I realized his doubt about the interaction between the bacteriophage (head) and a surface by electrostatic interactions. The magnesium ion, if complexed, you can change the phage load balancing and reduce this total charge, because the metal is positive and negative groups in the head, and even change some configuration of phage groups. The pH and ionic strength (salt concentration in liquid medium) should influence from the concentration mainly by water adsorbed in the phage and the electrical double layer that can block the interaction of the phage with an oppositely charged surface. This article in Portuguese can be used, wondering phage as a colloid and behaviors may be similar.

I'll think some more and I inform you, for what I am trying is to produce surfaces with carbon fibers and natural polymers for immobilization of the phages for control of surface charge and electrostatic interactions.

Hi dear Eduardo,

Thanks so much for your kind reply, 

 Actually, T4 phage head (and T-even phages) have impermeable capsids. It means that their capsid and the natural cations inside capsid (for neutralizing DNA) are impermeable to other external cations (from external salt or buffer), and also the internal cations can not go out with simple dialysis process. So, I think the reason for using buffer to keep phage is the existence of salts in buffer and keep the balance for entering water to the phage head. It means that if we remove salt from the external solution (from buffer), the water will penetrate more to the phage capsid and causing rupture.

So, I think that external salt has no influence on the phage head charge (it means that the phage head should be neutral), just the phage will be under high stress without salt which makes some phages being inactivate.

The other issue is that just in one situation the internal cations will be going out of phage head during dialysis. If before dialysing, we shock the phages with rapid dilution and then doing the dialysis for several hours, the cations inside phage can go out. Because the protein on phage head will be changing during dialysis and being permeable to the salt ions.

I am a little bit confused, because the papers that I found are so old and I could not find anything new for precise answer.

Till now most of papers were talking about negatively charge of phage head  due to the DNA negative charge, while I think that it should be just because of some proteins that are located outside of capsid, and their charge will change because of pH and ionic strength of the media.

Ima:

The process then is governed by internal and external concentration that is the osmotic pressure. The ion that is inside, possibly this complexed (or strongly coordinated) to a biological function and can be related to structural rigidity along with proteins and other constituents. This ion is not available due to their coordination and / or interaction with these components. So it's not a mobile ion may be removed by dialysis, it fulfills a structural function. Some of the articles I found the show organization and some influencing factors, including temperature. If the water molecule may enter the capsid there possibly channels or transport pathways that allows for mobility inwardly ... believe can understand the phenomenon by osmosis but not forgetting that there external solvation, even the coating is hydrophobic (water can be arranged externally even with unfavorable entropy).

Link1: http://www.nature.com/nrmicro/journal/v11/n3/full/nrmicro2988.html

Link2: http://www.engr.ucr.edu/~jwu/papers/Biophysical_737.pdf

Link3: http://schaechter.asmblog.org/schaechter/2011/09/phage-dna-going-with-the-flow.html

Link4: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1303066/

Link5: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0039793

Link6: http://www.sciencedirect.com/science/article/pii/S0042682205005829

Link7: http://www.rpgroup.caltech.edu/publications/purohit2005.pdf

Link8: http://www.sciencedirect.com/science/article/pii/S0006349508702261

Link9: http://arxiv.org/ftp/arxiv/papers/0806/0806.3835.pdf

Dear Ima,

I am not sure do I understand the question? But anyway, T4 surface charge is not directly connected to DNA charge. This is real surface charge of phage capsid. It is not true that phage capsid is impermeable to ions. Just analyze the protocol for preparation of T4 ghosts - phage particles are incubated in high salt solutions before dilution in deionized water. Old papers from 1950s are very useful. everything described there is truth!

Dear Rimantas,

Thanks for your answer,

I agree with you somehow that the capsid charge is not directly connected to the DNA charge, but presuming the phage head permeability to the salts, dilution increases the internal cations to go out. So, DNA negative charge should be predominant internal charge since there is not counterions inside to neutralize it. Hence, should this charge added to the net charge of capsid after dilution?

About the impermeability of T4 capsid, I read two old articles about the less permeability or in some cases impermeability of T-even phages to the ions in comparison to that of T-odd phages that is related to more heterogeneity of T-even capsid. 

"The Role of polyamines in the neutralization of bacteriophage deoxyribonucleic Acid", 1959

and 

"Effect of osmotic shock and low salt concentration on survival and density of bacteriophage T4B and T4Bo1", 1966

Best,

Ima

Dear Ima,

Even in the case of the ion-permeable capsid not so much Mg2+ will leak out because it (as an ion) will equilibrate according  ELECTROCHEMICAL gradient. When Mg2+ will start to leak out because of the chemical (concentration) gradient, the electrical gradient will start to increase and will stop the leakage of Mg2+. Only in such case if there will be some electroneutral exchange of Mg2+ to other cations from the medium (or some anion will leak out from phage head together with Mg2+, but  are such anions available in the phage head?) a considerable amount of Mg2+ will leak out. But not in the low ionic strength medium!

Dear Ima,

You might find this useful:

https://iram.iau.edu.sa/

Capsid's total charge as well as charge distrubution at a selected radii can be measured. Also, psf files (for further physical properties analysis, molecular dynamics, ..etc) are also generated and deposited at IRAM database.

Iman Almansour Dear Iman, Thank you very much for sharing such a useful database