Recent experiments have shown that the Taiwan mutation (D7H) slows the fibril formation of amyloid peptides A beta(40) and A beta(42). Motivated by this finding, we have studied the influence of D7H mutation on structures of A beta peptide monomers using the replica exchange molecular dynamics simulations with OPLS force field and implicit water model. Our study reveals that the mechanism behind modulation of aggregation rates is associated with decrease of beta-content and dynamics of the salt bridge D23-K28. Estimating the bending free energy of this salt bridge, we have found that, in agreement with the experiments, the fibril formation rate of both peptides A beta(40) and A beta(42) is reduced about two times by mutation.