The early formed oligomers of amyloid-beta proteins with 40 and 42 amino acids are believed to be the culprits of Alzheimer's disease. A beta 1-42 peptides with alanine and isoleucine mutations of glycine 33 are known to be much less toxic than the wild-type A beta-142 and promote the aggregation process in vitro. The fragment A beta 29-42; has also been shown to form fibrils, disrupt A beta 1-42 oligomerization, and inhibit A beta-42-induced neurotoxicity. As a first step toward understanding the impact of G33A and G33I mutations on the earliest steps along the A beta 1-42 aggregation pathway, we have studied the structures of the monomer and dimer of A beta 29-42 and its two G33 variants using coarse-grained replica exchange molecular dynamics simulations. These simulations, totaling 15 mu s, indicate that both substitutions impact the conformational ensemble of; A beta 29-42. For the monomer, the population of the beta-hairpin is high for:wild-type A beta 29-42, but marginal for A beta 29-42 G33I mutant. The three dimers are also stabilized by different patterns of interaction. The data are discussed in terms of the differences in the aggregation characteristics between wild-type A beta 1-42 and its two G33A.and.G33I variants.