Vertical alignment of molecules

Question

\begin{scheme}[h]
\centering
 %\setcrambond{3pt}{1pt}{1pt}
%\setbondoffset{3pt}
 \scriptsize\chemfig{(-[::60]R_1)(=[::180]O)-[::-60]O-[::-60]-[::60](-[::60]O-[::-60](-     [::60]R_2)(=[::-60]O))-[::-60]-[::60]O-[::-60](=[::60]O)(-[::-60]R_3)} + H_2O
 \chemrel{<->}
 \scriptsize\chemfig{(-[::60]R_1)(=[::180]O)-[::-60]O}
 \chemsign{+}
 \scriptsize\chemfig{(-[::60]R_1)(=[::180]O)-[::-60]O}
 \chemsign{+}
 \scriptsize\chemfig{(-[::60]R_1)(=[::180]O)-[::-60]O}
 \chemsign{+}
 \scriptsize\chemfig{(-[::60]R_1)(=[::180]O)-[::-60]O}
 \caption{reaction}
 \label{sn:reaction}
 \end{scheme}

How can I achieve that vertical alignment? I looked through the manual however there seemed no special command to mark the branch or molecule which everything else should align at. Also, why is the O italic?

I saw a solution with invisible arrows but it seems like it doesn't work here. \arrow{0}[,0] will result in [,0] being written before my equation.

Answer 1

I don't how how if this is the right way, to mark my questions as answered. however I found the solutionen. As mentionend above I already came across \arrow{0}[,0] the trick. I had to use:

\begin{scheme}
\schemestart
[...]
\schemestop
\end{scheme}

Original Question: here