section{Main structural statistical parameters for the conformers calculated}
begin{landscape}
setlengthLTleft{-2cm}
setlengthLTright{-4cm}
     begin{longtable}{r{4cm} |c|c|c|c|c|}
caption[Chemical shift list of PG-T4 in ce{H2O}, pH 3.5] {Main structural statistical parameters for the ensemble of the 20 lowest target function conformers calculated for PG-T4, PG-C6C15, PG-C8C13 in ce{H2O} and 30 mM DPC, pH 3.5}\
toprule
 &
multicolumn{3}{c}{textbf{ce{H2O}}}   &
multicolumn{3}{c}{textbf{30 mM DPC}}   \
cmidrule(lr){2-4}
cmidrule(lr){5-7}
          &
        textbf{PG-T4} &
        textbf{PG-C6C15} &
        textbf{PG-C8C13} &
        textbf{PG-T4} &
        textbf{PG-C6C15} &
        textbf{PG-C8C13}
\

endfirsthead
toprule
         & &
        textbf{PG-T4}  &
        textbf{PG-C6C15}  &
        textbf{PG-C8C13}&
        textbf{PG-T4}  &
        textbf{PG-C6C15}  &
        textbf{PG-C8C13}
        \
midrule
endhead
endhead
bottomrule
multicolumn{7}{r}{footnotesizeitshape continue on the next page} % <-- added
endfoot
endlastfoot

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Upper Limit distance restraints (from NOEs)             &       &       &       &       &       &       \
        tab Total  &   numbers &   numbers &   numbers &   numbers &   numbers &   numbers \
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    bottomrule
end{longtable}
end{landscape}
clearpage

documentclass{article}
usepackage{lscape}
usepackage{longtable}
usepackage{array}
usepackage{mhchem}
usepackage{booktabs}
deftab{}
begin{document}
section{Main structural statistical parameters for the conformers calculated}
begin{landscape}
setlengthLTleft{-2cm}
setlengthLTright{-4cm}
     begin{longtable}{p{4cm} cccccc}
caption[Chemical shift list of PG-T4 in ce{H2O}, pH 3.5] {Main structural statistical parameters for the ensemble of the 20 lowest target function conformers calculated for PG-T4, PG-C6C15, PG-C8C13 in ce{H2O} and 30 mM DPC, pH 3.5}\
toprule
 &
multicolumn{3}{c}{textbf{ce{H2O}}}   &
multicolumn{3}{c}{textbf{30 mM DPC}}   \
cmidrule(lr){2-4}
cmidrule(lr){5-7}
          &
        textbf{PG-T4} &
        textbf{PG-C6C15} &
        textbf{PG-C8C13} &
        textbf{PG-T4} &
        textbf{PG-C6C15} &
        textbf{PG-C8C13}
\

endfirsthead
toprule
         &
        textbf{PG-T4}  &
        textbf{PG-C6C15}  &
        textbf{PG-C8C13}&
        textbf{PG-T4}  &
        textbf{PG-C6C15}  &
        textbf{PG-C8C13}\
midrule
endhead
endhead
bottomrule
multicolumn{7}{r}{footnotesizeitshape continue on the next page} % <-- added
endfoot
endlastfoot
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Upper Limit distance restraints (from NOEs)             &       &       &       &       &       &       \
        tab Total  &   numbers &   numbers &   numbers &   numbers &   numbers &   numbers \
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    bottomrule
end{longtable}
end{landscape}
clearpage
end{document}