Photoreactive molecule

Some of the key parameters useful for planning a photochemical reaction are summarized in Table 1.1 and Figure 1.1. Thus, a lamp must be chosen that emits where the putative photoreactive molecule absorbs (check the absorption spectrum). 

Consider a photoreactive molecule M. If the triplet state of M is involved in any photochemical reaction, then addition of triplet acceptors to the reaction solution should quench triplet state reactions by the diffusion controlled process... 

Parenteral formulations often contain excipients considered to be chemically stable and inert however, all excipients in a formulation may influence the photochemical stability of the product. Dextrose and sodium chloride are used to adjust tonicity in the majority of parenteral formulations. Sodium chloride can affect photochemical processes by influencing solvation of the photoreactive molecules (see Section 14.2.3). The ionic strength is reported to affect the photochemical decomposition rate of minoxidil until a saturation level is reached (Chinnian and Asker, 1996). The photostability of L-ascorbic acid (vitamin C) in aqueous solution is enhanced in the presence of dextrose, probably caused by the scavenging effect of the excipient on hydroxyl radicals mediated by the photolysis of ascorbic acid sucrose, sorbitol, and mannitol have the same effect (Ho et al., 1994). Monosaccharides (dextrose, glucose, maltose, and lactose), disaccharides (sucrose and trehalose), and polyhydric alcohols (inositol, mannitol, and sorbitol) are examples of commonly used lyo-additives in parenterals. These excipients may also affect photochemical stability of the products after reconstitution. 

Phenomenologically, we can distinguish the following photochemical reactions photoisomerization, photocyclization (photoaddition), photocleavage, hydrogen abstraction, photo-concerted reaction, etc. For a photochemical reaction to occur, efficient absorption of ultraviolet or visible light is necessary, thus the photoreactive molecules should contain in their structure one of the bond types listed in Table 1.13. The characteristics of the typical photochemical reactions of C = C and C = O groups are summarized as follows. 

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