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arecharacteristicofmanysystems,suchasliquidsandamorphoussolids,
colloidalsystemsandalsoofbiologicalmacromolecules,theirassemblies
andstructuresformedbythem,biologicalmembranes.Theirphysicalchar-
acterizationandanalysisisaformidabletaskduetoverylimitedavailability
ofmethodsthatallowtheirstructuralanddynamicanalysis.Thereforeitis
extremelyimportanttocharacterizemoleculardisorderinbothconceptual
andquantitativeterms.
Wecandistinguishanddiscusssequentiallythreeeffectsofmolecular
disorder.Firstisthedistributionofmolecularinteractionsbetweenstructur-
allyidenticalfluorophoresanddisorderedenvironmentresultinginnon-
identityoftheirinteractionsites.Thisdistributiongeneratestheinhomogene-
ousbroadeningofthespectraandcanbedescribedinenergyterms(i.e.the
distributionsontheground-stateandexcited-stateinteractionenergy)
(Mazurenko1983,Rubinov&Tomin1983,Nemkovichetal.1991,Myers
1998).Secondisthedistributionamongdifferentsitesintheratesofmolecu-
larrelaxationsthatcanbeinducedbydifferentstimuliincludingelectronic
excitationofthefluorophore,whichrequiresfordescriptiontheinvolvement
oftime-dependentvariables.Andfinally,ifphotophysicalorphotochemical
reactivityispeculiartoparticularsite,thenthemembersofthedistribution
canexhibitthedifferencesintheirreactionkinetics.
Inallthesecases,theelectronicabsorptionandfluorescenceemission
spectraareusuallybroadwithvibrationalstructuresmoothenedorevenen-
tirelylost.Applicationofcryogenictemperaturesdoesnotresultinim-
provementinstructuralresolution.However,thereexistthepossibilitiesof
excitingwithininhomogeneouslybroadenedcontourofabsorptionoremis-
sionbandformedbyanensembleofevenidenticalspeciesthoseofthem
thatpossessdifferencesininteractionwiththeirmolecularenvironment.The
majoridealwhichisbehinddifferentapproachesusedinsite-selectionspectroscopyl
istheselectionandobservationofasmallpartofthefluorophorepopulationtogether
withstudyingthewholepopulationthatisresponsibleforaninhomogeneously
broadenedspectrum.Sinceinabsorptionspectrathebandcorrespondingto
the0-0electronictransitionisobservedatthelowestenergies(longestwave-
lengths),andthevibrationalstructure(oftennotobservable)propagatesin
thedirectionofhigherenergies,thebestsite-selectivitycanbeachievedat
thelow-energyslopeofabsorptionspectra(the“Red-Edge”).Incontrast,in
fluorescencespectrathe0-0bandislocatedathigherenergies,andvibra-
tionalstructurepropagatesinthedirectionoflowerenergies(longerwave-
lengths).Thus,inemissionspectrumthesite-selectivitycanbebestachieved
atthe“Blue-Edge”.Definitely,insomecasesthe0-0absorbanceisweakand
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