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Synthetic kinesin analogues: A transition metal complex that can walk

Final Report Summary - METALWALKER (Synthetic kinesin analogues: A transition metal complex that can walk)

Molecular motors are used throughout biology to drive chemical systems away from equilibrium and thereby, enable tasks to be performed, cargoes to be transported directionally and work to be done. Nature's use of such 'mechanical' molecular-level structures has inspired chemists to synthesise molecular analogues of some of the fundamental components of machinery from the macroscopic world. The ultimate objective of such studies is to create relatively simple synthetic devices or materials that can, like their far more complex biological counterparts, carry out tasks by controlled molecular-level mechanical motion. However, the wholly synthetic molecular machines prepared so far lack of the control over motion exhibited by biological systems. Most of this synthetic molecular machines simply switch between two, often equilibrium, states and only very few wholly artificial chemical systems which display the key mechanical property of sequential processivity of movement (progressive movement of one component directionally across, or through the cavity, of another) have been reported.

The aim of this research project was to design, synthesise, operate and fully characterise a wholly synthetic molecular machine mimicking the mechanical action of the kinesin motor protein, namely unidirectional, hand-over-hand (also known as 'passing leg'), mechanically processive motion to promote the progressive directional advance of the molecular walker along the pre-defined molecular 'track'. To accomplish this purpose, two sets of kinetically stable binding units must be present on the walker unit. These two units must have orthogonal binding requirements and the relative binding affinities of one of the units within the binding points along the track must be switchable by a stimulus that does not affect the other binding unit. To achieve this goal, metal-binding events of the motor molecule along the functionalised track are exploited to control the thermodynamics and kinetics of biased Brownian motion in a functional molecular system. In concrete, the binding units of the walker contain two metallic centres, a palladium(II) complex and a platinum(II) complex, which can undergo a pH-mediated ligand exchange based on two orthogonal mechanisms, because while the exchange of the palladium(II) complexes is achieved thermally, the one of the platinum(II) centres is photochemical.

An optimised synthetic route towards the components, the bimetallic walker unit and the track, of the target system was developed, including a protocol for the sequential attachment of the Pd(II) and Pt(II) complexes of the walker to the half track unit to obtain the walker-half track conjugate which was unambiguously characterised by the standard techniques in synthesis, including X-ray crystallography. The coupling to the second half of the track led to synthesis of the target walker-full track conjugate system which was fully characterised. A screening for suitable conditions for the first step of the operation of the machine was undertaken using a model system and a set of reaction conditions that led to a good directionality and high processivity, the two main requirements for the successful operation of the machine was found.