arrow(@U--V){->[$\mathrm{k_{uv}}$]} V
\schemestop


\end{document}

This results in the following output:

enter image description here

The following code applies the color change to all <=> arrows.

\documentclass{article}
\usepackage{chemfig}

\catcode\_=11 \definearrow3{&lt;=&gt;}{% \CF_arrowshiftnodes{#3}% \path[allow upside down](\CF_arrowstartnode)--(\CF_arrowendnode)% node[pos=0,sloped,yshift=1pt](\CF_arrowstartnode u0){}% node[pos=0,sloped,yshift=-1pt](\CF_arrowstartnode d0){}% node[pos=1,sloped,yshift=1pt](\CF_arrowstartnode u1){}% node[pos=1,sloped,yshift=-1pt](\CF_arrowstartnode d1){};% \begingroup \pgfarrowharpoontrue \expandafter\draw\expandafter[\CF_arrowcurrentstyle](\CF_arrowstartnode u0)--(\CF_arrowstartnode u1);% \expandafter\draw\expandafter[\CF_arrowcurrentstyle,red](\CF_arrowstartnode d1)--(\CF_arrowstartnode d0);% &lt;---------------modified line \endgroup \CF_arrowdisplaylabel{#1}{0.5}+\CF_arrowstartnode{#2}{0.5}-\CF_arrowendnode% } \catcode_=8


\begin{document}


\schemestart
P \arrow(P--Q){<=>[$\mathrm{k_{pq}}$]}[,,,] Q
\arrow{<=>[$\mathrm{k_{qs}}$]}[30,,,] S \arrow(S--T){<=>[][$\mathrm{k_{st}}$]}[-30,,,] T
\arrow(@Q--R){<=>[][$\mathrm{k_{qr}}$]}[-30,,,] R
\arrow(@R--@T){<=>[][$\mathrm{k_{rt}}$]}[,,,]
\arrow(@T--U){<=>[][$\mathrm{k_{tu}}$]}[,,,] U
\arrow(@U--V){->[$\mathrm{k_{uv}}$]} V
\schemestop


\end{document}

I have copied the definition of <=> from chemfig.tex and added ,red in the marked line in the code.

This results in the following output:

enter image description here

A previous version of these MWEs which work with older versions of the chemfig package (prior to v 1.4) can be accessed through the revision history of this post.

leandriis
  • 62,593
  • I had to modify the question. I need it for only some reactions; not all reactions. – nxkryptor Dec 11 '17 at 17:07
  • 1
    @nxkryptor: I have updated my answer according to your request. – leandriis Dec 11 '17 at 18:38
  • The color changes, however the harpoon arrows are missing and I get the following error:

    ! Undefined control sequence. <=r> ... @d1){};\begingroup \pgfarrowharpoontrue \expandafter \draw \expand... l.24 P \arrow(P--Q){<=r>[$\mathrm{k_{pq}}$]}[,,,]

     – nxkryptor Dec 12 '17 at 11:01
  • @nxkryptor: I have tried to reproduce the error you described by using the above MWE, but unfortunately I was not able to. Did you make sure, there are no typos in the code? – leandriis Dec 12 '17 at 21:54
  • I check for typos; everything looks fine. The interesting thing is if I comment \pgfarrowharpoontrue, the error no longer exists. However, the harpoon arrows are missing. – nxkryptor Dec 13 '17 at 07:10
  • I am compiling it on Ubuntu 14.04 LTS, just for information. I don't think it matters. I have checked for every possible source of error. I just can't figure out what is wrong. It just doesn't display the harpoon ends and gives 'Undefined control sequence.'. – nxkryptor Dec 17 '17 at 05:26
  • Do you encounter the error while compiling the MWE or with your actual document? I have again made some tests and, at least on my system, the MWE works fine with latex, pdflatex and xelatex, as well as with versions 1.2d and 1.2e of chemfig. – leandriis Dec 17 '17 at 20:27
  • The first MWE of your answer. – nxkryptor Dec 18 '17 at 04:00