Question

Plastocyanin, a copper-containing protein found in photosynthetic systems, is involved in electron transport, with the copper ion switching between the +1 and +2 oxidation states. The copper ion is coordinated with two histidine residues, a cysteine residue, and a methionine residue in a tetrahedral configuration. How does the crystal field splitting \((\Delta)\) change between these two oxidation states?

Short answer

The crystal field splitting increases from the +1 to the +2 oxidation state.

Step-by-step solution

Understand the oxidation states

The copper ion in plastocyanin alternates between two oxidation states: +1 and +2. In the +1 oxidation state, the electron configuration is \[\text{Cu}^{+} : [ \text{Ar} ] \, 3d^{10}\]. For \([ \text{Cu}^{2+} : [ \text{Ar} ] \, 3d^{9}]\), there is one less electron in the 3d subshell compared to \[\text{Cu}^{+}\]. This change in electron count affects the crystal field splitting.

Examine electron configuration effects

In the +1 oxidation state, the 3d orbitals are fully occupied (\[3d^{10}\]), resulting in a passive crystal field with minimal splitting. In contrast, in the +2 oxidation state (\[3d^{9}\]), there is one vacancy in the d-orbitals, which increases interactions with the surrounding ligands, enhancing the crystal field splitting.

Consider crystal field theory

According to crystal field theory, the crystalline environment created by ligands affects the d-orbitals. An electron removal from the 3d orbitals, as seen in the transition from +1 to +2 oxidation state, typically increases the differences in energy among the d orbitals. Therefore, moving from Cu(+1) to Cu(+2) results in greater crystal field splitting (\(\Delta\)).

Relate oxidation state to coordination

The ligands coordinating with the copper ion—two histidines, one cysteine, and one methionine—remain constant; therefore, any change in (\(\Delta\)) is solely due to the change in oxidation state. The increased oxidation state intensifies ligand to metal repulsion, further influencing the splitting magnitude.

Conclusion on crystal field splitting

Overall, the transition from the +1 to the +2 oxidation state results in a more pronounced crystal field splitting due to the electron removal in the 3d orbitals.

Key concepts

Oxidation States

Oxidation states refer to the charge of an atom after accounting for the electrons lost or gained compared to its elemental state. In the context of transition metals, like copper, understanding oxidation states is crucial, as they influence how a metal interacts with other elements and compounds.
Plastocyanin, a copper-containing protein, showcases copper in two states: +1 and +2. This alternation is vital for its role in photosynthesis. In the +1 state, copper has one additional electron compared to the +2 state. This change alters its ability to bond with other elements. The alternation impacts the electron orbitals, mainly affecting the crystal field splitting. The oxidation state heavily dictates how metals like copper participate in redox reactions, showcasing their importance in biological systems such as photosynthetic electron transport.

Electron Configuration

Electron configuration describes the distribution of electrons among an atom's shells and subshells. For copper, in its +1 oxidation state, it follows \[\text{Cu}^{+}: [\text{Ar}] 3d^{10}\]. This configuration means that all 3d orbitals are filled. Conversely, in the +2 oxidation state, we have \[\text{Cu}^{2+}: [\text{Ar}] 3d^9\], indicating a vacancy in the 3d subshell.
This vacancy plays a significant role in the interactions with ligands, influencing the magnitude of the crystal field splitting (\(\Delta\)). Such configurations matter greatly in coordination chemistry because they determine the shape and strength of bonds formed with coordinating ligands, impacting properties like color, magnetic behavior, and electron transport capabilities.

Ligands in Coordination Chemistry

Ligands are atoms or molecules that bind to a central metal atom to form a coordination complex. In plastocyanin, ligands include two histidines, a cysteine, and a methionine, coordinating in a tetrahedral arrangement around copper. Ligands significantly influence the metal's properties, including its electronic transition states and stability.
The nature of these ligands and their environment affects the energy levels of the d-orbitals in the metal, leading to crystal field splitting. For copper in plastocyanin, a change from Cu(+1) to Cu(+2) alters the interaction strength with these ligands, affecting the electron distribution and resulting in varied crystal field splitting. The study of ligands and their effects on metals forms the bedrock of coordination chemistry, crucial for understanding many biological and industrial processes.

Photosynthetic Electron Transport

Photosynthetic electron transport is a sequence of reactions in plants and algae that convert light energy into chemical energy. Plastocyanin, containing copper ions, plays a pivotal role by shuttling electrons between complexes in the electron transport chain.
During this process, the copper ion oscillates between +1 and +2 oxidation states. This switching is not just a valence change; it provides the drive for electron transport. The alternation influences copper's electron configuration, directly affecting how it transfers electrons. Such abilities highlight the sophistication of biological systems in using inorganic chemistry principles to sustain life through processes like photosynthesis.

Copper-Containing Proteins

Copper-containing proteins, like plastocyanin, are essential in biological functions including electron transport, oxidase reactions, and cellular respiration. These proteins utilize copper ions to participate in redox activities, often switching between different oxidation states.
The presence of such proteins in organisms showcases the versatility of copper—a metal capable of complex electronic configurations and oxidation states. In photosynthesis, plastocyanin illustrates copper's ability to participate in electron transfer processes efficiently. By continually transitioning between states, these proteins play a critical role in maintaining homeostasis and harnessing energy throughout biological systems.