[Tex/LaTex] Unequal equilibrium arrows with overhead text

Question

\documentclass[12pt]{article}
\usepackage[version=3]{mhchem}
\begin{document}
   \ce{CH2OH + ATP <=>>[\text{Hexokinase}] + \text{glucose 6-phosphate} + ADP + H+} \\
   \ce{CH2OH + ATP <=>> + \text{glucose 6-phosphate} + ADP + H+}
\end{document}

See the resulting Latex output, and compare. In [1] the difference between the two arrows of the unequal equilibrium sign becomes less perceptible because the sign has been stretched by the text. Compare to the same sign without the text [2]. I want the the differences between the upper and lower arrows to be exacerbated a bit more. How can I achieve this in LaTEX?

Answer 1

Another answer because I misunderstood your question. Digging around in the source code there are several macros which affect the length of the arrows.

From mhchem.sty we have

\newcommand*\mhchem@xlongrightshortleftharpoons[2][]{\mathrel{%
  \sbox{\mhchem@arrows@box}%
    {$\mkern9mu\ext@arrow 3399\mhchem@leftharpoondownfill@%
     {#1}{\hphantom{#2}\mkern-6mu\mkern-12mu}$}%
  \rlap{\protect\raisebox{-.22ex}{\usebox{\mhchem@arrows@box}}}%     
  \protect\raisebox{.22ex}{$\ext@arrow 3399\mhchem@rightharpoonupfill@%
    {\hphantom{\usebox{\mhchem@arrows@box}}\mkern-6mu}{#2}$}%
  }}

It's basically a front end to amsmath's \ext@arrow macro:

\def\ext@arrow#1#2#3#4#5#6#7{%
  \mathrel{\mathop{%
    \setbox\z@\hbox{#5\displaystyle}%
    \setbox\tw@\vbox{\m@th
      \hbox{$\scriptstyle\mkern#3mu{#6}\mkern#4mu$}%
      \hbox{$\scriptstyle\mkern#3mu{#7}\mkern#4mu$}%
      \copy\z@
    }%
    \hbox to\wd\tw@{\unhbox\z@}}%
  \limits
    \@ifnotempty{#7}{^{\if0#1\else\mkern#1mu\fi
                       #7\if0#2\else\mkern#2mu\fi}}%
    \@ifnotempty{#6}{_{\if0#1\else\mkern#1mu\fi
                       #6\if0#2\else\mkern#2mu\fi}}}%
}

So playing around with the arguments to \ext@arrow in lines 3-4 will affect the length of the lower arrow. I could not grok the \ext@arrow code enough to figure out how.

Instead I tried the option of drawing arrows with pgf. You have to put

\usepackage{tikz}
\mhchemoptions{arrows=pgf}

in the preamble. Then the relevant macro is:

\newcommand*\mhchem@longrightshortleftharpoons@pgf[1]{%
  \begin{tikzpicture}[baseline]%
    \draw[use as bounding box] (0,0) (#1,1ex);
    \draw[cap=round, join=round, line width=0.09ex]
        (0.0cm,0.575ex+0.22ex) -- ++(#1,0cm) arc (250:198:0.9ex);  
    \draw[cap=round, join=round, line width=0.09ex]
        (#1+0.5em-1em,0.575ex-0.22ex) -- (0.5em,0.575ex-0.22ex) arc (70:18:0.9ex);
  \end{tikzpicture}%
}% 

From this you can see that the bottom arrow is 1em shorter than the top one. To make it even shorter, change the last line to something like:

(#1-0.75em,0.575ex-0.22ex) -- (0.75em,0.575ex-0.22ex) arc (70:18:0.9ex);

This will make the bottom arrow 1.5em shorter.

To make this configurable, define a key and invoke it. Here's the complete example:

\documentclass[12pt]{minimal}
\usepackage[version=3]{mhchem}
\usepackage{tikz}
\mhchemoptions{arrows=pgf}

\makeatletter
\newlength{\mhchem@shortarrow@reduction}
\define@key{mhchem}{shortarrowreduction}{\setlength{\mhchem@shortarrow@reduction}{#1}}
\renewcommand*\mhchem@longrightshortleftharpoons@pgf[1]{%
  \begin{tikzpicture}[baseline]%
    \draw[use as bounding box] (0,0) (#1,1ex);
    \draw[cap=round, join=round, line width=0.09ex]
        (0.0cm,0.575ex+0.22ex) -- ++(#1,0cm) arc (250:198:0.9ex);  
    \draw[cap=round, join=round, line width=0.09ex]
       % was: (#1+0.5em-1em,0.575ex-0.22ex) -- (0.5em,0.575ex-0.22ex) arc (70:18:0.9ex);
        (#1-\the\mhchem@shortarrow@reduction,0.575ex-0.22ex) 
        -- (\the\mhchem@shortarrow@reduction,0.575ex-0.22ex) arc (70:18:0.9ex);
  \end{tikzpicture}%
}
\AtBeginDocument{\mhchemoptions{shortarrowreduction=0.5em}}
\makeatother


\begin{document}

\noindent%
   \ce{CH2OH + ATP <=>>[\text{Hexokinase}] + \text{glucose 6-phosphate} + ADP + H+} \\
   {\mhchemoptions{shortarrowreduction=2em}
   \ce{CH2OH + ATP <=>>[\text{Hexokinase}] + \text{glucose 6-phosphate} + ADP + H+}} \\
\end{document}