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Coherent optical systems are well known for performing certain operations such as Fourier and Fresnel transformation and pattern recognition. The basis for pattern recognition in many coherent optical systems is the correlator. This paper discusses the effect of undersampling in an optical joint transform correlator which has a sampled-data input, detectors of size 1 × i resolution elements in the Fourier plane, and detectors of size 1 × 1 resolution elements in the correlation plane, where i is an integer. After the undersampling, it is shown that the correlation plane still contains the needed data, the shape of the correlation surfaces are unchanged, and the different correlation terms just move closer together. It is shown that the penalty for undersampling is that the amount of input data must be reduced by a factor of i. It is also shown that a digital simulation using a discrete Fourier transform gives an accurate prediction of the optical correlator performance. Results from the simulation are given to verify the theory.

Incubation with a mixture of Sepharose-bound peptidases was shown to result in the quantitative release of amino acids from certain peptides and S-aminoethylated proteins. Subtraction of the low background values of amino acids generated by the enzymes enables amino acid ratios of corticotrophin-(1–24)-tetracosapeptide to be determined with a standard deviation on repeat digestions of 3–5%. Good values were obtained for amino acids that are completely or partially destroyed on acid hydrolysis, i.e. tryptophan, tyrosine, serine, asparagine and glutamine. Experiments with peptides containing d-amino acids showed that the enzyme mixture is stereospecific and could therefore be used to detect the presence of d-residues in peptides. The enzyme mixture completely hydrolyses peptide fragments obtained after Edman degradation and should therefore be useful for determining sequences of peptides containing acid-labile amino acid residues. The activities of the bound enzymes were unaltered over a period of 7 months and they provide a simple, reproducible procedure for the quantitative determination of amino acids in peptides and proteins containing l-amino acids.

IN a previous paper1it was shown that DL-N-benzoyallothreonine ethyl ester was converted into the corresponding oxazoline ester with inversion of configuration at the β-carbon atom by the action of thionyl chloride.

IN a communication to the editor of the Journal of the American Chemical Society1which has just been received are described the conversion of DL-N-benzoylallothreonine methyl ester (I, R = Me; R′ = H) into the corresponding oxazoline (II, R=Me) by the action of thionyl chloride and hydrolysis of the oxazoline with acid to give DL-threonine (III). This discovery was anticipated by Attenburrow, Elliott and Penny2, who described not only the thionyl chloride method of inverting the configuration of DL-N-benzoylallothreonine ethyl ester (I, R=Et;, R′=H), but also a second and closely related method consisting of the treatment of the p-toluenesulphonyl derivative (I, R=Et; R′=Ts) with potassium acetate in boiling alcohol, followed by acid hydrolysis. The same oxazoline (II, R=Et) was produced in both cases, and the view was advanced that inversion of configuration took place simultaneously with ring closure. The trans structure was therefore assigned to the oxazoline on the basis of the known configuration of threonine3.

THE isolation and purification of a hypertensin peptide, resulting from the action of rabbit renin on ox serum, have been described recently by one of us. Its amino-acid composition was consistent with a decapeptide structure containing the following eight amino-acids: leucine (1), phenylalanine (1), tyrosine (1), proline (1), aspartic acid (1), arginine (1), valine (2) and histidine (2). Application of the fluorodinitrobenzene technique of Sanger showed that the N-terminal residue was aspartic acid1. The amino-acid sequence in this peptide has been determined by the following methods:

THE value of lithium aluminium hydride as a reagent for the characterization of the C-terminal residues of proteins has been investigated by several workers1; but, except in the case of insulin2, the potentialities of lithium borohydride have been less fully explored. Grassmann, Hörmann and Endres3have described a method for the characterization of the C-terminal residues of small peptides which involved esterification followed by reduction with lithium borohydride hydrolysis and identification of the amino-alcohols produced. When a similar technique was applied to procollagen4, amino-alcohols were obtained even from the un-esterified protein and it was concluded that carboxylic ester groupings were present. Experiments carried out in this laboratory and described below show, however, that cleavage of peptide bonds can be brought about by lithium borohydride, those bonds involving the carboxyl group of glycine or of alanine being particularly susceptible. At present, therefore, little confidence can be placed in lithium borohydride as an end-group reagent for large peptides or proteins.