What was the first piece of work in computational biology?
I don't believe you'll ever find the first work in bioinformatics (or computational biology, as you put it), however the field really began in the times of accumulating data about protein biochemistry. Computational biologists (before they had access to the computer) would be writing and analyzing morphologies and types of proteins with pencil and paper. 

But you can go even further back than this. "Alignment" is another old technique used in bioinformatics. This is establishing the amount of similarity between two DNA sequences, or rather the degree of similarity between any two objects or data sets in computational biology.

Your question, though, asks for a scientific paper (one of the oldest) on bioinformatics. This is arguably  M. O. Dayhoff's paper: "A Computer Program to Aid Primary Protein Structure Determination." (1962)

This is the first formal paper that limits the scope of computational biology to the definition provided by this BioPlanet article and generally agreed by many:


  Bioinformatics is the application of computer technology to the management of biological information.