The Stereochemistry and Reactivity of Metal-Schiff Base Complexes. VII. Contribution of Hydrophobic Interligand Interaction to Chiral Recognition of Phenylalaninate and Tryptophanate with (1<i>R</i>,2<i>R</i>)-<i>N</i>,<i>N</i>′-Disalicylidene-1,2-cyclohexanediaminecobalt(III) Complex
作者:Yuki Fujii、Yuzo Yoshikawa、Masato Syoji、Homare Shinohara
DOI:10.1246/bcsj.63.138
日期:1990.1
The stability constants, K1, of Δ-β2-diastereomers of mixed ligand cobalt(III) complexes with a chiral quadridentate Schiff base (sal-(R,R)-chxn), derived from salicylaldehyde and (R,R)-1,2-cyclohexanediamine, and d- or l-amino acidate (aa−=gly, ala, val, leu, thr, phe, trp, pro, asp, asn, and glu) were determined spectrophotometrically in water–methanol (2:3 by volume) containing acetate buffer (0.3 mol dm−3) at 22°C: trans-[Cosal-(R,R)-chxn}(H2O)2]++aa−\oversetK1\ightleftharpoonsΔ-β2-[Cosal-(R,R)-chxn}(aa)]. The K1 values range from 5.6×106 to 1.2×109 mol−1 dm3 and obey a linear free energy relationship except for d-phe, d-trp, d-asp, d-asn, and d-pro. In the cases of d-phe, d-trp, d-asp, and d-asn, their stability constants are 5–30 times as high as those for the corresponding l-aa. Extraordinary stabilization of the d-phe and d-trp complexes is discussed in terms of the interligand stacking of the aromatic rings between the Schiff base ligand and a side chain of amino acidate on the basis of conformational analysis and 1H NMR spectra.
Δ-β2-型混配体钴(III)配合物的稳定常数K1是通过光谱法在22°C下,在含有乙酸盐缓冲液(0.3 mol dm−3)的水-甲醇(2:3体积比)混合溶液中测定的。这些配合物由手性四齿席夫碱(sal-(R,R)-chxn)和d-或l-氨基酸根(aa− = gly、ala、val、leu、thr、phe、trp、pro、asp、asn和glu)组成。sal-(R,R)-chxn是由水杨醛和(R,R)-1,2-环己二胺衍生而来。
K1值范围在5.6×106到1.2×109 mol−1 dm3之间,除了d-phe、d-trp、d-asp、d-asn和d-pro外,都符合线性自由能关系。对于d-phe、d-trp、d-asp和d-asn,它们的稳定常数是相应l-氨基酸的5-30倍。
基于构象分析和1H NMR光谱,文章讨论了d-phe和d-trp配合物的特殊稳定性,认为这与席夫碱配体和氨基酸侧链之间的芳香环层间堆积有关。