Kimura Eiichi, Gotoh Teruhiro, Aoki Shin, Shiro Motoo
Department of Medicinal Chemistry, Faculty of Medicine, Hiroshima University, Kasumi 1-2-3, Minami-ku, Hiroshima, 734-8551 Japan.
Inorg Chem. 2002 Jun 17;41(12):3239-48. doi: 10.1021/ic020087k.
To elucidate intrinsic recognition of carboxamides by zinc(II) in carbonic anhydrase (CA) (as inhibitors) and carboxypeptidase A (CPA) (as substrates), a new series of Zn(2+)-carboxamide-appended cyclen complexes have been synthesized and characterized (cyclen = 1,4,7,10-tetraazacyclododecane). Two types of Zn(2+)-carboxamide interactions have been found. In the first case represented by a zinc(II) complex of carbamoylmethyl-1,4,7,10-tetraazacyclododecane (L(1)), the amide oxygen binds to zinc(II) at slightly acidic pH (to form ZnL(1)), and the deprotonated amide N(-) binds to zinc(II) at alkaline pH (to form ZnH(-1)L(1)) with pK(a) = 8.59 at 25 degrees C and I = 0.1 (NaNO(3)), as determined by potentiometric pH titrations, infrared spectral changes, and (13)C and (1)H NMR titrations. The X-ray crystal structure of ZnH(-1)L(3) (where L(3) = N-(4-nitrophenyl)carbamoylmethyl cyclen, pK(a) = 7.01 for ZnL(3) <==> ZnH(-1)L(3)) proved that the zinc(II) binds to the amidate N(-) (Zn-N(-) distance of 1.974(3) A) along with the four nitrogen atoms of cyclen (average Zn-N distance 2.136 A). Crystal data: monoclinic, space group P2(1)/n (No. 14) with a = 10.838(1) A, b = 17.210(2) A, c = 12.113(2) A, b = 107.38(1) degrees, V = 2156.2(5) A(3), Z = 4, R = 0.042, and R(w) = 0.038. These model studies provide the first chemical support that carboxamides are CA(-) inhibitors by occupying the active Zn(2+) site both in acidic and alkaline pH to prevent the occurrence of the catalytically active Zn(2+)-OH(-) species. In the second case represented by a zinc(II) complex of 1-(N-acetyl)aminoethylcyclen, ZnL(6), the pendant amide oxygen had little interaction with zinc(II) at acidic pH. At alkaline pH, the monodeprotonation yielded a zinc(II)-bound hydroxide species ZnL(6)(OH(-)) (pK(a) = 7.64) with the amide pendant remaining intact. The ZnL(6)(OH(-)) species showed the same nucleophilic activity as Zn(2+)-cyclen-OH(-). The second case may mimic the Zn(2+)-OH(-) mechanism of CPA, where the nucleophilic Zn(2+)-OH(-) species does not act as a base to deprotonate a proximate amide.
为阐明碳酸酐酶（CA）（作为抑制剂）和羧肽酶A（CPA）（作为底物）中锌（II）对羧酰胺的内在识别作用，已合成并表征了一系列新的锌（2+）-羧酰胺连接的轮环配合物（轮环 = 1,4,7,10-四氮杂环十二烷）。发现了两种类型的锌（2+）-羧酰胺相互作用。在第一种情况中，以氨甲酰甲基-1,4,7,10-四氮杂环十二烷（L(1)）的锌（II）配合物为代表，在略酸性pH下，酰胺氧与锌（II）结合（形成ZnL(1)），在碱性pH下，去质子化的酰胺N(-)与锌（II）结合（形成ZnH(-1)L(1)），25℃、I = 0.1（NaNO(3)）时pK(a) = 8.59，通过电位pH滴定、红外光谱变化以及(13)C和(1)H NMR滴定确定。ZnH(-1)L(3)（其中L(3) = N-(4-硝基苯基)氨甲酰甲基轮环，ZnL(3) <==> ZnH(-1)L(3)的pK(a) = 7.01）的X射线晶体结构证明，锌（II）与酰胺酸根N(-)（Zn-N(-)距离为1.974(3) Å）以及轮环的四个氮原子（平均Zn-N距离2.136 Å）结合。晶体数据：单斜晶系，空间群P2(1)/n（编号14），a = 10.838(1) Å，b = 17.210(2) Å，c = 12.113(2) Å，b = 107.38(1)°，V = 2156.2(5) Å(3)，Z = 4，R = 0.042，R(w) = 0.038。这些模型研究提供了首个化学证据，表明羧酰胺通过在酸性和碱性pH下占据活性锌（2+）位点来阻止催化活性锌（2+）-OH(-)物种的出现，从而成为CA的抑制剂。在第二种情况中，以1-(N-乙酰基)氨基乙基轮环的锌（II）配合物ZnL(6)为代表，在酸性pH下，侧链酰胺氧与锌（II）几乎没有相互作用。在碱性pH下，单去质子化产生一个与锌（II）结合的氢氧化物物种ZnL(6)(OH(-))（pK(a) = 7.64），侧链酰胺保持完整。ZnL(6)(OH(-))物种表现出与锌（2+）-轮环-OH(-)相同的亲核活性。第二种情况可能模拟了CPA的锌（2+）-OH(-)机制，其中亲核的锌（2+）-OH(-)物种不作为碱使邻近酰胺去质子化。
