Peptide Reconstitution

Peptides are stored in a lyophilized (a freeze dried fine powder) state and so need reconstitution before they can be used. Bacteriostatic water is generally used although some peptides for example IGF1 type peptides Acetic acid is used. Both extends the storage life of the reconstituted product and also allows multiple doses to be drawn up out of the vial, which is more convenient and cost-effective in a research context. However, for IGF1 DES and IGF1 Long R3, Acetic Acid 0.9% is favoured for reconstitution due to buffering issues and poor solubility when using bacteriostatic water. 
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Looking at the various diluting agents inmore detail: 

Bacteriostatic Water 

This is a sterile, non pyrogenic preparationof water for injection containing 0.9% (9 mg/mL) of benzyl alcohol added as a bacteriostatic preservative. It is supplied in a multiple-dose container from which repeated withdrawals may be made to dilute or dissolve drugs for injection. The pH is 5.7 (4.5 to 7.0). Bacteriostatic water is used to reconstitute all peptides, although for IGF1 derivatives Acetic Acid is recommended. 

Acetic Acid Water

Acetic acid water benefits over Bacteriostatic water is it can help increase solubility of certain peptides and keep them more stable.