recent studies have shown that only quinones with a 2-methoxy group 
 can act simultaneously as the primary ( qa ) and secondary 
 ( qb ) electron acceptors in photosynthetic reaction centers 
 from purple bacteria such as rb . 
 sphaeroides . 
 13c hyscore measurements of the 2-methoxy group in the semiquinone 
 states , sqa and sqb , were compared with dft 
 calculations of the 13c hyperfine couplings as a function 
 of the 2-methoxy dihedral angle . 
 x - ray structure comparisons support 
 2-methoxy dihedral angle assignments corresponding to a redox potential 
 gap ( em ) between qa and 
 qb of 175193 mv . a model having a methyl group 
 substituted for the 2-methoxy group exhibits no electron affinity 
 difference . 
 this is consistent with the failure of a 2-methyl ubiquinone 
 analogue to function as qb in mutant reaction centers with 
 a em of 160195 
 mv . 
 the conclusion reached is that the 2-methoxy group is the principal 
 determinant of electron transfer from qa to qb in type ii photosynthetic reaction centers with ubiquinone serving 
 as both acceptor quinones .