304 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS posed upon a disulfide bond. Prior to our ultraviolet spectral interpreta- tion of disulfide strain there were two methods employed to measure this strain at best these methods were little more than qualitative. The "alkali lability" method used by Clarke (50), Schbberl (51), and others, is based on the time required to produce a precipitate of lead sulfide when a sulfur containing compound is treated with an alkali solution of a soluble lead salt. A second method employed by Aflqeck and Dougherty (52) in the case of cyclic disulfide was to observe the apparent ease with which polymer- ization of the disulfide compound occurred in the presence of AICl:•. In our method not only have we been able to estimate the disulfide strain from ultraviolet spectra analysis, but we have been able to quantitatively inter- pret the spectra in terms of the specific chromophores responsible for the disulfide absorption in the ultraviolet. We have shown that the categori- cal assertion that alkali of itself cleaves the S--S bond is not necessarily true. Many tertiary disulfides are stable in alkali. Actually ultraviolet spectra do not reveal the strain existing in a disulfide bond as such, but rather they reveal the presence of a chromophore. This chromophore is produced as a consequence of the desire on the part of the strained struc- ture to assume a non-strained configuration. The structure invariably assumed by strained disulfides is the conjugated linear form --C•S-- S=C--. This particular chromophoric form will occur in either acid or alkali and depends solely on the structure of the disulfide compound in question. In addition to the apparent strain imposed on the disulfide linkage by pH or structural distortion of the sulfur bond angles, there is another factor affecting S--S cleavage, namely, the steric factor. Here we will attempt to distinguish between internal and external steric factors. What we imply by an internal steric factor may be understood by the following considera- tion. When large or numerous bulky groups are present on both sides of an S--S linkage, these groups will serve to barricade the disulfide against attack by various disulfide-specific reagents. In the case of penicillamine disulfide, reagents which will normally cleave cystine such as sulfite, cya- nide, etc., are now ineffective. This we interpret in terms of the presence of the four methyl groups. Schbberl (53) has similarly indicated in the cases of tetra methyl and tetra phenyl dithiodiglycolic acids, that these com- pounds are not attacked by reagents that ordinarily cleave S--S. Arnold (54) in addition, has reported that tertiary butyl disulfide is extremely diffi- cult to reduce. In all instances cited here bulky groups within the mole- cule are preventing an attack upon the disulfide group. The external steric factor in connection with disulfide cleavage is only manifest in three-dimensional structures such as proteins. The proteins, keratin, cortocin, and insulin, have the common feature that all contain

PROGRESS IN THE CHEMISTRY OF DISULFIDES 305 about 13 per cent/-cystine in addition to qualitatively possessing the same amino acids. These proteins differ in the sequence in which the amino acids occur along the peptide chains. The proteins most likely differ in the identity of the amino acid joined to the cystine peptide link at the site of the disulfide cross link. In the case of insulin, Sanger (5) has shown that one important sequence involving cystine is the tri-peptide unit--glycine- cystine-alanine-. In the case of keratin the large amounts of dicarboxylic acids present as aspartic and glutamic acids lead us to believe that in a good number of chains several of these dicarboxylic acids are joined to cystine at the site of the crosslink. There occurs in nature a tripepride, glutathione, which consists of glutamyl-cystyl-glycine. It has been established by Phillips and co-workers (55) that about 25 per cent of the combined cystine in wool keratin is resistant to the attack of chemical reagents such as sulfite, alkali, cyanide, permanganate, etc. This resistant portion has been designated as the D fraction. Reed and others (56) have reported that commercial alkaline thioglycolate solu- tions effectively reduce only about 75 per cent of the cystine present in hair. The remaining resistant fraction seems to correspond to the D fraction observed in wool. Studies of peptides derived from wool hydrolyzates reveal that there is a juxtaposition of glutamic and cystine residues (57). The presence of the bulky side chain group of the former amino acid residue may well serve as an effective barrier to chemical attack upon the disulfide bond. Benesch and Benesch (58) attribute the greater resistance ofgluta- thione to oxidation as compared with cysteine as being due to the glutamyl residue with which the sulfur can hydrogen bond. If this proves to be the case then certain disulfide linkages in hair would be shielded by this ex- ternal steric effect and hence may account for the D fraction. Aside from the academic interest in the external barrier effect, what are the practical consequences of this phenomenon ? It might well be that the essential success of the hair waving process is due largely to these few resist- ant bonds which make up the D fraction. These resistant disulfide link- ages serve to maintain the skeletal structure of the hair when it is treated with reagents that cleave the other disulfide links. If these resistant links were not present, application of a reducing agent would then cleave all crosslinks and thus produce a material capable of undergoing plastic flow. Hence the hair will have lost its fiber properties. Severely damaged hair may well be characterized by a destruction of a good number of these resist- ant groups. It may now be seen that the danger inherent in employment of too strong a thioglycolate solution or too long a processing time is that all crosslinking disulfide bonds will be broken, yielding a plastic non-fibrous material. It is thus not necessary and, in fact, potentially detrimental to cleave all S--S bonds. Just enough bonds of the non-resistant, non-skeletally important