The structural transition of a nonionic peptide aggregate Boc-Leu-Asn-OEt (1) in aqueous medium was studied by H-1 NMR, UV, and fluorescence spectroscopic techniques. Fluorescence studies of pyrene in the peptide aggregate indicate a considerable change in the interior dielectric constant above the transition temperature 40 degrees C (T-m), while fluorescence studies of ANS reveal significant changes in micellar microviscosity on structural transition. ANS binding studies were also carried out to study the nature of interaction in the two types of aggregates. The results suggest that the binding of ANS at lower temperature (<40 degrees C) is entropically driven and at higher temperature(>40 degrees C) the interaction is enthalpic in nature. The temperature coefficients of amide proton chemical shifts of the peptide indicate that the hydrogen bonding pattern changes during the structural transition. The results indicate that the structural transition is due to the melting of "solid-like" aggregates to the "molten state" of the peptide in aqueous medium. This study has been extended to other nonionic peptides such as Boc-Ala-Asn-OEt (2) and Boc-(Ile)(2)NH(CH2CH2O)(3)CH3 (3). Aggregation studies of peptides 2 and 3 have been carried out, and their thermodynamic parameters such as Delta G(m)degrees, Delta H(m)degrees, Delta S(m)degrees and Delta C(p)degrees are determined from the critical micellar concentration of the respective peptides. Here we present the comparative studies on the aggregate-forming characteristics and structural transition of peptides 1, 2 and 3.