Undesired spaces in newenvironment break commands

I created a new environment to change the behavior of the toc for appendices. The commands work fine when using them outside newenvironment, but not within newenvironment.

The root cause seems the line addtocontents{toc}{letprotectl@chapterprotectl@section}: Outside the environment, the corresponding line in MWE.toc is let l@chapter l@section while the same command within the new environment leads to the toc line let l @chapterl @section (please notice the spaces before the @ symbols).

How can I get rid of the space before the @ sign?

MWE:

documentclass{scrbook} 
usepackage{appendix}

newenvironment{myappendices}{
makeatletter
addappheadtotoc
addtocontents{toc}{begingroup}
addtocontents{toc}{letprotectl@chapterprotectl@section}
makeatother}{
addtocontents{toc}{endgroup}
}

flushbottom
begin{document}

tableofcontents

mainmatter
chapter{One}
chapter{Two}

appendix

% version without newenvironment: works
makeatletter
addappheadtotoc
addtocontents{toc}{begingroup}
addtocontents{toc}{letprotectl@chapterprotectl@section}
makeatother
chapter{Appendix One}
chapter{Appendix Two}
addtocontents{toc}{endgroup}

% version with newenvironment: broken toc
begin{myappendices}
chapter{Appendix Three}
chapter{Appendix Four}
end{myappendices}
end{document}

makeatletter
newenvironment{myappendices}{%
  addappheadtotoc%
  addtocontents{toc}{begingroup}%
  addtocontents{toc}{letprotectl@chapterprotectl@section}%
}{%
  addtocontents{toc}{endgroup}%
}%
makeatother

documentclass{scrbook} 
usepackage{appendix}

makeatletter
newenvironment{myappendices}{%
  addappheadtotoc%
  addtocontents{toc}{begingroup}%
  addtocontents{toc}{letprotectl@chapterprotectl@section}%
}{%
  addtocontents{toc}{endgroup}%
}%
makeatother

flushbottom
begin{document}

tableofcontents

mainmatter
chapter{One}
chapter{Two}

appendix

% version without newenvironment: works
makeatletter
addappheadtotoc
addtocontents{toc}{begingroup}
addtocontents{toc}{letprotectl@chapterprotectl@section}
makeatother
chapter{Appendix One}
chapter{Appendix Two}
addtocontents{toc}{endgroup}

% version with newenvironment: works now, too
begin{myappendices}
chapter{Appendix Three}
chapter{Appendix Four}
end{myappendices}
end{document}