生化反应之溶剂水课件

Chapter2:BiomoleculesinWater水做為溶劑之特殊性非共價作用力解離常數與pH緩衝溶液Chapter2:BiomoleculesinWat水為生物的首選溶劑Biologicalsolvent:amediumformetabolicreactionsaquaporins:membranechannelsforwatertransportEssentialbuffertoregulatetemperatureandpH:highspecificheatcapacityandpH-regulatingsubstancesdissolvedinitParticipantinmanybiochemicalreactions:photosynthesisandhydrolysisofATP水為生物的首選溶劑Biologicalsolvent:a水是威力強大的小分子水是威力強大的小分子WeakChemicalBondsNon-covalentinteractions:Hydrogenbonds氫鍵Ionicinteractions離子交互作用(靜電吸引力)Hydrophobicinteractions(厭水性交互作用)vanderWaalsinteractions(凡得瓦爾力)

這些弱的維繫力重要性何在?*WeakChemicalBondsNon-covalen“Themorecomplexthesystem,theweakeraretheforcesthatgovernitsbehavior”

-J.R.Platt

較強

較弱

Forces:

核間力 原子力共價鍵 非共價鍵 重力

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Structure:

原子 核原子簡單化合物生物巨分子宇宙

簡單複雜“Themorecomplexthesystem,TheFourWeakInteractions*共通特質：非共價可逆專一TheFourWeakInteractions*共通特ThePolarNatureofWaterGiventhedifferenceinelectronegativitybetweenoxygenandhydrogen(3.5-2.1=1.4)anditsshape,waterisapolarmoleculewithadipolemomentof1.85Dthenetchargeonoxygenis-0.66andthatoneachhydrogenis+0.33ThePolarNatureofWaterGiven1.HydrogenBonds<5%ofthebondstrengthofaO-H(20vs.460kJ/mole)Muchlongerbonddistance:0.18vs.0.096nmHighlydirectionalBasedonthedipolemomentofwaterAccountfortheunusualpropertiesofwaterasbothsubstanceandsolvent1.HydrogenBonds<5%ofthebCommonHydrogenBonds*PeptidesX-H:::AX=HydrogenbonddonorN,O,SA=HydrogenbondacceptorO,NCommonHydrogenBonds*PeptidesHydrogenbondinginice冰為何浮在水上？Icecrystal:staticWater:dynamicbreakingandformingHydrogenbondinginice冰為何浮在水上重要生物分子間氫鍵重要生物分子間氫鍵HydrogenBondStrengthisHighlyDirectionalFig.2-5

HydrogenBondStrengthisHigh

2.IonicInteractionsTheyareabout5-10%thestrengthofacarbon-carbonbond(20-40vs.350kJ/mole)Ionicallystabilizedcompounds(likeNaCl)arereadilydissolvedinsolventswithahighdielectricconstant(likewater)

Why?F=Q1Q2/er2

e:dielectricconstant r:distancebetweenthechargegroups2.IonicInteractionsTheyareSolventPropertiesofH2OIoniccompounds(e.g.,KCl)andlow-molecular-weightpolarcovalentcompounds(e.g.,C2H5OHandCH3COCH3)tendtodissolveinwaterTheunderlyingprincipleiselectrostaticattractionofunlikecharges;thepositivedipoleofwaterforthenegativedipoleofanothermolecule,etc.ion-dipoleinteraction:e.g.,KCldissolvedinH2Odipole-dipoleinteractions:e.g.,ethanoloracetonedissolvedinH2Odipoleinduced-dipoleinteractions:weakandgenerallydonotleadtosolubilityinwaterSolventPropertiesofH2OIonicDipole-DipoleIon-DipoleDipole-DipoleIon-DipoleSaltsDissolveinAqueous“CagesofHydration”Fig.2-6還有什麼因素讓鹽很容易溶於水？SaltsDissolveinAqueous“Cag氧既是非極性，又如何支持生命？氧既是非極性，又如何支持生命？3.HydrophobicInteractionsTheyareabout2%thestrengthofacarbon-carbonbond(8vs.350kJ/mole)WaterbecomeshighlyorderedaroundhydrophobicgroupsGreasychainsstaytogethertominimizeentropylossbywater3.HydrophobicInteractionsThe生化反应之溶剂水课件生化反应之溶剂水课件生化反应之溶剂水课件水的退出有利酵素與受質作用水的退出有利酵素與受質作用4.vanderWaalsInteractionsTheyareabout1%thestrength