• Energy-based reconstruction of a protein backbone from its alpha-carbon trace by a Monte-Carlo method

For local interaction energy, use is made of a two-dimensional Fourier series expansion of the energies of model terminally blocked amino acid residues, calculated with the Empirical Conformational Energy Program for Peptides (ECEPP/3) force field in the angles lambda(1) and lambda(2) defining the rotation of peptide groups adjacent to a C-alpha carbon atom about the corresponding C-alpha ...C-alpha virtual-bond axes.

What does it mean by terminally blocked amino acid residue?