106 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS : NH• H: NH• I I [ I c c //% //% 0 OH 0 0: © This ionization prevents the desired transition by bonding the unshared electron pair of the nitrogen atom in the ammonium ion, so that the quin- oidal resonance structure cannot be formed. Thus, PABA is a relatively poor sun screen, and the absorption coefficient is low. However, when ionization is prevented by esterification of the carboxylic group, the quinoidal resonance is no longer impeded. The major electronic transition in glyceryl-p-aminobenzoate is shown in the following scheme. : NH= •NHz I c c //% O OCH=--CHOH--CHzOH ©:O OCH= CHOH CH=OH Here the absorption maximum is at 297.5 m•, which corresponds exactly to the erythematogenic maximum of sunlight. To compare effectiveness, more than a 15 per cent concentration of PABA is required to equal the screening efficiency of 2 per cent glyceryl-p-aminobenzoate. The relative positions of the polar groupings in the molecule greatly affect the energy and efficiency relationships. A change from the para to the ortho isomer in aminobenzoate esters results in a decrease in the chromophore cross-sectional area. Because of this the ortho isomers (anthranilates) are much weaker absorbers than the corresponding p- aminobenzoates. When the meta-aminobenzoate is tested, it is found that the peak absorption has shifted to lower wavelengths, and that the material is a very weak absorber. This is a consequence of the structure: electronic shifts involving both polar groups are not readily possible in " o/NH,, I [•///NH= .--.'• • c c c O OR O OR ©:O OR

DESIGN OF COSMETIC SUN SCREENS 107 the meta isomer since they are located at nonconjugating positions in the molecule bottom of page 106. This transition is simila rto that of the simple benzoate esters, and to aniline. These considerations lead to the conclusion that the para isomers afford the optimum cosmetic screen. Substitutions of various groups on the amino radical of the para amino- benzoates lead to considerable modification of the transition possibilities, and hence change the screening characteristics of the original chomophore. Acetylation of the amino radical greatly reduces the basic character of the nitrogen atom and hence decreases its electron-releasing tendencies. Thus the probability of the transition o o II I1 NH--C--CHa •NH--C--CHa I c c //% 0 OR ©: 0 OR is smaller than that in the parent free amine, and requires greater energy. As a result, absorption maximum is lowered from 297.5 mu to about 270 mu, and resulting p-acetamidobenzoate ester is not a good cosmetic screen. Another method of modifying the amino group is through Schiff-base formation by condensation with an aldehyde group. NH2 N=CH--R• q- R•--CH:O --• + H20 I c c o OR O OR The resulting aldimino structure allows a transition to occur which lies within the erythematogenic range of energy, N:CH--R• N--CH--R• I I c c //N /% 0 OR ©: 0 OR