Saha Abhijit, Manna Swarup, Nandi Arun K
Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700 032, India.
Langmuir. 2007 Dec 18;23(26):13126-35. doi: 10.1021/la7026219. Epub 2007 Nov 15.
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 (<or=25 degrees 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) pi-pi-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-pi* 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 pi-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 degrees C. Arrhenius treatment of the rate constant values of the CD data indicates an activation energy of approximately 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 degrees 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.
摩尔比为3:1的核黄素（R）和三聚氰胺（M）超分子复合物（RM31）在水性介质中形成热可逆凝胶。通过光学显微镜、电子显微镜和原子力显微镜进行形态学研究，并结合时间相关圆二色性（CD）和光致发光（PL）光谱，阐明了凝胶化机制。光学显微镜显示，在较低凝胶化温度（≤25℃）下为球晶形态，但在较高温度下会形成纤维状网络形态。电子显微镜和原子力显微镜表明，纤维中存在左旋螺旋结构。利用圆二色性（CD）和光致发光（PL）光谱对凝胶形成进行动力学研究表明，有三个步骤：（1）RM复合物形成；（2）构象有序化；（3）有序构象体的π-π堆积。RM复合物形成的第一步已通过傅里叶变换红外（FTIR）光谱确定（Manna, S.; Saha, A.; Nandi, A. K. Chem. Commun. 2006, 4285），第二步可从CD光谱中检测到。在此，凝胶形成过程中n-π*跃迁的椭圆率值增加了600倍。椭圆率的急剧增加归因于核糖醇链的构象有序化，随后形成螺旋纤维。第三步是通过荧光光谱得出的，荧光强度也增加了30倍。PL强度的大幅增加是由于复合物π堆积过程中形成疏水核心所致。椭圆率和PL强度均随时间呈S形增加，使用阿弗拉米方程对数据进行分析表明，前者的n值接近1.5，后者接近2。两种方法从阿弗拉米图截距获得的速率常数不同。CD光谱的速率常数数据显示出较小的正温度系数，但PL数据的速率常数除30℃和35℃的数据外显示出负温度系数。对CD数据的速率常数进行阿累尼乌斯处理表明活化能约为13 kcal/mol，这表明构象转变是椭圆率增加的原因。从荧光数据获得的速率常数的负温度系数归因于球晶形成，而30℃和35℃时速率常数的增加归因于纤维形成。荧光强度和峰位置随温度以及凝胶中RM复合物的浓度而变化。文中给出了基于纤维厚度的一种可能解释。
