Of even larger importance than nearby polar or ionic groups, could be the
binding of single water molecules to charge sites (Fig. 2.6) thereby diminishing
the electron delocalisation (shorter “box lengths”). However, the actual role of
water molecules is not in general easily deduced as charge localisation can imply
Fig. 2.3 Strong versus weak charge transfer. The oscillator strength increases with the degree of
coupling
Fig. 2.5 Examples of negative charge sources: (a) C-terminal carboxylate group, (b) aspartate
(Asp) side chain, (c) glutamate (Glu) side chain
Fig. 2.6 Charge localisation by a single water molecule
Fig. 2.4 Examples of positive charge sources: (a) N-terminal ammonium group, (b) lysine (Lys)
side chain, (c) histidine (His) side chain, (d) arginine (Arg) side chain
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S.B. Nielsen