Figure 5.

(Center) LoProp charge population analysis (with hydrogen atom charges summed onto the heavy atoms to which they are connected) for lumiflavin in its ground (S₀) and first singlet excited (S₁) states. Red circles indicate negative charge density, and blue circles indicate positive charge density. The area of the circles is directly proportional to the charge on the corresponding atom. The S₁ and S₀ states have slightly different charge distributions that are difficult to discern without close inspection. Therefore, the difference in the atomic charges (S₁ – S₀) is also shown in the middle. In the S₁ – S₀ difference plot, the areas of the circles are proportional to the magnitude of the charge difference between S₀ and S₁, with red circles indicating reduced charge (higher electron density) on the atom after excitation from S₀ to S₁ and blue circles indicating increased charge (lower electron density). (Left) Scheme illustrating the effect of placing a positively charged lysine side chain close to the flavin N5/C4a, which would result in a red shift (see the text for details). (Right) Scheme illustrating the effect of placing a negatively charged glutamate side chain close to the flavin N1, which should also result in a red shift (see the text for details).