Mero stabilization

Electron-withdrawing substituents stabilize such neutral radicals considerably. Mero stabilization is found, for example, in the pyrazolyl derivative (260) <->(261) (74JCS(P1)1422). 

Radicals are particularly strongly stabilized when both an electron-attracting and an electron-donating substituent are present at the radical site. This has been called mero-stabilization" or " capto-dative stabilization. This type of stabilization results from mutual reinforcement of the two substituent effects. Scheme 12.3 gives some information on the stability of this type of radical. 

Myosin. Rabbit muscle myosin is a long, thin molecule (VI400 X 20-50 A) with a molecular weight of 5 X 10. It is composed of two heavy chains and four light chains as demonstrated by SDS-polyacrylamide disc gel electrophoresis. On tryptic digestion, myosin is split into the subunits, H-meromyosin (HMM) and L-mero-myosin (LMM). HMM is further split into S-l and S-2 subunits. While LMM is a rod of V)0% a-helical content, the a-helical content for HMM, S-l and S-2 fragments is 46%, 33% and 87%, respectively. The ATPase activity is localized in the S-l subunit (33,34). Although fish myosins appear to have the same structural profile (10,22,35-40) and similar amino acid composition as rabbit myosin (39,41,42), fish myosin is different from rabbit myosin in physicochemical properties such as solubility, viscosity and stability (10,22,35-40). 

The coil helix transition proceeds rapidly within seconds, whereas the back reaction requires several hours for full conversion. Notably, in this case, the photochromic behavior of the spiropyran groups is opposite to that observed in other solvents (see example (b) in Table 5-1). The reverse photochromism is due to the high polarity of hexafluoro-2-propanol, which stabilizes the charged mero-cyanine form better than the uncharged spiropyran form. 

---