CD and infrared spectroscopic studies were performed on (i) the inhibitory effects of equimolar quantities of LPFFD-OH and LPYFD-NH(2) on the time-dependent aggregation of amyloid beta-protein (Abeta) (1-42) and (ii) the beta-sheet-breaker effects of two-fold molar excess of the pentapeptides on aggregated Abeta(1-42) aged 1 week. The data obtained from the time-dependent studies demonstrated that LPFFD-OH did not significantly influence, whereas LPYFD-NH(2) exerted some inhibitory effect on the aggregation of Abeta(1-42). When added to a solution of Abeta(1-42) aged 1 week, LPFFD-OH accelerated, while LPYFD-NH(2) delayed, but did not prevent further fibrillogenesis. The difference in the effects of these two pentapeptides on the aggregational profile of Abeta(1-42) is probably due to the difference in their conformational preferences: LPFFD-OH adopts a beta-turn and extended structures, while LPYFD-NH(2) adopts a prevailing beta-turn conformation.