Functionalized Keggin- and Dawson-Type Cyclopentadienyltitanium Heteropolytungstate Anions: Small, Individually Distinguishable Labels for Conventional Transmission Elactron Microscopy. 2. Reactions

Article abstract of DOI:10.1021/ja00285a013

Our goal is to develop a series of small, highly electron dense reagents to label substrate molecules covalently and chemoselectively for subsequent visualisation by using conventional transmission electron microscopy (CTEM).Starting with the organic functionalized cyclopentadienyltitanium (CpTi) substituted Keggin- and Dawson-type heteropolytungstate (HPT) anions prepared in the accompanying paper, it was first established that the HPT unit as well as the Cp-Ti bond in those anions are stable under a variety of reaction conditions that lead to modification (esterification, acylation, reduction, reductive amination, oxidation, and cycloaddition reactions) of the organic appendage.A Diels-Alder reaction between either Keggin HPT diene 34 or Dawson HPT diene 18 and one of several substituted N-phenylmaleimides (27-33) was a versatile method for the attachment of a variety of protein-reactive groups to the HPT anions.Thus prepared were HPT maleimides 20 and 35, bromoacetamides 21 and 36, biotin derivative 22, isothiocyanate 24, and N-hydroxysuccinimide esters 37 and 40.Additionally, the new heterobifunctional dienophiles 29, 30, 32 and 33 should act as protein cross-linking agents, complementing those already available.Acylating agent 40 is noteworthy in that two Dawson HPT units are tethered in close proximity to each other in this reagent.By analogy to the EM image of "dimeric" HPT 23, the EM image of 40 is expected to be recognizable morphologically as dumbells.HPT-labeled ATP derivative 42 was prepared by a reductive amination of Dawson benzaldehyde 10 with N⁶-<<(aminohexyl)carbamoyl>methyl>ATP (Li salt).Both Keggin and Dawson HPTs are visible individually by using CTEM.Their stability in the electron beam is high.