RAMAN-ACTIVE BANDS SENSITIVE TO MOTION AND CONFORMATION AT THE CHAIN TERMINI AND BACKBONES OF ALKANES AND LIPIDS
RAMAN-ACTIVE BANDS SENSITIVE TO MOTION AND CONFORMATION AT THE CHAIN TERMINI AND BACKBONES OF ALKANES AND LIPIDS
复制标题
DOI：
10.1021/j100296a066
复制
发表时间：
1987-06-04
影响因子：
--
通讯作者：
LADJADJ, M
中科院分区：
文献类型：
--
作者：
BROWN, KG;BICKNELLBROWN, E;LADJADJ, M
本文比较了两端带有甲基的长链烷烃和一端带有取代基的长链烷烃的拉曼光谱。拉曼活性带出现在890和900 cm-1之间，并且先前被指定为与末端CC键的伸展偶联的CH 3摇摆和长链酯中的酰基C1-C2伸展，为了确定分配的准确性和每个波段的相对贡献，（如果两种分配都正确）与在长链酯的光谱中在约895 cm-1处观察到的总拉曼强度的比值。这一观察到的谱带的归属在合成和生物聚合物的研究中具有重要意义，因为甘油酯中895 cm-1处的谱带已被证明对分子状态敏感，特别是局部运动自由度和构象随机性。895-cm-1拉曼谱带的强度显示为实际上完全由链端CC拉伸组成。此外，它示出，对于长链酯，末端酰基CC拉伸和末端甲基端CC拉伸叠加。然而，酰基端CC伸缩带比甲基端CC伸缩带对总强度的贡献大得多。液体烷烃的光谱在约875和895 CHT 1处包含加宽的谱带。875-CHT 1谱带先前已被指定为C2C，拉伸无序的链端。晶体二棕榈酰磷脂酰胆碱（DPPC）和在链2的C1原子处用13 C标记的晶体DPPC的拉曼光谱的比较证实了谱带归属于酰基末端CC伸缩。此外，895-cm-1的拉曼谱带被指定为反式CrA旋转异构体的伸缩模式，而875-cm-1的谱带被指定为偏置C2-C1旋转异构体的伸缩模式。因此，895-和876-cm-1拉曼谱带的相对强度可用于确定链端的反式和侧式旋转异构体的相对数量。比较了液态烷烃和熔融DPPC的CH伸缩带和骨架CC伸缩带的强度。假设液体的2885-CHT 1 CH 2反对称拉伸峰高取决于反式骨架CC的数量，而“全反式”骨架CC拉伸模式在1130 CHT 1处的峰高取决于全反式链段的数量，对谱图的解释得出结论，与沿链烷烃链的分布相比，沿链烷烃链的分布更为随机。熔融DPPC的酰基链。
The Raman spectra of long-chain alkanes with methyl groupsat both ends are compared with those having a substituted group at one end of the chain. A Raman-activeband that appears between 890 and 900 cm" 1, and was previously assigned to CH3 rocking coupled with stretching of the terminal CC bond andto acyl C,-C2 stretching in long-chain esters, was examined in detail in order to determine the accuracy of the assignments and the relative contributionsof each band (if both assignments are correct) to the total Raman intensity observed in spectra of long-chain esters at about 895 cm" 1. The assignment of this observed band assumes significance in studies of synthetic and biological polymers because a band at 895 cm" 1 in glycerides has been shown to be sensitive tomolecular state, in particular local motional freedom and conformational randomness. The intensity of the 895-cm" 1 Raman band is shown to consist virtually entirely of chain end CC stretching. Further, it is shown that, for long-chain esters, terminal acyl CC stretching and terminal methyl end CC stretching are superimposed. However, the acyl end CC stretch band contributes much more to the total intensity than does the methyl end CC stretch band. The spectra of the liquid alkanes contain broadened bands at about 875 and 895 cm" 1. The 875-cm" 1 band has been assigned previously to C2C, stretching for disordered chain ends. Comparison of Raman spectra of crystalline dipalmitoylphosphatidylcholine (DPPC) and crystalline DPPC labeled with 13C at the C, atom of chain 2 confirms the assignment of the band to acyl terminus CC stretching. Further, the 895-cm" 1 Raman band is assigned to the stretching mode for trans CrA rotamers, while the 875-cm" 1 band is assignedto the stretching mode for gauche C2-C, rotamers. Therefore, the relative intensities of the 895-and876-cm" 1 Raman bands may be used to determine the relative populations of trans and gauche rotamers at chain ends. The intensities of CH stretch bands and skeletal CC stretch bands are compared for the liquid alkanes and melted DPPC. With the assumption that the 2885-cm" 1 CH2 antisymmetric stretch peak height for liquids depends on trans skeletal CC population while the peak height of the “all-trans” skeletal CC stretch mode at 1130 cm" 1 depends on the populationof all-trans segments, the interpretation of thespectra leads to the conclusion that gauche CC rotamers are more randomly distributed along the liquid alkane chains than along the acyl chains of melted DPPC.