膠原蛋白的結構是由三條左旋第二型聚脯胺酸螺旋結構，沿著一個相同的主軸右旋轉組成，序列中每三個胺基酸有一個 Gly 、高含量的亞胺基酸和氫鍵是促進其穩定性的重要因素。另一方面， 正電荷與芳香環四極矩間的非共價鍵作用力 - cation-π 作用力，也陸續被發現其在蛋白質的 ligand 鍵結等生物辨識上的重要性。因此，我們利用一系列的主客胜□系統去探討 cation-π 作用力對膠原蛋白三股螺旋的影響；我們合成了一系列胜□： Ac-(POG)3-(PYGXOG)-(POG)3-NH2 ，在 X 位置為 Phe、 Tyr 、 Trp 、 Phe(4-F), 、 Phe(F5) 和 Phe(4-Me)； Y 位置為 Lys 、 Arg。 而 X 位置的 Trp 由於其立體障礙的緣故，使其形成 cation-π 作用力不如預期的大。而當 Y 位置為 Arg 的 Tm 值比 Lys 時高，因為 Arg 側鏈形狀的幾何特性，所以比 Lys 易產生 cation-π 作用力，與之前的文獻結論一致。此外，非自然胺基酸取代則顯現出，當芳香環側鏈上有電子基的去活化者 – 氟，會減少膠原蛋白模擬胜□的穩定性，因其降低 π 電子密度而減弱了 cation-π 作用力，證實 cation-π 作用力對於膠原蛋白三股螺旋的穩定性有重要的貢獻。
更進一步，我們設計膠原蛋白模擬胜□序列： Ac-RG(POG)8F-NH2 和 Ac-RG(POG)10F-NH2 ，發現 cation-π 作用力對促進膠原蛋白模擬胜□的自組裝速率的影響。最後，我們亦試著利用 cation-π 作用力去形成異源三聚體膠原蛋白模擬胜□。
以上這些結果顯示出，強健的非共價鍵作用力 – cation-π 作用力，可以被應用在穩定膠原蛋白模擬胜□，及促使膠原蛋白模擬胜□自組裝成高階結構。
Collagen is a right-handed triple helix composed of three left-handed PPⅡ helices. The presence of Gly as every third residue, a high content of imino acids and hydrogen bonding are significant characteristics to improve the stability of collagen. Cation-π interactions are considered as an electrostatic attraction between a positive charge and the quadrupole moment of the aromatic ring and are important in a variety of proteins that bind to ligands or substrates. Here, we used a host-guest peptide system to study the cation-π interactions in the collagen triple helix. A series of Ac-(POG)3-(PYGXOG)-(POG)3-NH2 peptides were synthesized and characterized, where X is Phe, Tyr, Trp, Phe(4-F), Phe(F5), Phe(4-Me), and Y is Lys, Arg. Owing to the steric hindrance, the cation-π interactions formed by Trp are not as stable as what we predicted. The peptides with Arg in the Y position have a higher Tm value than those with Lys at this position consistent with the previous conclusion that Arg is more likely than Lys to be in a cation-π interaction due to its geometric features. Moreover, the unnatural residue substitutions showed that an electron-withdrawing deactivator, fluorine, in the aromatic ring will reduce the thermal stability because it would reduce the π-electron density as well as the cation-π interaction. These results suggest that cation-π interactions significantly contribute to the collagen triple helix stability.
Furthermore, we design another two peptides, Ac-RG(POG)8F-NH2 and Ac-RG(POG)10F-NH2 to promote the rate of self-assembly by cation-π interactions. We have also tried to use cation-π interactions to form the heterotrimeric collagen helices. These results exhibit that the strong and non-covalent force, cation-π interactions, could be applied to stabilize collagen triple helices and assemble them into a higher order structure.

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