– 1992: NO named ‘Molecule of the Year’ by the journal Science
– 1997: Creation of the journal Nitric Oxide (impact factor = 4.4)
– 1998: Nobel Prize awarded to NO research
The best NO source agents are ruthenium-nitrosyl complexes (RuNO) which release NO under light irradiation, according to the following reaction:
[RuIIL5NO]n+/- + solvent → [RuIIIL5-(solvent)]n+/- + NO
The intense research activity resulting from their study is illustrated in Figure 1. There has been a fivefold increase in the annual rate of publications over the last 20 years.
Figure 1: Evolution of the annual rate of publications on RuNO complexes.
1. Optimizing the molecular property
There are two obstacles to the development of the application of RuNO complexes in biological media: (i) the inherent toxicity of NO which requires it to be delivered locally and quantitatively and (ii) the need to use radiation of wavelength = 300 – 500 nm, i.e. outside the relative transparency range of biological tissues ( = 600 – 1300 nm). These pitfalls can be avoided by using two-photon absorption (TPA) in which the transition (300-500 nm) is made by absorbing two photons of double wavelength (600-1000) (see Figure 3). The TPA technique has the added advantage of being localized to the focal point of the radiation, which offers the possibility of working on a targeted cell, thus avoiding collateral damage to healthy tissue.
Figure 3: NO release by one-photon absorption (left) or two-photon absorption (right).
An example of the synthesis of a RuNO complex capable of two-photon NO release is shown in Figure 4.
Figure 4: Synthesis of the [FTRu(bipy)(NO)]3+ complex
The TPA properties of these systems are studied by Z-scan, a technique that gives access to the “molecular cross section” (σ) which quantifies the ability of the molecule to absorb two photons in a manner similar to the molar extinction coefficient used for one-photon absorption. σ is expressed in Goppert-Mayer (GM). In the case of monometallic RuNOs the values of vary between 100 and 200 GM. An even more ambitious approach is to study polynuclear RuNOs like those shown in Figure 5.
Figure 5: Bimetallic species (up) in C2 symmetry and trimetallic species (down) in C3 symmetry showing enhanced cross sections up to a factor of 16 vs. the reference monometallic species [FTRu(bipy)(NO)]3+. The electron donor fragments are shown in blue and the electron acceptors in red.
3. Compatibility with biological media
The first research on RuNOs carried out in the laboratory aimed to consider these systems only as switchable optical materials. They were then studied in a purely organic medium (acetonitrile) or in the solid state. Looking at them as biological molecules requires studying them in water
The [FTRu(bipy)(NO)]3+ complex is only stable in acidic conditions and transforms to [FTRu(bipy)(NO2)]+ after few minutes at pH=7. However, complexes based on the [Ru(terpy)(Cl)2(NO)]+ stabilize as [Ru(terpy)(OH)(Cl)(NO)]+ (highly stable over a long period in the dark). It is interesting to note that the final compound is the same regardless of the starting isomer, thus avoiding the need for a separation method that can be very difficult to implement. The reaction is illustrated in Figure 7. It is observed for all tridentate substituted terpyridine ligands.
Figure 7: trans(NO,OH) isomer stable in aqueous medium