手性糖基锌卟啉对手性氨基酸甲酯的分子识别

合成了手性四糖基取代锌卟啉(Zn-A)的三种阻转异构体αβαβ-Zn-A、ααββ-Zn-A和αααβ-Zn-A以及一种单糖基取代锌卟啉(Zn-B), 并通过可见光谱滴定法和圆二色(CD)光谱滴定法研究了它们对手性氨基酸甲酯(L/D-LeuOMe, L/D-ThrOMe, L/D-ValOMe和L/D-PheOMe)客体的分子识别行为. 研究发现, 三种锌卟啉对L型氨基酸甲酯的缔合常数均要高于D型, 其中ααββ-Zn-A的对映体选择性(K_L/K_D)最高可达4.75, 可用于L型氨基酸甲酯的选择性识别. 不同的Zn-A 阻转异构体对手性氨基酸甲酯的缔合常数给出相同的顺序:K^θ(LeuOMe) > K^θ(ValOMe) > K^θ(ThrOMe) > K^θ(PheOMe);主体Zn-B对手性氨基酸分子的缔合常数顺序为K^θ(PheOMe) > K^θ(LeuOMe) > K^θ(ValOMe) > K^θ(ThrOMe). 同时, 以咪唑为探针分子研究了非手性分子对Zn-A构象的影响, 发现非手性分子咪唑与手性主体结合后, 也可对主体的构象产生影响. Zn-A的三种阻转异构体与氨基酸甲酯和咪唑类客体的缔合常数关系均为K^θ(ααββ-Zn-A) > K^θ(αβαβ-Zn-A) > K^θ(αααβ-Zn-A).

Molecular Recognition of Glycoconjugated Porphyrin with Chiral Amino Acid Methyl Ester

Three atropisomeric tetrasaccharide-substituted zinc porphyrins (αβαβ-Zn-A, ααββ-Zn-A and αααβ-Zn-A) and one monosaccharide-substituted zinc porphyrin (Zn-B) were synthesized. We explored the interactions of these zinc porphyrins with a set of amino acid methyl esters (L/D-LeuOMe, L/ D-ThrOMe, L/D-ValOMe, and L/D-PheOMe)using visible titration and circular dichroism (CD) spectroscopy. We observed that all of them bind to L-amino acid methyl esters more closely than to those with D-configuration. In particular, ααββ-Zn-A gives a high enantioselectivity, K_L/K_D of 4.75, and could be used for chiral recognition of amino acid methyl esters. The binding constants of the three Zn-A compounds with methyl amino acid methyl esters are in the same order, i.e., K^θ(LeuOMe) > K^θ(ValOMe) > K^θ(ThrOMe) > K^θ(PheOMe), but the order of the binding constants for K^θ(PheOMe) > K^θ(LeuOMe) > K^θ(ValOMe) > K^θ(ThrOMe). In addition, we used imidazole as a probe to study the effects of achiral molecules on the conformations of the glycoconjugated porphyrins. The experimental results show that binding with imidazole causes adjustments in the conformations of the Zn-A compounds. The binding constants of the Zn-A compounds with imidazole and with methyl amino acid methyl esters are in the same order: K^θ(ααββ-Zn-A) > K^θ(αβαβ-Zn-A) > K^θ(αααβ-Zn-A).