How to modify just some nodes' styles in chemfig

Question

In the following molecular drawing, I want to only circle the rightmost OH of the first molecule and the leftmost H of the second, to show that they are the ones that are involved in the formation of water:

\documentclass{article}
\usepackage{chemfig} % for typesetting molecules

\begin{document}

\chemfig{CH(-[4]H_2N)(-[6]CH_2(-[6]C(=[5]O)(-[7]OH)))-C(=[2]O)-OH} % I want to circle this last OH
\chemsign{+}
\chemfig{CH(-[4]H_2N)(-[6]CH_2(-[6]*6(=-=-=-)))-C(=[1]O)(-[7]O-CH_3)} % I want to circle the H of the H_2N near the beginning
\chemrel{<>} \\
\chemfig{CH(-[4]H_2N)(-[6]CH_2(-[6]C(=[5]O)(-[7]OH)))-C(=[2]O)-N(-[2]H)-CH(-[6]CH_2(-[6]*6(=-=-=-)))-C(=[1]O)(-[7]O-CH_3)}
\chemsign{+}
\chemfig{H_2O} \\

\end{document}

I know that if I put

\setnodestyle{draw,circle}

on the very first line after \begin{document}, then I can make all the nodes (atoms) circled, but I only want to circle the two I mentioned, but I'm not having success inserting a \setnodestyle inside the \chemfig argument. Does anyone know how to custom modify just one node's style in chemfig?

Answer 1

Since you can use tikzpictures inside of tikzpictures you can for example define some \circleatom command and use it inside \chemfig:

\documentclass{article}
\usepackage{chemfig} % for typesetting molecules
\newcommand*\circleatom[1]{\tikz\node[draw,circle]{#1};}
\begin{document}

\chemfig{CH(-[4]H_2N)(-[6]CH_2(-[6]C(=[5]O)(-[7]OH)))-C(=[2]O)-\circleatom{OH}} % I want to circle this last OH
\chemsign{+}
\chemfig{CH(-[4]\circleatom{H}_2N)(-[6]CH_2(-[6]*6(=-=-=-)))-C(=[1]O)(-[7]O-CH_3)} % I want to circle the H of the H_2N near the beginning
\chemrel{<>} \\
\chemfig{CH(-[4]H_2N)(-[6]CH_2(-[6]C(=[5]O)(-[7]OH)))-C(=[2]O)-N(-[2]H)-CH(-[6]CH_2(-[6]*6(=-=-=-)))-C(=[1]O)(-[7]O-CH_3)}
\chemsign{+}
\chemfig{H_2O}

\end{document}

Edited: a better definition of \circleatom would be

\newcommand*\circleatom[1]{\tikz\node[circle,draw]{\printatom{#1}};}

The \printatom will ensure that that aroms are displayed the same if you change it to, say, have sans serif printed atoms.

BTW: I would probably use chemfig's scheming commands \schemestart, \arrow and \schemestop to typeset the reaction.

\documentclass{article}
\usepackage{chemfig}
\newcommand*\circleatom[1]{\tikz\node[circle,fill=green!30]{\printatom{#1}};}
\setatomsep{2em}
\setcompoundsep{7em}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\begin{document}

\schemestart
 \chemfig{CH(-[4]H_2N)(-[6]CH_2(-[6]C(=[5]O)(-[7]OH)))-C(=[2]O)-\circleatom{OH}} % I want to circle this last OH
 \+
 \chemfig{CH(-[4]\circleatom{H}_2N)(-[6]CH_2(-[6]*6(=-=-=-)))-C(=[1]O)(-[7]O-CH_3)} % I want to circle the H of the H_2N near the beginning
 \arrow{<=>}[-90]
 \chemfig{CH(-[4]H_2N)(-[6]CH_2(-[6]C(=[5]O)(-[7]OH)))-C(=[2]O)-N(-[2]H)-CH(-[6]CH_2(-[6]*6(=-=-=-)))-C(=[1]O)(-[7]O-CH_3)}
 \+
 \chemfig{\circleatom{H_2O}}
\schemestop
\end{document}