Role of hydration in nanopore sensing: Hofmeister series, preferential solvation, osmotic stress, and all that

Transcription

1 Role of hydration in nanopore sensing: Hofmeister series, preferential solvation, osmotic stress, and all that Sergey M. Bezrukov Laboratory of Physical and Structural Biology, NICHD National Institutes of Health together with: Philip A. Gurnev, Daniel Harries, and V. Adrian Parsegian

2 Salting in Salting out Preferential solvation Chaotropes Kosmotropes Franz Hofmeister ( ) Osmotic stress, etc. kosmotropes chaotropes

3 Surface hydration changes upon molecular complexation I II I II I II I II

4 Nature 370, pp (1994)

5 Nature 398, pp (1999) +

6 Cyclodextrin: carbohydrate with a non-polar cavity Cyclodextrin (CD): Adamantane carboxylate 1 nm

7 Solutes change water activity and complexation equilibrium When is a solute effect osmotic? Changes in (binding) free energy are linear in water s chemical potential (solute osmolality) Implies a constant number of released solute excluding waters for all salt concentrations d G dµ water = N Salt n water KBr 9 ± 1 KCl 14 ± 1 Na 2 SO 4 47 ± 2 waters G osm mcosolute slope N excluding waters Harries, Rau, Parsegian 2007, JACS

8 Probing association of molecules using nanopore α-hemolysin channel 0 mv 80 mv αhl L1 50 pa 50 pa 0.5 s 0.5 s αhl + γcd αhl + γcd + AD L1 L2 L1 L2

9 Probing association of molecules using nanopore α-hemolysin channel 0 mv 80 mv αhl L1 Gu, Braha, Conlan, Cheley, Bayley pa 50 pa 0.5 s 0.5 s αhl + γcd αhl + γcd + AD L1 L2 L1 L2

10 Probing association of molecules using nanopore α-hemolysin channel 0 mv 80 mv αhl L1 50 pa 0.5 s αhl + γcd L1 L2 + Gu, Braha, Conlan, Cheley, Bayley pa 0.5 s αhl + γcd + AD L1 L2

11 Probing association of molecules using nanopore α-hemolysin channel 0 mv 80 mv αhl L1 50 pa 20 pa 0.5 s 5 ms L pa αhl + γcd + AD L1 L2 Gu, Braha, Conlan, Cheley, Bayley s

12 How about the involvement of water restructuring in this timeresolved molecular complexation?

13 AD-CD interaction is sensitive to the added salt 1.00 M KCl; α-hl + γ-cd no AD Increasing [KCl] 0 pa 0 pa 1.00 M KCl; α-hl + γ-cd + AD 2.25 M KCl; α-hl + γ-cd + AD 20 pa 2.5 ms 40 pa 20 pa 2.5 ms Spectral density, A 2 /Hz M KCl; α-hl + γ-cd + AD 1.00 M KCl; α-hl + γ-cd + AD 1.00 M KCl; α-hl + γ-cd no AD Frequency, Hz 0 pa 2.5 ms Gurnev, Harries, Parsegian, Bezrukov 2009, ChemPhysChem, in press

14 AD-CD interaction is sensitive to the added salt 0 pa 1.00 M KCl; α-hl + γ-cd no AD 1.00 M KCl; α-hl + γ-cd + AD 20 pa 2.5 ms Equilibrium binding constant, M M KCl 1.25 M KCl 1.75 M KCl 2.25 M KCl Increasing [KCl] 0 pa 2.25 M KCl; α-hl + γ-cd + AD 40 pa 20 pa 2.5 ms δ G, kj / mol δ G, kjm Voltage, mv [KCl], Osm 0 pa 2.5 ms Gurnev, Harries, Parsegian, Bezrukov 2009, ChemPhysChem, in press

15 Single molecule vs. bulk measurements 0 α-hemolysin channel KBr KCl Na 2 SO 4 Isothermal Titration Calorimetry 0 KBr KCl Na 2 SO 4 δ G, kj / mol δ G, kj / mol [Salt], Osm Salt n water KBr 10 ± 1 KCl 18 ± 2 Na 2 SO 4 85 ± [Salt], Osm Salt n water KBr 9 ± 1 KCl 14 ± 1 Na 2 SO 4 47 ± 2 Gurnev, Harries, Parsegian, Bezrukov 2009, ChemPhysChem, in press

16 Salts affect differently on- and off-rates of the reaction KBr KCl Na 2 SO 4 ln (τ ο / τ ο 0) KBr KCl Na 2 SO 4 ln (τ r / τ r 0) [Salt], Osm [Salt], Osm Salt N waters KCl 6 ± 2 KBr -1 ± 1 Na 2 SO 4 71 ± 12 Salt N waters KCl 11 ± 1 KBr 13 ± 2 Na 2 SO 4 31 ± 6 Gurnev, Harries, Parsegian, Bezrukov 2009, ChemPhysChem, in press

17 Main Results Sensing with a nanopore allows to probe the dynamic side of the Hofmeister effect Anions affect equilibrium molecular association according to their position in the Hofmeister ranking Br < Cl < SO 4 2 Remarkably, both on- and off-rates are affected by these anions; sulfate affects association on-rate more effectively than off-rate Gurnev, Harries, Parsegian, Bezrukov 2009, ChemPhysChem, in press

18 How strong is this effect in practical terms? Experiments with maltoporin and sugars (unpublished, Gurnev et al.)

19 How strong is this effect in practical terms? Experiments with α-hemolysin and PEGs (Krasilnikov, Rodrigues, Bezrukov 2006, Phys. Rev. Lett.) 2 µm PEG 1 M KCl: 2 µm PEG 4 M KCl:

20 Adrian and Daniel, collaborators not present in the audience

21 Role of hydration in nanopore sensing: Hofmeister series, preferential solvation, osmotic stress, and all that Sergey M. Bezrukov Laboratory of Physical and Structural Biology, NICHD National Institutes of Health together with: Philip A. Gurnev, Daniel Harries, and V. Adrian Parsegian

22 Melting temperature for the ribonuclease transition as a function of concentration of various added salts K Phos (ph 6.6) (NH 4 ) 2 SO 4 Melting temperature, C KCl CaCl 2 KSCN NaCl LiCl NaBr LiBr Concentration, moles / liter von Hippel and Wong, Science, ,