I was reading this paper (https://www.tandfonline.com/doi/full/10.1080/10717540801905124) and I was wondering about this part below and I had a few questions.

“Synthesis of Aminated Gelatin

Aminated gelatin was synthesized by the reaction of native cationic gelatin with 1, 2-ethylenediamine (Wang et al. 2000). Briefly, native cationic gelatin was dissolved in phosphate buffer (pH 5.3), and 1, 2-ethylenediamine, was added under stirring. After adjusting the pH to 5.0 with 5 M HCl, 1-ethyl-3-(3-dimethyl-aminopropyl)-carbodiimide hydrochloride was added. The final concentrations of native gelatin, ethylenediamine, and 1-ethyl-3-(3-dimethyl-aminopropyl)-carbodiimide hydrochloride in the resultant solution were 20, 56, and 10.7 mg/mL, respectively. The resultant solution was incubated at 37°C for 18 hr, followed by dialysis against purified water for 48 hr, and then freeze-dried to obtain the aminated gelatin. The numbers of primary amino groups introduced into the native cationic and aminated gelatin were measured by the TNBS method (Snyder and Sobocinski 1975). The amino group contents of the native cationic gelatin and aminated gelatin were 0.33 and 0.64 mmol/g, respectively.

Preparation of Gelatin Microspheres

NGMS and CGMS were prepared by glutaraldehyde cross-linking in the dispersed state of an aqueous gelatin solution in the oil phase by a modification of the method of Tabata and Ikada (1989). Briefly, 1 mL native cationic or aminated gelatin aqueous solution (10 wt%, preheated to 40°C) was added dropwise to 25 mL olive oil and then emulsified by vortex-mixing to yield a water-in-oil emulsion. The emulsion temperature was dropped to 4°C, then 7.5 mL cold acetone was added to the emulsion and stirring was continued for 1 hr to induce gel formation. The resulting microspheres were collected by centrifugation (2500× g, 4°C, 5 min) and washed three times with acetone. The cross-linking of the microspheres was carried out using 0.06% glutaraldehyde in acetone/0.01 M HCl solution (7:3) at 4°C for the indicated time. Following collection by centrifugation (2500× g, 4°C, 5 min), the microspheres were agitated in 10 mM aqueous glycine solution at room temperature for 1 hr to block the residual aldehyde groups of the unreacted glutaraldehyde. The resulting microspheres were finally washed three times with purified water by centrifugation and then freeze-dried.”

From Millipore Sigma’s website “Physical Properties: Type A gelatin has 78-80 millimoles of free carboxyl groups per 100 g of protein and a pI of 7.0-9.5.”

What does it mean when there’s a range for the isoelectric point? Does it mean that using the procedure from the paper, it can increase type A gelatin’ s isoelectric point to be closer to 9.5? From the second paragraph, are microspheres just powders? Would it be soluble in water? Lastly, is an isoelectric point the same as a pH point of zero charge? Thanks so much!