面向固氮酶铁钼辅基加氢模式的柠檬酸钼研究

Molybdenum citrate towards the protonation of FeM o-co in nitrogenase

柠檬酸钼配合物的光谱和结构研究,为固氮酶铁钼辅基中的高柠檬酸和柠檬酸突变种的配位性质提供了重要的信息.本文利用水热法,用钼酸铵、柠檬酸(H_4cit)、α,α,α-三联吡啶(tpy)和盐酸肼在酸性条件下分离得到了两个不同价态的双核柠檬酸钼配合物[Mo^(IV)_(2)O(Hcit)_(2)(tpy)_(2)]·3H_(2)O(1)和(H_(2)tpy)_(2)[Mo^(VI)_(2)O_5(Hcit)_(2)]·7.5H_(2)O(2).通过元素分析、红外光谱、紫外光谱、核磁共振、电子顺磁共振、键价计算和X射线单晶衍射等手段对这两个配合物进行了表征.单晶结构分析表明,1和2中的柠檬酸配体通过α-烷氧基、α-羧基和β-羧基与钼三齿螯合,另一个游离的β-羧酸基团与周围的水分子形成氢键.核磁碳谱表明2在溶液中发生了解离.对1、2和已报道的柠檬酸钼配合物的键长进行统计分析显示,端氧的反位效应、羟基质子化效应和金属钼的价态是影响键长的主要因素.其中,Mo-O(α-烷氧基/α-羧基)键长与金属钼的价态呈现一定的线性关系.线性拟合计算得到的Mo(III)-O(α-羧基)键长与钼铁蛋白结构中Mo(III)-高柠檬酸和柠檬酸的配位键长接近,而计算的Mo(III)-O(α-烷氧基)键长远小于蛋白中的键长,但质子化作用下的Mo-O(α-羟基)键长与之接近.由此进一步支持固氮酶铁钼辅基中的高柠檬酸和柠檬酸通过α-羧基和未去质子的α-羟基与钼配位.该加氢配位模式对于研究高柠檬酸和柠檬酸在固氮和CO还原反应中的加氢作用具有重要意义.

The structure of Fe Mo-cofactor(Fe Mo-co)in nitrogenase has been clarified as Mo Fe_(7)S_(9)C[R-(H)homocit](Hhis)(cys)(H_(4)homocit=homocitrate,Hhis=histidine,Hcys=cysteine),where homocitrate chelates to Mo(III)viaα-alkoxy/α-hydroxy andα-carboxy groups.Recent model comparisons and theoretical calculations suggested a hydroxy coordinated homocitrate in Fe Mo-co,which may play an important role in storing and transferring hydrogen source for the reduction of substrate.Herein,two molybdenum citrates[Mo^(IV)_(2)O(Hcit)_(2)(tpy)_(2)]·3 H_(2)O(1)(H_(4)cit=citrate,tpy=α,α,α-terpyridine)and(H_(2)tpy)2[Mo^(VI)_(2)O_(5)(Hcit)_(2)]·7.5H_(2)O(2)have been obtained via hydrothermal reactions under the reduction of hydrazine hydrochloride in acidic condition.1 and 2 were fully characterized by elemental analyses,IR,UV-vis,EPR and ^(13)C NMR spectra,bond valence calculations and X-ray single crystal diffractions.Structural analysis indicates that 1 is a binuclear complex,which seven coordination sites are occupied by oneμ2-O atom,three nitrogen atoms of terpyridine,three oxygen atoms from citrate.Citrate chelates to Mo viaα-alkoxy,α-carboxy and β-carboxy groups,the uncoordinatedβ-carboxylic acid group interacts with water molecules through strong hydrogen bonds.2 is a common binuclear Mo(VI)complex counterbalanced by protonated terpyridine cations.The coordination of citrates in 2 is similar to that of 1.^(13)C NMR spectrum indicates that 2 partially dissociates in solution.We have also analyzed Mo–O distances of 1 and 2,as well as previously reported molybdenumα-hydroxycarboxylates.It is found that trans-effect of Mo=O group,protonation ofα-alkoxy group and oxidation state of Mo are the three major factors that affect the distances between Mo and coordinated atoms fromα-hydroxycarboxylates.Mo–O distances are elongated about 0.05-0.13Afrom trans-effect,while this effect forα-carboxy group is stronger than that ofα-alkoxy group.Mo–O(α-hydroxy)distance is about 0.11?longer than that of Mo–O(α-alkoxy)due to protonation effect.Mo–O(α-alkoxy/α-carboxy)distances show negative correlation with oxidation state of Mo.Linear fit gives a Mo^(II)I–O(α-carboxy)distance that is close to those of wild-type Fe Mo-co and citrate-substituted cofactor of variant Mo-nitrogenase,while the calculated Mo^(III)–O(α-alkoxy)distance is much shorter.The difference is well in agreement with the protonation effect.This result gives a quantitative conclusion for the protonation mode of homocitrate in Fe Mo-co.A new protonated model is also deduced for citrate-substituted cofactor of variant Mo-nitrogenase.It seems more structural data of model compounds should be included for statistical analysis,especially those complexes in low valences.