I have been trying to resolve this for days and now I surrender. I need a table that can span multiple pages, so I started using longtable instead of tabularx. I tried to do a fixed width column (the first one) but I still get it too wide. when I try to shrink it to 3 or 4cm, it doesn't work.
\section{Main structural statistical parameters for the conformers calculated}
\begin{landscape}
\setlength\LTleft{-2cm}
\setlength\LTright{-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}{\footnotesize\itshape 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
If you can help me to find what's happening here, I would be very thankful.
r{4cm}
doesn't exist. probably you looking for>{\raggedleft\arraybacslash}p{4cm}
. for numbers are advisable useS
columns fromsiunitx
package ...