ent ▁bond ▁is ▁formed ▁between ▁a ▁conser ved ▁c yst e ine - res id ue ▁and ▁the ▁FM N . ▁This ▁causes ▁a ▁conform ational ▁change ▁in ▁the ▁protein ▁that ▁is ▁necessary ▁for ▁signal ▁propag ation ▁and ▁also ▁leads ▁to ▁the ▁loss ▁of ▁flu ores c ence . ▁The ▁c oval ent ▁bond ▁is ▁ener get ically ▁unf avor able ▁and ▁is ▁cle aved ▁with ▁a ▁protein ▁specific ▁time ▁constant ▁r anging ▁from ▁seconds ▁to ▁hours . ▁In ▁order ▁to ▁make ▁better ▁use ▁of ▁the ▁flu ores c ence ▁properties ▁of ▁these ▁prote ins , ▁the ▁natural ▁phot oc ycle ▁of ▁these ▁LO V - dom ains ▁was ▁abol ished ▁by ▁ex ch anging ▁the ▁conser ved ▁c yst e ine ▁against ▁an ▁al an ine ▁on ▁a ▁gen etic ▁level . ▁Thus , ▁upon ▁blue ▁light ▁ir rad iation , ▁the ▁protein ▁remains ▁in ▁the ▁flu ores cent ▁state ▁and ▁also ▁exhib its ▁a ▁br ighter ▁flu ores c ence . ▁ ▁The ▁first ▁F b FP s ▁that ▁were ▁generated ▁in ▁this ▁fashion ▁were ▁subsequently ▁further ▁optimized ▁using ▁different ▁methods ▁of ▁mut agen esis . ▁Es pecially ▁the ▁bright ness ▁ ▁but ▁also ▁the ▁phot ost ability ▁ ▁of ▁the ▁prote ins ▁were ▁en hanced ▁and ▁their ▁spectral ▁characteristics ▁alter ed . ▁ ▁Spect ral ▁characteristics ▁▁ ▁Typ ically ▁F b FP s ▁have ▁an ▁exc itation ▁maximum ▁at ▁a ▁w avel ength ▁of ▁approximately ▁ 4 5 0   nm ▁( blue ▁light ) ▁and ▁a ▁second ▁distinct ▁exc