We examine the effect of deletion of the amino-terminal (residues 1-9) on the structure and energetics of A beta(1-40) peptides. To this end, we use replica exchange molecular dynamics to compare the conformational ensembles of A beta(1-40) and amino-truncated A beta(10-40) monomers and dimers. Overall, the deletion of the amino-terminal appears to cause minor structural and energetic changes in A beta monomers and dimers. More specifically, our findings are as follows: (1) there is a small but discernible conversion of beta-strand structure into helix upon amino-terminal deletion, (2) secondary structure changes due to truncation are caused by missing side chain interactions formed by the amino-terminal, and (3) the amino-terminal together with the central sequence region (residues 10-23) represents the primary aggregation interface in A beta(1-40) dimers. The amino-truncated A beta(10-40) retains this aggregation interface, which is reduced to the central sequence region. We argue that the analysis of available experimental data supports our conclusions. Our findings also suggest that amino-truncated A beta(10-40) peptide is an adequate model for studying A beta(1-40) aggregation.