\documentclass%
% [landscape]
{article}

\usepackage[utf8]{inputenc}%utf8
% OR
% \usepackage[latin1]{inputenc}
\usepackage[T1]{fontenc}%pour taper directement les accents
\usepackage[frenchb]{babel}% typographie francaise

\usepackage{geometry}
\geometry{a4paper,%twoside,
left=2.0cm,right=2.5cm,marginparwidth=1.5cm,%
  marginparsep=3mm,top=2cm,bottom=2cm}

\usepackage{chemfig}
\usepackage[babel=true,kerning=true]{microtype}%pour eviter le clash entre 

\begin{document}

\setbondstyle{semithick} % make bonds thicker than the default
\setdoublesep{2pt} % seperate double bonds a little bit more
\setcrambond{3pt}{0.5pt}{1pt} % not needed here
\setatomsep{1.6em} % make the default bond length shorter
\setarrowoffset{1em} % this is the distance between arrow and compound
\setcompoundsep{6em} % this is the (new default) distance between compounds
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}} % we want sans serif printed atoms
\begin{minipage}{.9\linewidth}
\chemname{\chemfig{H_2N-R-C(-[:-30]OH)=[:30]O}}{Acide aminé 1}
\chemsign{+}
\chemname{\chemfig{H_2N-R'-C(-[:-30]OH)=[:30]O}}{Acide aminé 2}
\chemrel{->}
\chemname{\chemfig{H_2N-R-C(=[:30]O)(-[:-30] N(-[:-90]H)-R'-C(-[:-30]OH)=[:30]O)}}{Peptide}
\chemsign{+}
\chemname{\chemfig{H_2O}}{Eau}
\end{minipage}

\end{document}