A mechanistic approach on the self-organization of the two-component thermoreversible hydrogel of riboflavin and melamine

Abstract

The riboflavin (R) and melamine (M) supramolecular complex in the mole ratio of 3:1 (RM31) produces a thermoreversible gel in aqueous medium. The gelation mechanism has been elucidated from morphological investigations using optical, electron, and atomic force microscopy together with time-dependent circular dichroism (CD) and photoluminescence (PL) spectroscopy. Optical microscopy indicates spherulitic morphology at lower gelation temperature (≤25 °C), but at higher temperature fibrillar network morphology develops. Electron and atomic force microscopy indicate the presence of left handed helical structures in the fibrils. Kinetic study of gel formation using circular dichroism (CD) and photoluminescence (PL) spectra indicates that there are three steps: (1) RM complex formation, (2) conformational ordering, and (3) π-π-stacking of ordered conformers. The first step of RM complex formation is already established from Fourier transform infrared (FTIR) spectroscopy (Manna, S.; Saha, A.; Nandi, A. K. Chem. Commun. 2006, 4285), and the second step is detected from the CD spectra. Here, the ellipticity value of the n-π^∗ transition increases by 600 times during gel formation. The dramatic increase of ellipticity is attributed to conformational ordering of the ribityl chain followed by helical fibril formation. The third step is concluded from fluorescence spectroscopy, which also shows a 30 times increase in intensity. The substantial increase in PL intensity is caused by hydrophobic core formation during π-stacking of the complex. Both the ellipticity and PL intensity show a sigmoidal increase with time, and analysis of data using the Avrami equation shows n values close to 1.5 for the former and close to 2 for the latter. The rate constant values obtained from the intercepts of Avrami plots are different in the two methods. The rate constant data from the CD spectra show a small positive temperature coefficient, but the rate constant values from the PL data show a negative temperature coefficient except the data at 30 and 35 °C. Arrhenius treatment of the rate constant values of the CD data indicates an activation energy of ~13 kcal/mol, signifying that the conformational transition is the cause of ellipticity increase. The negative temperature coefficient of the rate constant obtained from the fluorescence data has been attributed to the spherulitic crystal formation, and the increase of the rate constant at 30 and 35 °C has been attributed to fibril formation. The fluorescence intensity and peak position change with temperature and with the concentration of the RM complex in the gel. A probable explanation from fibrillar thickness is offered.