Matt, how well does it work in practice? Did you check the colligative predictions against the measurements that you originally made?
I agree that the PEG has virtually no effect on the drop equilibration. This can be seen in comparisons of batch and v.d. e.g. http://www.douglas.co.uk/convert.htm. Therefore I always thought that equilibration in high-PEG experiments was driven by any salts that may be present - including the salt in the protein, which is easy to overlook. I remember a paper by Luft and De Titta about "chaperone salts". However, unless you know the relative effects of different salts, the result is hard to predict! On 23 November 2012 16:05, Boaz Shaanan <bshaa...@exchange.bgu.ac.il> wrote: > > > > > *Boaz Shaanan, Ph.D. > Dept. of Life Sciences > Ben-Gurion University of the Negev > Beer-Sheva 84105 > Israel > > E-mail: bshaa...@bgu.ac.il > Phone: 972-8-647-2220 Skype: boaz.shaanan > Fax: 972-8-647-2992 or 972-8-646-1710 * > ** > ** > * > > * > ------------------------------ > > >This leads to a counter-intuitive observation - it is only the number > of molecules in solution that affect the RH and not the type >of > molecule/ion - therefore one molecule of glycerol has the same contribution > as a chloride ion or anything else. This means >that there is no effect > for charge etc. What does matter is how many species the salt dissociates > into - this means that a given >concentration of sodium malonate (3 > species) will have a lower RH than ammonium sulphate (2 species (NH4+ and > >(NH4SO4)-) and not 3 as might be expected). > > This means that relatvie humidity is a colligative property: > http://en.wikipedia.org/wiki/Colligative_properties > > Doesn't it? So it should not be too surprising. > > Boaz > > On 22/11/2012 18:32, Patrick Shaw Stewart wrote: > > > Matt > > My old "Rubber Book" (Handbook of Physics and Chemistry, 1976) has a > table (attached) showing the "lowering of vapor pressure by salts in > aqueous solution", taken from the Smithsonian Tables. > > I've never been able to make any sense of it in terms of cations, > anions, valency, or charge density (position in Hofmeister series), whether > concentrations are expressed as M or N solutions. > > For example, of the salts mentioned on your website, MgSO4 seems to be > anomalous. > > My spreadsheet also has a little converter that my colleague wrote many > years ago to convert vapor diffusion conditions to batch. This might be of > interest to people who have to work in batch, e.g. people making crystals > for X-FEL data collection. You can download it as a program from > http://www.douglas.co.uk/vdtomb/vdtomb.htm - it seems to work pretty well. > > Best wishes > > Patrick > > > > > > On 22 November 2012 12:11, Matthew Bowler <mbow...@embl.fr> wrote: > >> Dear All, >> after a few requests, I have now added equations to the online >> calculator that uses Raoult's law to calculate the relative humidity >> equilibria for precipitant solutions (see http://go.esrf.eu/RH). The new >> equations (4 and 5) allow the calculation of salt concentrations that will >> be in equilibrium with a certain PEG or other molecule solution - this will >> allow slow and controlled dehydration experiments to be designed in vapour >> diffusion plates, by slowly increasing the salt concentration in the >> reservoir above the equilibrium and thereby reducing the amount of water in >> the crystallisation drop by a controlled amount. Hope it is useful, >> cheers, Matt. >> >> -- >> Matthew Bowler >> Synchrotron Science Group >> European Molecular Biology Laboratory >> BP 181, 6 rue Jules Horowitz >> 38042 Grenoble Cedex 9 >> France >> =================================================== >> Tel: +33 (0) 4.76.20.76.37 >> Fax: +33 (0) 4.76.88.29.04 >> >> http://www.embl.fr/ >> =================================================== >> > > > > -- > patr...@douglas.co.uk Douglas Instruments Ltd. > Douglas House, East Garston, Hungerford, Berkshire, RG17 7HD, UK > Directors: Peter Baldock, Patrick Shaw Stewart > > http://www.douglas.co.uk > Tel: 44 (0) 148-864-9090 US toll-free 1-877-225-2034 > Regd. England 2177994, VAT Reg. GB 480 7371 36 > > > -- > Matthew Bowler > Synchrotron Science Group > European Molecular Biology Laboratory > BP 181, 6 rue Jules Horowitz > 38042 Grenoble Cedex 9 > France > =================================================== > Tel: +33 (0) 4.76.20.76.37 > Fax: +33 (0) 4.76.88.29.04 > http://www.embl.fr/ > =================================================== > > -- patr...@douglas.co.uk Douglas Instruments Ltd. Douglas House, East Garston, Hungerford, Berkshire, RG17 7HD, UK Directors: Peter Baldock, Patrick Shaw Stewart http://www.douglas.co.uk Tel: 44 (0) 148-864-9090 US toll-free 1-877-225-2034 Regd. England 2177994, VAT Reg. GB 480 7371 36
