Wie definert man einen neuen Tikz nodestyle zur Verwendung in chemfig ?

Wie im Beispiel möchte ich eine neudefinierten node verwenden. Wie müßte der Code denn lauten um eine schwarze, umrandende Kreisline zusätzlich zur grünen Füllfarbe zu bekommen ?

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\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}
 
 
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