Question

Picket-fence porphyrin. When free heme is exposed to oxygen, an oxo-bridged dimeric species forms readily and irreversibly. Chemists have designed and synthesized a special heme derivative based on a "picket fence" porphyrin that binds oxygen reversibly without forming dimeric species.Propose a basis for the design and an explanation for the reversible oxygen binding.

Short answer

Design a porphyrin with bulky side groups to prevent dimerization and facilitate reversible oxygen binding.

Step-by-step solution

Understanding the Problem

The goal is to propose a design for a heme derivative that allows reversible oxygen binding without forming dimeric species. This involves understanding the limitations of free heme, which forms oxo-bridged dimers irreversibly upon exposure to oxygen.

Significance of Picket Fence Porphyrin

The term 'picket fence' refers to strategic modifications of the porphyrin ring, where steric hindrance is introduced to prevent the formation of dimeric species. This structure mimics biological environments where proteins can prevent unwanted reactions by shielding active sites.

Design Basis: Steric Hindrance

Introduce bulky side groups to the porphyrin structure, creating a physical barrier ('picket fence') that prevents two heme molecules from coming close enough to form oxo-bridged dimers. These groups should allow space for the binding of oxygen molecules while blocking heme dimerization.

Reversible Binding Explanation

By introducing steric hindrance, the newly designed heme derivative allows an oxygen molecule to bind to the iron center without causing significant structural or electronic changes that would lead to irreversible binding. This design mimics the structural features seen in myoglobin or hemoglobin, which naturally bind oxygen reversibly.

Key concepts

Heme Derivative Design

The design of heme derivatives involves strategic modifications to improve their functional properties. One innovative design is the picket-fence porphyrin. This concept builds on altering the traditional heme structure by adding bulky side groups. These side groups act as barriers, preventing unwanted interactions, such as the formation of oxo-bridged dimers. These dimers occur when two heme molecules link together through oxygen, which is usually irreversible.

The goal of these modifications is to allow oxygen binding without forming these problematic dimers. By mimicking proteins in biological systems, the picket-fence creation mimics the shielding effect of proteins encasing heme groups, avoiding unwanted reactions.

Reversible Oxygen Binding

Reversible oxygen binding is an essential feature for heme derivatives, especially in biological contexts like myoglobin and hemoglobin. This process means that oxygen can attach to and detach from the heme group without causing permanent changes to the molecule's structure.

In picket-fence porphyrins, reversible binding is achieved by introducing bulky side groups that prevent structural and electronic changes upon oxygen binding. These changes are avoided by blocking the close approach of another heme, ensuring that the binding and release of oxygen occur seamlessly. Such a design helps in maintaining a balance in systems where oxygen needs to be transported and released efficiently.

Steric Hindrance in Porphyrins

Steric hindrance refers to the prevention of molecules interacting due to spatial obstruction caused by large side groups attached to a molecule. In the context of porphyrins, especially the picket-fence type, steric hindrance plays a vital role in preventing unwanted reactions.

By adding bulky groups around a porphyrin's edge, chemists create a physical 'fence' that stops two heme molecules from getting close enough to form oxo-bridged dimers. It allows the porphyrin to still participate in desirable reactions, such as binding oxygen. This selective blocking is key in designing heme derivatives that function optimally, much like naturally occurring proteins that carefully regulate reactions.