Efficiencies of the intramolecular triplet-triplet energy transfer (ITET) in various bichromophoric amino acids (glycine, valine, phenylalanine, and sarcosine), dipeptides (glycylglycine, phenylalanylphenylalanine), and a simple diester, with the benzoyl and naphthyl terminal groups serving as donor and acceptor, respectively, have been determined by the steady-state photokinetic measurements. The magnitude of the transfer rate constants (>108 s-1) and the number of bonds separating the chromophores (8 or 11 atoms) suggest a through-space exothermic exchange mechanism in all cases. The influence of interchromophore distance, the character of the connecting chain as well as of side chains, was evaluated. While the most efficient energy transfer was found in a flexible diester and in valine- and sarcosine-based molecules due to the steric effect of the side hydrocarbon groups, the benzyl groups in the phenylalanine and phenylalanylphenylalanine-based bichromophores had a suppressing effect on ITET. Rigidity of the peptide bond in short bichromophoric compounds causes that a large number of favorable geometries preexist already before excitation; thus the intramolecular processes are controlled by ground-state conformational distribution. Replacing this bond by a less rigid ester moiety would allow that certain unfavorable conformations may coil to favorable ones within the excited-state lifetime (a rotation-controlled photochemical model). Some conclusions were supported by a conformational search of the potential energy surface and molecular dynamics simulations.
各种双色基氨基酸(甘氨酸、缬氨酸、苯丙氨酸和硫氨酸)、二肽(甘氨酰甘氨酸、苯丙氨酰苯丙氨酸)和一个简单的二酯的分子内三重态-三重态能量转移效率(ITET)通过稳态光动力学测量确定。传递速率常数的大小(>10^8 s^-1)和色团之间的键的数量(8或11个原子)表明在所有情况下都存在一种通过空间的放热交换机制。评估了色团间距离、连接链的性质以及侧链的影响。尽管在柔性二酯和以缬氨酸和硫氨酸为基础的分子中发现了最有效的能量转移,这是由于侧链烃基的立体效应,但苯丙氨酸和苯丙氨酰苯丙氨酸双色基色团中的苄基对ITET有抑制作用。短双色基化合物中肽键的刚性导致在激发之前已经存在大量有利的几何构型,因此分子内过程受基态构象分布控制。用较不刚性的酯基团取代这种键将允许在激发态寿命内某些不利的构象转变为有利的构象(一个受旋转控制的光化学模型)。通过对潜在能量表面的构象搜索和分子动力学模拟支持了一些结论。